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It appears

tion **

P. 24.


arguments on which he founded this opinion are the fol- acidifiable bases, wbich are to unite with it, are found Or Irritability. lowing.

to be already present in the fibre.'

Irritability. 1. The irritability of organized bodies is always in a He found that every thing that augments too much direct ratio to the quantity of oxygen they contain. the quantity of the acidifiable bases diminishes the irri

2. Every thing that augments the quantity of oxygen tability; and that every thing that increases too much
in organized bodies augments at the same time their ir the quantity of oxygen, likewise diminishes it; and bę

thinks it very probable, that the same takes place with
3. Every thing that diminishes the quantity of oxy respect to the proportion of the galvanic fluid.
gen diminishes likewise their irritability.

It is therefore only in a just equilibrium of these prin.
He distinguishes the organized fibre by three differ. ciples that the necessary irritability of the parts consists.
ent states :

Upon these principles this philosopher ibus explains 1. A state of health, or the tone of the fibre, in which the production of muscular motion." In a state of re the oxygen exists in its proper quantity.

pose, the nerve being inserted in the muscles, the gal.
2. A state of accumulation, in which the fibre is over vanic fluid is put into equilibrium in organs that touch
charged with the oxygen or irritable principle.

each other. The spontaneous motion is made by a sur-
3. A state of exhaustion, in which the fibre is more or charge of galvanic fluid into the nerve.
less deprived of it.

that the instant we wish to make a motion, the galHe likewise arranges the substances, that are capable vanic fluid produced in the brain, is carried en masse of coming into contact with the irritable fibre, into towards the part that ought to move, and surcharges the three classes.

nervous fibres. A discharge from the nerve is then
The first comprehends those substances that have the made into the muscles. The particles of these last, ani-
same degree of affinity for the irritable principle or oxy mated by increased affinities, approach cach other, and * Johnson's
gen, as the organized fibre itself; hence the substances it is this that constitutes the plienomena of muscular mo- Animah
produce no effect



vol öö The second comprehends those substances that have Dumas lays down the following fundamental laws rea less degree of aflinity for oxygen than the organized fi- specting animal irritability. bre has : hence these, when they come into contact with 1. The essential characteristic of irritability consists Laws of ivit, surcharge it with oxygen, and produce a state of ac in a series of contractions and dilatations, determined ritability cumulation. They are called negative stimuli. either by the impression of an external stimulus, or by by Damas.

The third comprehends substances for which oxygen the simple exertion of the will. has a greater affinity than it bas for the organized fi 2. Irritability is independent of the action of the bre. These, therefore, deprive the fibre of its oxygen, nerves; and though generally diffused throughout the and produce a state of exhaustion. They are called po animal organization, it belongs rather to the muscular sitive stimuli.

fibre than to any other structure. Its action is in proBy way of answer to this fanciful doctrine, we may portion to the number of fibres upon which the irritalobserve, that if oxygen were so essential to irritability ing causes can exert their influence. as is supposed in Girtanner's positions, those animals .3. Irritability is a relative faculty which is not indiswhich respire most oxygen should possess most irritabi criminately obedient to every species of excitation, but lity, and those which are capable of living for a long only to those which have some relation to it in the diftime in deoxygenated air, should have their irritability ferent parts of the living body. very low. Now, the reverse of this is found to take 4. There belongs to each organ a specific irritability

place. The muscular fibres of birds which respire more which requires a peculiar stimulus, accommodated to * Johnson's oxygen than most other animals, possess but little irri its nature, and to the kind of functions which it exerAnimal

cises. tahility, while reptiles and worms, which can live for a Chemistry, val. üi.

long time without oxygen, are universally and strongly 5. Irritability has certain vicissitudes of diminution irritable *.

and increase, which vary in the different species of aniHypothesis The other opinion is that of Humboldt, wbo consi mals, in the different organs of the same animals, and

ders the galvanic fluid. the source of nervous power, under the different circumstances that successively occur boldt.

and the primary cause of irritability. He lays down in the life of an individual.
three principles as necessary to excite irritability; viz. 6. Irritability is developed with most energy at the
1. Oxygen, which forms combinations with different moment of death, and immediately after this has taken
acidifiable bases. 2. The acidifiable bases (carbone, place.
hydrogen, azote, and phosphorus), of the fibre, with 7. It is multiplied and revived in proportion as the
which the

oxygen may combine. And, 3. The galvanic organ which has lost it is divided into a greater number fluid.

of pieces. The galvanic fluid produces, according to Humboldt, 8. It diffuses itself in each part with a velocity prothe same effect in the animal economy, as the electric portioned to the activity, number, and duration, of the fluid in the mixture of azote and oxygen. It is this gal irritations by wbich it is excited. vanic fiuid that, being conveyed by the nerves, brings 9. There exist mutual relations with respect to influ. about the co:nbinations of the oxygen

with the different ence between sensibility and irritability, though each of acidifiable bases of the fibres; but when the nerve of a them is essentially distinct from the other. part is tied, it prevents the fluid from passing, which ex 10. The exercise of this faculty supposes in the organs plains the reason of the irritability being destroyed. a moderate degree of cohesion, above or below which * Prheijas The

oxygen necessary for these unions is carried by the action of this force is enfeebled, obstructed, or opthe arterial blood in the course of circulation; and the

posed *.'

ül. 53 3



of Hum.

de Plussa

logi, tom.



Qr Animal

as to bring the point C to H, the point A would be of Animal Motion. CHAP. IV. Of Animal Motion.

drawn but a little beyond the middle point C, so that Motion.

although this latter fibre is contracted to as great an Organs of

The organs of motion vary considerably in their na extent as the former, it has not brought the extremities ture and connection in the different classes of animals. of the tendon so near together. In some tribes, as in the animalcules and polypes, no 2. When two fibres enter a tendon on opposite sides distinct organs can be observed. In all above these, and contract at the same time, they will draw the tenhowever, there are evident muscular fibres, and in many don in the diagonal; and the more nearly the angles there are hard parts or strong membranes, which serve which they form with the tendon approach to right as points of attachment and fulcra of motion to these angles, the more will the length of the muscle be shortfibres. The muscular fibres are to be considered as the ened in proportion to the degree of contraction of the essential moving organs, while the parts to which they fibres. are attached are merely the passive functions of this or Let the fibres BC, BD, BE, BF, BG, (fig. 2.) be Fig. 2: gan. It would be out of place here to enter on a com- fleshy fibres, inserted into the tendon AB, at the point parative account of the organs of motion; and there is the B, and let us suppose that all these fibres co-operate in less occasion for it, as they have been more or less fully bringing the point B to the point G, in the straight described in the former part of the work. The bones, line BG. Now the straight fibre BG will be so much ligaments, muscles, and tendons, with their appendages, shortened when B comes to G, as to be obliterated, As they appear in man, have been amply described in while the oblique fibres EB and FB will be shortened the first and second chapters of the First Part of ANA- only to E a and Fb, and the more oblique fibres CB TOMY; and those of other animals have been briefly and DB will remain of the length of C c and D d. noticed in the Second Part of that article. Such of 3. All muscles that are inserted into bones, are thereour readers as wish for a more particular account, may by furnished with levers, and as in the action of all

consolt Cuvier's Lectures, vol. i. or Blumenbach's Com levers there are also a fulcrum, a power, and a resist130

parative Anatomy, chap. 1, 2, 3, 4, 5, and 22. ance, these in different cases will be differently situated
Principles Many of the phenomena of muscular motion, as they with respect to one another.
of mascular take place in man, have also been related under ANA a. In the motions of the head backward and forward

TOMY, N° 85 and 86. We shall here therefore only on the atlae, the fulcrum is situated between the power
enumerate and briefly illustrate these phenomena, and and the resistance; or the lever is of what is called in
shall then proceed to consider a most interesting part mechanics, the first kind. See MECHANICS, No 33.
of the physiology of motion, the progression of different b. When the tibia rests upon the astragalus, and tle

heel is raised by the muscles of the calf of the leg act-
Dr Barclay, in his late excellent work on the mus- ing on the tendo achillis, the resistance (which in this
cular motions of the human body, las considered the case is the pressure of the tibia) is situated between the
general subject of muscular action under the following power and the fulcrum, which are here respectively at
heads, which may be considered as fundamental prin- the heel and at the toes; or the lever is of the second

1. Fleshy fibres that are continued into tendon by a c. In raising a weight at the palm of the hand, and
straight line, shorten the muscle which they compose, bending the arm at the joint of the elbow, the

in the same degree in which they shorten themselves ; of action in this joint is situated between the resistance
those fibres which enter the tendon obliquely, shorten and the fulcrum, which are here respectively at the
it more, and still more in proportion to their degree of palm of the band and the distal extremity of the hu-
contraction, as they deviate more from the line of the merus (D), or the lever is of the third kind.
tendon, and approach nearer to the perpendicular, in The shortness of the lever, and the consequently great
which last direction they would sborten the muscle most force of the muscular power required to overcome the
with the least contraction.

resistance in this last case, may be thus illustrated. Let This may be illustrated in the following manner. AB (fig. 3.) represent the radius articulated at B with Fig. 3Plate Let AB (fig. 1.) represent a tendon, and CD a fleshy the humerus BC; let DFE represent the biceps flexor ccccxvir. fibre ; and let us suppose that AB is the diameter, and muscle running along the humerus, and attached to the fig. i. CD the radius of the same circle ADB. It is evident radius at E; and suppose a weight W hung to the dis

that if the fibre CD should contract so as to bring the tal extremity A of the radius. Now, BĖ will repre-
point C of the tendon to the point G in the straight sent the lever of resistance, and BG perpendicular to it
line, the extremities of the tendon A, B, (which are the lever of the muscle, which is in tbis case extremely
supposed to be moveable) would come respectively to E short.
and F; and the situation of the tendon itself would be 4. As, other things being equal, all muscles pro-
represented by the angle EGF. If the fibre could be duce a greater extent of motion by a less proportion-
supposed to contract so as to bring the point C to D, the al degree of contraction, and consequently a less propor-
two parts of the tendon CA and CB, would come in tional change in their fibres, than if they were shorter;
contact. If, on the other hand, the fibre CH, which those muscles which follow a direct course are seldom
enters the tendon obliquely, were to contract to H, só attached at the nearest points of the two bones with


(D) In Dr Barclay's nomenclature, that extremity of a bone which is towards the trunk is called proximal, and that extremity which looks from the trunk is called distal.


Fig. 4.

of Animal which they are connected. Hence, beside the advan. with a less shortening of their fibres, than any straight of Animal Motion. tages already mentioned, relations are thus formed be muscles attached to the same parallel surfaces. tween parts at a distance, and the mutual dependence of Let AB and CD (fig. 6. and 7.) be parts of two

Fig. 6.367. the functions and their organs is extended and strength- ribs that are parallel, and that will continue parallel till ened. On the contrary, those muscles that are not ex they are brought in contact by the action of the straight tended along the surface of the bones to which they are muscles AC, EF, and BI), or by the action of the attached, are observed to follow an oblique direction, by oblique muscles CE and DE (fig. 7.) and FA and FB which they acquire not only contractibility and length, (fig. 6.). It is evident, that when the point E comes but at the same time a shorter lever than if they had in contact with F, the length of the straight muscles been inserted at the same place with a less obliquity. must be obliterated, while that of the oblique muscles

5. Of muscles attached to ribs that are parallel, will only be shortened by c E and d E in fig. 7. and / A equally moveable, and at right angles to the vertebral and g B in fig. 6. column, those that follow a direct course from one to 9. As, however, no two bones can approach one anthe other, will act on each by equal levers, and make other in a parallel direction, at least by the action of a them approach with the same velocity; while those that single muscle, and as no muscle can continue to act in a observe an oblique course will act on each by different direction perpendicular to their two approximating surlevers, and make them approach with different veloci- faces ; a muscle entering them at right angles, when ties.

they are parallel, may be placed so near to the centre of Let AB and CD (fig. 4.) represent two parallel motion as to carry the bones threugh a given space, ribs, articulated with the vertebral column at A and C, with a less shortening of fibres than any oblique muscle where they are equally moveable ; and let DB and DE that has the same origin, but is inserted at a distance, be two muscles, the former observing a direct, and the and acts through the medium of a longer lever. Furlatter an oblique course. The levers of

DB will be ther, a muscle with a less obliquity may be so situated AB and CD, which, as AC is parallel to BD, are evi as to carry the bones through a given space, with a dently equal; but the levers of DE will be CF and less shortening of fibres than any other muscle of the AG, which being of different lengths, the muscle must same origin, but of a much greater obliquity. act with different degrees of force on the different ribs, Let AB and CD (fig. 8.) be two ribs, of which AB Fig. & so that it will make CD, on which it acts with the is moveable about the centre A; and suppose that by longest lever, approach AB, faster than it will make the shortening of the straight muscle EF, and of the two this latter approach the former.

oblique muscles, EG and EH, AB is brought into the Corollary.—When bones are not parallel, the muscles position Ab. The points of attachment, after moving in that cross in the interval between them, must fall o- the segments Ff, Gg, and Hh, will now be respectively bliquely on both, as it is impossible for a straight line at f, g, and h.Now, on the centre E, with the radii to be at the same time perpendicular to two other lines, EJ, Eg, and Eh, describe three different circular segunless these be parallel.

ments. The difference between the present and former 6. As all bones move on a centre or axis of motion, lengths of the most oblique muscle EH, will be eH, while the muscular attachments move in a circun) while the differences between the present and former ference, the muscles, in changing the relative position lengths of the muscles EG and EF, will be only - G of any two bones, must, at the same time, be changing and , F respectively. the direction of their own action, and varying their le 10. The shortenings which any muscle suffers in car

rying round the point of its attachment through a given Let AB and CD (fig. 6.) represent parts of two pa- space, will partly depend on the length of its lever, rallel ribs, and let AB be moveable on the centre A, partly upon its degree of obliquity, partly on its drawand let CF and GE be two muscles inserted obliquelying peripherad or centrad, and partly on its acting withinto AB at F and E. Now suppose that by the action out or with a pulley (E). of these muscles, AB is brought into the position A b. 11. The lever of a muscle, which is varied with every The points of attachment of the two muscles to AB, degree of obliquity, is also varied by every change in will now be f and e, and the muscles will be C f and the centre of motion. Where bones are connected by Ge, having changed their length, situation, obliquity, large surfaces, the centre of motion frequently shifts and lever.

from one part to another; but in general it approaches 7. All muscles where the points of attachment move towards that aspect whither the bone is moving at the in a circle, draw either towards the centre, or towards time; and as it advances, the muscles recede, to inthe circumference.

crease their force. 8. If any two bones could, by the action of their a. The lever of resistance, as well as of the power, is muscles, be made to approach in a parallel direction, the varied by the several changes of position ; is sometimes oblique muscles attached to their parallel and approach shortened at the time that the lever of the power is ing surfaces, would perform a greater extent of motion lengthened ; and vice versa.



Fig. 5.

(E) The terms peripheral and central are employed, by Dr Barclay, to denote the aspects of any organ, according as they respect the circumference or the centre of the organ; and when the termination of these words is changed from / into d, they denote, like the other terms of his nomenclature, the direction in which the action of these parts is exerted. See Barclay's Anatomical Nomenclature.



Of Animal If AB (6. 9.) represent the radius, BC the hume gans. Some can only creep, as torms, and many mol- of animal Motiun. ruj, DE the biceps flexor muscle, and R the resistance lusca ; others can only swim, as all fishes, many of the hung to the distal extrenity of the radius, it will be mollusca, and some of the testacea.

Most birds can evident that, when BA is, by the action of the flexor both fly, walk, and run, wlile a few do not possess the muscle, brought into the position B a, the lever of resis- power of exercising the first of these nyotions. All the tance will no longer be BA, but BH, equal to a perpen mammalia, and most reptiles, properly so called, can dicular straight line drawn from B, the centre of mo. walk, run, climb, leap, and perform a variety of other tion, to the plane of resistance; and, as the lever of re. motions; and a few of the former class can imitate the sistance has been shortened, the lever of the muscle has flying of birds. We shall briefly examine the mechabeen proportionably lengthened. Were the radius to nism of these different actions, but by way of introducresume its former position, the reverse of these circum- tion, we shall first consider how the action of standing stances would take place.

is performed. b. Sometimes again, the lever of the power and of the Standing, in most animals, is solely the effect of the Standing resistance are lengthened or shortened at the same time. continued action of the extensor muscles of all the

Let AB (fig. 10.) represent the tibia, BC the fe- joints, as is evident from the circumstance, that if an mur, and DEF the crureus muscle ; and that the fe animal, while standing, suddenly dies, or in consequence mur, with the weight of the body, is to be raised to the of some powerful cause, as a strong electric shock, ceases situation Bc; the centre of motion will, during exten- to make the necessary efforts for preserving the upright sion, approach towards the muscle at the rotular aspect, position, all the articulations of the legs yield to the while the plane of resistance, as is evident from the fi- weight of the body, and bend under it. In some anigure, will be approaching to the centre of motion. mals, however, the extension of the muscles is so much

c. In the changes of attitude, while a bone is turning assisted by powerful ligaments attached to the articulaon its centre of motion, the centre itself is often at the tions of the legs, that they are enabled to continue standsame time describing, either the segment of a circle, or ing for a much longer time, and with much less fatigue a line composed of circular segments.

than most others. This is the case with birds that Let AB (fig. 11.) represent the foot, BC the tibiu, perch, and it is particularly remarkable in the stork, CD the thigh bone, and DE the trunk; and let us sup which by means of this peculiar mechanism is able to pose that it is required to bring the three last, by the stand on one foot for several days together. action of their muscles, to the perpendicular BF, so The action of standing is somewhat different, accordthat BC shall occupy the situation of BG, CD the si ing as the animal stands on two feet or on four. tuation of GI, and DE the situation of IF; the point C That a body may be supported in a vertical position, Standing on the centre B will move in the segment CG, and as it is necessary that it be so disposed as to be in a state on two fect. C is changing its position in CG, the point D, which of equilibrium, or that it be so balanced that a perpenmoves round the point C as its centre, will, if the ex dicular line from the centre of gravity shall fall within tensions be regularly performed in the same time, de- its base. See MECHANICS, N° 193, et seq. It is evi. scribe such a curve as DI; for as the point D must dent that the more extensive the base is on which the necessarily move atlantad and sternad, in order to body stands, the less is the danger of its losing its bapreserve the centre of gravity, the general direction of lance. Man can very easily preserve himself in the verits course must be known ; and if CG be divided into tical position, from the broad basis formed by his feet, equal parts, and at each of the divisions a circle describ- and from the great power he possesses of separating these ed with the radius CD, the points in DI corresponding to a considerable distance. This latter depends chiefly in number with the points in CG, and at equal distances on the greater weight of his pelvis, and the length and in the sternal direction, will each be found in the cir- obliquity of the neck of the thigh bone, by which this cumference of one of the circles described successively bone is carried more out ward, and removed farther in round the point C as it passes along the segment CG. its articulation, than in any other animal. In man, too,

In like manner, if the extensions of CD and DE be the foot is peculiarly adapted to stand firmly on the
regularly performed in the same time, the point E will ground, from the flatness of its inferior surface, and
describe such a curve as EF, the points in EF being in from baving the heel bone so formed as to come in per-
the circumferences of the several circles successively fect contact with the ground. The muscles that move
described round the point D as it moves along the curve the foot are also very advantageously inserted, and the

extensor muscles of the heel are proportionably thicker
12. When we examine the structure of the animal than in most of the mammalia.
system, we shall generally find that the motions of the The thigh of man, when in the erect posture, is in a
bones, as produced by the muscles, are the combined straight line with the trunk and the leg, whereas in
effects of different forces, and hence that a small num- quadrupeds, it is situated close upon the flank, and forms

ber of muscles is enabled to produce, with steadiness an acute angle with the spine. On this account, the * Barclay and accuracy, an almost infinite variety of changes * thigh bone of quadrupeds is flat, and proportionally on Muscue

For more on the general subject of muscular action, weaker than that of man. The extensor muscles of the iar lotion. and for an account of the principal motions of the hu- thigh are proportionally stronger in man than in the Part ä. chap 3.

man bady, we must refer to Dr Barclay's publication. other animals ; and as the thigh bone moves upon the

One of the most interesting enquiries rexpecting ani- pelvis in every direction, these extensors are in man so
mal motion, is that of the progression of different ani- considerable, that he is the only animal that possesses
mals, or of the powers of loco-motion.

what are properly called bips.
Progressive Those animals which possess the faculty of changing In consequence of this structure, the human sacral

their place, exercise this faculty by very different or extremities are furnisbed with a sufficient base, and form
VOL. XVI. Part II.



3 P


two feet

The ground

of Animal very solid bodies for supporting the trunk. Man also up on the approach of danger, as the hedgehog, the of Animal Motion. possesses several advantages for maintaining the gene armaulillos, and the pangolins.

ral equilibrium of the body, especially the facility with Oviparous quadrupeds or reptiles, have their thighs which be holds bis head in the erect posture, owing to directed outward, and the inflections of the limbs take the position of the occipital bone, and the horizontal place in planes that are perpendicular to the spine. In direction of the eyes and mouth. See the article Man, these, therefore, the weight of the body must act with No 5. and 6.

a much longer lever, in opposing the extension of the The quadrupeds that sometimes try to stand on their knee-joints; and accordingly they have the knees alhind foet only in order that they may either employ ways bent, and the belly dragging on the ground betheir fore feet in taking hold of some object, or avoid tween their legs, whence the name of reptiles.

135 keeping their head too low, seem rather to sit than to In walking on a fixed surface, the centre of gravity Walking stand. Their trunk rests at the same time on their hind is alternately moved by one part of the extremities, feet, as far as the heel, and on the buttocks ; it is still and sustained by the other, the body never being at necessary, however, that their head and neck should be any time completely suspended over the ground.

136 proportionally small, as in monkeys, squirrels, oppossums, Animals which can stand erect on two legs, such as

Walkido c. otherwise the weight of those parts would be too man and birds, walk also on two legs. But several great for the force employed in their elevation ; but quadrupeds that cannot stand on two feet but with great even when seated, the animal is generally obliged to rest difficulty, may yet move in that posture for some time on the fore feet, as may be observed in dogs, cats, fc.

with sufficient ease. This arises from its being in geneSome quadrupeds use their tail as a third foot, to en ral less painful to walk than to stand, the same muscles large the base of the body: and when it is strong, it is not being continued so long in action. And also it is capable of contributing to their support for some time. less difficult to correct the unsteady motions by conWe find examples of this in the kangaroos and jerboas. trary and alternate vacillations (a thing easy in walk

We have already noticed the mechanism in the feet ing), than it is to prevent them altogether. of birds, which enables these animals to support them Wben man intends to walk on even ground, he first selves on two legs, though they do not stand in a verti. advances one foot; his body then rests equally on both cal position, and though the atlantal part of their bodies legs, the advanced leg making an obtuse angle with the is adva.ced more beyond the centre of gravity than the tarsus, and the other an acute one. 9ıcral part. Other advantages possessed by birds in not yielding to the point of the foot, the beel and the this respect are, the great flection of the thigh bone rest of the leg must of necessity be raised, otherwise the and tarsus; the length of the anterior toes, and the beel could not be extended. The pelvis and trunk are length and flexibility of the neck.

consequently thrown upward, forward, and somewhat Standing on An animal which stands on four feet is supported on

in a lateral direction. In this manner they move round four feet. a very considerable base ; but from the great weight of the fixed foot as a centre, with a radius consisting of a

the head and neck in these animals, their centre of leg belonging to that foot, which, during this operagravity is nearer to the atlantal than to the sacral ex tion, continually diminishes the angle formed with the tremities (F). It is evident from this, that in quadru tarsus. The leg which communicated this impulse is peds, the former must sustain almost the whole weight then thrown forward, and rests its foot upon the ground; of the body; and we find, accordingly, that they are while the other, which now forms an acnte angle with furnished with very strong muscles. In short, all that its foot, has the beel extended in its turn, and in like the sacral extremities seem to want in muscular force, manner makes the pelvis and trouk turn round upon the appears to be transferred to the atlantal.

former leg As in most quadrupeds the bead inclines towards the As each leg supports the body in its turn, as in standhorizon, and the neck is often very long, very powerful ing on one foot, the extensor muscles of the thigh and means are required to sustain the former. These means knee are brought into action, to prevent these articu. are furnished by the great size, and extensive attach lations from yielding ; and the flexors act immediately ments of the muscles of the neck, and especially in ma

after, when the leg having thrown the weight of the ny quadrupeds by the cervical ligament. In the mole, body on its fellow must be raised before it can again be which empioys its lead to raise considerable burdens carried forward. As the undulatory motion that neof earth, the cervical muscles are peculiarly strong, cessarily attends a man's walking, cannot be perfectly and the ligament is converted into bone.

regulated on both sides, he cannot walk in a perfect The body of a quadruped hangs between the four straight line, nor can he walk in a direct course with legs, and by its weight tends to draw the spine down bis eyes shut. wards. This is counteracted by the abdominal mus In walking down an inclined plane, or descending a cles, especially by the straight muscles, which produce staircase, as the advanced leg is placed lower than that a curvature in the opposite direction. The abdominal which remains bebind, the extensors of the leg must act muscles act with peculiar force in arching the spine more powerfully to prevent the body from falling backupwards in those mammalia that are covered with wards. Again, on ascending such situations it is reşcales or spines, and are accustomed to roll themselves quisite at each step, not only to transport the body hori



(F) These terms signify the same as superior and inferior in man, anterior and posterior in quadrupeds; but are more convenient, as applying indiscriminately to both. Atlantal denotes what is next the atlus; sacral what is next the sacrum. See Barclay's Nomenclature.

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