Of

The exercise of distinct vision depends chiefly on the Sensation. following circumstances: 1.The perfect transparency of the cornea, and the several humours of the eye; 2. on the just proportional distance between the cornea and Requisites for distinct the crystalline lens, and on their degree of convexity; vision. 3. on the sensibility of the retina; 4. on the degree of illumination of the visible object; 5. on the colour of the pigmentum between the choroid and the retina; and, 6. on the contraction and dilatation of the pupil.

102

Action of

system.

The action of light on living beings is not confined light on the to its effects in producing vision. It seems to act on the system in general as a moderate but constant stimulus. When the light of day is vivid, as in bright sunshine, the body is more active, and the mind more vigorous, than under a cloudy sky. Those climates which are frequently obscured by clouds and vapours, are notoriously the birthplaces of seriousness and gloom; and Boeotian dulness and English melancholy have long become proverbial; while on the contrary, the serene brightness of an eastern sky has been considered as peculiarly favourable to the exertions of imagination, and the flights of fancy. Mr Stuart, a famous pedestrian traveller, told Dr Rush, that during a summer which he passed in a high northern latitude, where the sun is visible for several months together, he enjoyed an uncommon share of health and spirits, which he attributed to the long continuance of the light of the sun. state of nature most animals retire to rest when the light fails, and few people can sleep soundly, unless light be excluded.

103

In a

The stimulating effects of light are peculiarly evident on persons whose nervous system is unusually sensible; they cannot bear strong light, which not only hurts their eyes, but produces considerable agitation on their whole frame. The same effects are produced on those who have been confined in a dark prison. The countenance of these unfortunates is pale and sallow. This latter effect of the absence of light is similar to what takes place in vegetables, as we know that the colour, taste, and smell of plants depend on their being exposed to a due degree of light.

Absence of It has been remarked, that those animals which have light sup- been long confined in a dark situation, are universally posed to fa-disposed to grow fat; and this has been found to take vour obe- place even in condemned criminals, in whom we would sity. least expect it. This obesity has been attributed chiefly to the absence of light. We are disposed to think that the absence of this stimulus can have no immediate effect, but that the disposition to obesity depends rather on the indolence of the confined animals, which is favoured by the absence of light.

104 Bats supposed to have a

For an account of the principal morbid affections of vision, see MEDICINE, No 81.

6. Is there in some animals a sixth sense? From the experiments of Jurin and Spallanzani on the flight of bats that have been deprived of sight, (see MAMMALIA, N° 38.) it has been supposed by some sixth sense. that the accuracy with which these animals in their flight avoided the obstacles that were placed in their

Of

way, is owing to some additional sense which they possess. Others have conceived that the sense of hearing, which Sensation, appears to be very acute in the species on which these experiments were made, is sufficient to supply their want of sight. It is scarcely possible to ascertain which of these two opinions is the more probable; but the writer of this article is rather inclined to adopt the latter, from having observed that when he was walking in an unfrequented street, when it was very dark, he was enabled to avoid running against the common stairs that projected into the street, from a certain sensation that he perceived, when he approached the wall of the stair, which he cannot better describe than by saying that the air at these points appeared to be unusually still.

With respect to sensation in general, we may lay down the following laws, which are considered by Dumas as fundamental principles of this function.

105 1. As activity is an essential character of sensation, Laws of this cannot exist without a certain action of the organs, sensation. and must be proportioned to the degree of attention bestowed on the external objects, or ideas by which it is produced.

2. A repetition of the same sensations tends to render the sensibility less acute, and less capable of receiving new impressions. By repose its energy is restored.

3. As sensibility cannot be employed on two impressions at the same time, it must hold a certain balance throughout all the organs, and it cannot be acutely excited in one part, without being proportionably diminished in another.

4. Sensibility is a relative faculty, which is not equally obedient to all kinds of excitations, but only to those which have some relation to it in the different parts of the living body.

5. It is increased and accumulated in the direct proportion to the defect or weakness of stimulus.

6. It is not proportioned to the number, arrangement, or distribution of the nerves, and its changes of increase or diminution are not susceptible of calculation.

7. It is inconstant, variable in its progress, and unconfinable.*

organs.

* Dumas,

To these we may add the following facts respecting Principes this function and its de Physiologie, tom. 1. The nerves which are principally distributed to ii. p. 151. the organs of the external senses, arise from that part of the sensorium that is within the head.

2. The sensations produced in any part by the contact of external bodies, are more perfect, according as the nerves which terminate in that part arise more immediately from the common sensorium.

3. When a ligature is fastened on a nerve, the parts on which the nerve is distributed are deprived of sensation as far as depends on that nerve.

4. Compression of the brain diminishes general sensation in proportion to its intensity. Slight compression produces numbness.

5. Though sensation probably takes place only in the central parts of the sensorium, it is commonly referred to the extremities of the nerves. Thus, a gouty person who has lost his leg, will suppose that he sometimes feels

ΟΙ

feels the pain of the gout in the toe of the amputated Sensation. limb.

106

Internal senses be

long to me

6. A sympathy takes place between those parts which are supplied by branches of the same nerve. Thus, a violent scratching of the head often produces sneezing; powerful odours snuffed at the nose produce a flow of tears; the head sympathises with the stomach; the mammæ with the uterus, &c.

These are all the phenomena respecting sensation which we can at present notice; we shall mention others when we come to consider the relation between this function and those of motion, digestion, circulation, &c. What have been called the internal senses, as memory, imagination, and judgment, are rather qualities of taphysics. the mind, than operations of the brain; and the consideration of them belongs rather to metaphysics than physiology. To that article, therefore, we refer the reader; and we shall conclude this account of the phenomena of sensation with the following comparative view of that function in the inferior animals.

107 Compara- In all animals that have nerves, voluntary motions tive physio- and direct sensations take place by the same means as logy of sensation. in man. The differences in their motions depend partly on the intrinsic mobility of their fibres, and partly on the disposition of their muscles, and the parts to which they are attached.

The differences in their sensations depend on the number of their senses, and the perfection of the organs belonging to each sense. The animals that approach nearest to man have their senses equal in number to his. In certain species, some of these senses are even more perfect in the structure of their organs, and susceptible of more lively and delicate impressions than ours; on the contrary, in proportion as animals are removed from us, the number of their senses and the perfection of certain organs are diminished; but perhaps some animals, at the same time, possess senses of which we can form no idea.

We know not whether there are differences in the intrinsic sensibility of the nervous system of different animals, i. e. whether an equal impression made on an organ equally perfect, would affect every animal with the same force.

The animals next in order to man have, like him, spontaneous, or what we call internal, sensations. Images are excited in them at times, when they receive no immediate impression from external objects. Thus, dogs and parrots dream. We are not certain, indeed, that the more inferior animals experience similar sensations.

The passions produce effects in animals similar to those which they excite in man. Love is manifested in the same manner in all classes; fear occasions a discharge of excrements in quadrupeds and birds; it makes them tremble, and even renders insects immoveable; but the other animals afford fewer examples of these kind of phenomena than man, because they are not masters of their imagination, cannot direct it towards certain objects, and create for themselves factitious passions. We are even ignorant whether their imaginations can, like ours, be wrought up to such a pitch as to make them experience emotions of anger, desire, or fear, from simple ideas or simple recollections; and whether the real presence of the objects which cause these passions, is not always necessary to excite them in the inferior animals; we know, however, that those which approach nearest

to us, the mammalia and the birds, have their sorrows. The affliction they feel on the absence or loss of a com- Sensation. panion, friend, or benefactor, is manifested by evident signs, in the same manner as they testify their attachment without any temporary inducement.

The same animals exhibit frequent proofs of a very perfect memory; sonre even appear to possess a certain degree of judgment. But does any thing similar exist in the inferior classes, and particularly in the lowest? Of this we shall probably remain always ignorant.

With so much resemblance in the structure of the nervous system, in its mode of action, and in the number and structure of the principal external organs, why is there so vast a difference, as to the total result, between man and the most perfect animal?

Is this owing to a more accurate proportion in the relative perfection of the external organs, so that one does not so much surpass another? Or has the internal organ, in which are performed all the intermediate operations between the sensation received and the movement executed, that is to say, the organ of perception, memory and judgment, greater differences than we have yet observed? Or, finally, is the substance by which these processes are effected of a different nature? These, however, are not anatomical questions.

The sympathies or effects resulting from the connec- * Cuvier's tions of nerves with each other, and the influence of the Lectura, nerves on vegetative functions, are subject to the same vol. ii laws in man and the other animals *.

108

sensation.

The theory of sensation is perhaps more imperfect Theory of than that of any other function. On this subject we can derive little light from the structure of the brain and nerves, accurately as this has been examined. Anatomy has taught us, that the principal part of these organs consists of very delicate fibres, intermixed with a medul lary pulp, and incased in membranes; and that they are furnished with a great proportion of blood-vessels; but whether the seat of sensation resides in the fibrous or medullary part, we cannot ascertain.

It was formerly the opinion, that the nervous power Vibration. was propagated between the brain and the external organs, by vibrations of the nerves; but as the structure of these chords, and their connection with surrounding parts, must wholly disqualify them for such vibrations, this theory has long been abandoned.

Another hypothesis that has been very generally re- Nervous ceived is, that the nervous fibres are the conductors of fluid. a very subtile fluid, called the nervous fluid, the motions of which are the cause of sensation. This was the opinion of Dr Haller, (First Lines, chap. x.) and was stre nuously maintained by Dr Cullen. We shall present our readers with the following modification of it, as given by an able disciple of Cullen.

"It is probable, (says this writer), that in each nervous fibril, an elastic fluid is inherent, forming, from the moment of animation, a part of it; differing, however, according to the state of the constitution, in power, mobility, and, perhaps, in other qualities. Of this fluid the the nerves are conductors, and are surrounded in their course by non-conducting membranes, while the same membrane lines every part of the brain, and is carried into the deepest cavities, guarding with particular attention the slightest aperture. In this view sanguiferous vessels are chiefly useful in nourishing this medullary substance, and they appear to be necessary also in adapting

the

Of

the nerves to their office; for, when the circulation is Sensation. greatly increased, the sensibility is more acute; and when it languishes, or is destroyed, the nervous energy soon shares the same fate.

"This fluid must necessarily be an elastic one; and impressions are apparently conveyed through it by vibrations. It does not follow from hence, that the nerves vibrate like muscular cords; or that, in every the slightest motion, a portion is conveyed from the brain. The elasticity of the fluid is proved from the momentary continuance of the impression after the cause is removed; and vibration is a term employed in many branches of philosophy as a means of communicating motion, without any distinct application. If we touch an object with a stick, or with a metallic rod, we perceive through it the impression, and, in a general way, the nature of the substance. The impression must be conveyed by something; and whatever that something is, it may as well convey impressions through the nerves as through the rod. But through the nerves only can it affect the brain, and produce an idea, or some change in the brain, or its fluid connected with the nature of the ob ject, and which couveys to the mind some peculiar and discriminated impression which it afterwards retains ." A third hypothesis, which is at present very fashionMedzcal able, is, that sensation is produced by a change in the Dictionary, substance of the brain and nerves. M. Cuvier is an advocate for this doctrine, which he illustrates in the following manner.

*New

Land on

vol. i. P. 39.

The nervous system is susceptible of two kinds of action; one which is confined to our sensitive faculty, and another which affects our vital and vegetative functions only. External sensations are produced by the impressions of external bodies, on our senses; internal sensations, by changes which take place, in the state of the internal parts of the body, to which the nerves are distributed; and spontaneous sensations are caused by a change in the nerves, or in the brain itself, without any external excitement.

These circumstances, added to the phenomena arising from the cutting or tying of nerves, show, that sensation does not reside in the external organs, but nearly in the centre of the nervous system, and that the external organs serve only to receive the action of the external bodies, and to convey it to the nerves, by which it is propagated to a greater distance. They also demonstrate, that this propagation is not produced by any matter or concussion, but by a change in the state of the nervous substance. This change may arise from internal causes, or it may be produced by external causes, different from those which usually occasion it. The nerves are not merely passive agents, nor the conductors or reservoirs of any particular matter; but it appears that the substance which produces sensation, is liable to be consumed, or to lose its activity by exertion.

There are phenomena which shew that the general susceptibility of the nerves, for receiving sensations, may vary in consequence of causes external to the nerves themselves, and which can operate only by altering their substance. Certain medicines weaken or revive that susceptibility;-inflammation frequently increases it to an excessive degree. Does this take place in consequence of an increased secretion of the nervous matter? The most remarkable change that occurs in the susceptibility of nerves, is sleep. It is not unnatural to suppose

Of

that this change may be occasioned by the temporary loss of the substance which is essentially sensitive. But Sensation. how does it happen that sleep depends, in a certain degree, on the will? Why do we awake suddenly, or from causes which do not appear calculated to restore that substance? Why does cold produce sleep? From these observations, may it not rather be supposed that this state is the effect of a change in the chemical nature of the nervous substance?

But whether the substance contained in the nerves is exhausted by sensations, or whether it merely undergoes an alteration in its chemical composition, and becomes, as it were, neutralized, it must remain in the nerve throughout the whole of its course, and leave it only at one of its extremities. It does not, however, resemble the blood in the vessels, either as to the manner in which it is retained, or in which it moves in the nerve. There is no evidence of the nerves being tubular. No phenomena indicate that any matter escapes from them when they are divided. Besides, what vessels could have parietes sufficiently compact to retain so subtile a fluid as that of the nerves must be. It is far more probable that it is retained in the nerves, in the same manner as the electric matter is in electric bodies, by communication and insulation; and that the nervous system is its only conductor, while all the other parts of the animal body are, with respect to it, cohibent substances *, The theory of sensorial power, brought forward by Lectures, Dr Darwin, has already been noticed.

CHAP. III. Of Irritability.

* Cuvier's vol. ii.

III

1. WHEN any part of a living animal body that con- General tains muscular fibres, as a part of its composition, is phenomena touched with a sharp instrument, with a hot iron, or of irritabiwith a corrosive liquor, or when a shock of electricity lity. or galvanism is made to pass through it, a contraction takes place in the part; and this contraction is discontinued when the stimulus is removed, but is renewed on repeating the application.

2. The same contractions take place in certain parts of a living animal body, from an exertion of the will.

3. Many parts in which the presence of muscular fibres has not been ascertained, possess the same capacity of being excited to motion by stimuli. Such are the ureters, the biliary ducts, the small blood-vessels, and probably the lymphatics; all of which, though not evidently muscular, have a fibrous structure.

4. Some parts of the living animal body which appear rather nervous than muscular, possess a contractile power, as the retina.

5. When the nerves which form a communication be tween a contractile part and the brain, in the higher orders of animals, are divided or compressed, those parts which before contracted in obedience to the will, lose this power; but,

6. These parts, as well as every muscular part, still contract on the application of stimuli, particularly eletricity and galvanism.

7. Such parts of an animal body as have muscular fibres, are thrown into contraction on the application of stimuli, for some time after having been separated from the living body, provided that nervous filaments remain connected with the muscular fibres.

8. It has been found, that the fibrine of the blood is 302 suceptible

112 Definition

9. In some animals in which a nervous system has not been detected, as polypes, this contractile power seems to pervade every part of the animal.

10. Plants, in a greater or less degree, possess the power of moving on the application of stimuli; and in some species this motion is very remarkable. See N° 57•

The above are some of the principal phenomena which take place in organized beings with respect to irritability. They are so analogous, that we may attribute them to the same cause or the same vital power. This susceptibility of being thrown into contraction on the application of stimuli is called irritability; and it is possessed in a greater or less degree by every organized being with which we are acquainted.

We have restricted the term irritability to denote the of irritabili- susceptibility of the fibrous structure to contraction on the application of stimuli; but it is proper to remark that this term has not always been used in the same

ty.

113 Different applications: of this

term.

sense.

Irritability has long been employed in medicine, as in common language, in reference to the passions, especially that of anger; and this appears to have been the original meaning of the term.

Multo fero ut placem genus irritabile vatum. HOR. It is perhaps still more common to apply it to a morbid sensibility of the system; and we speak of a person being of a very irritable habit, or possessing a great degree of irritability, when we mean to say that he possesses a more than ordinary share of sensibility, liable to a more keen sensation of the same impressions.

"Or are your nerves too irritably strung."

ARMSTRONG.

Even the accurate Dr Whytt, to whom the proper distinction between irritability and sensibility must have been familiar, and by whom it is in general strictly regarded, sometimes falls into this inaccuracy. He speaks, in his work on nervous diseases, of "a delicate or easily irritable nervous system." In fact, this confusion of irritability with sensibility, appears to be a stumbling block to most physiological writers. We shall presently inquire how far they are independent of each other.

The term irritability, in its most received acceptation, as a property of the muscular fibre, seems to have been first employed by Glisson, about the middle of the * Glisson 17th century. He distinguishes two kinds of irritabide Ventri- lity, primary or direct, and secondary or sympathetic*. culo et In- Haller was, however, the first, who treated of irritability, with any degree of accuracy. He confines it to the muscular fibre; though at the same time he will not allow it to many parts, the muscularity of which has never been questioned, and which, since his time, have, by decisive experiments, been proved to possess a considerable degree of contractile power. He completely di

testinis.

Of

stinguishes irritability from sensibility, with which he will have it to be totally unconnected; and he attempts Irritability. to make a distinction between the irritability of the living, and that of the dead fibre †,

+ Haller's

Dr Whytt, whose controversy with Haller respecting First Lines the nature of irritability and sensibility is famous in the of Physioannals of medical warfare, admits three kinds of irrita-logy, chap. bility: 1. That power of alternate contraction and dila- '. tation which is peculiar to those organs we call muscles; 2. That uniform contraction which takes place in the dartos (one of the coats of the scrotum) and the pores of the skin; and, 3. That redness and inflammation, which is excited in every sensible part of the body, as often as acrid things are applied to it; although this last is allowed by him to be only an effect of the first kind of irritability taking place in the small vessels of the part ‡. Thus he reduces the three kinds to two, and Why's we may perhaps consider his second kind only as a mo-cal Essay, Physiologi dification of the first. Essay

Among those who seem to have a sufficiently just part 2. idea of the nature of irritability, the word itself is not unfrequently misapplied. Thus, Vicq d'Azyr ||, and Encyclop. Dumas §, in enumerating the functions of the animal Method. body, called those of motion and sensation, irritability to and sensibility. These latter are powers or capacities of § Principes living beings, and as such should be distinguished from de Physisthe functions that depend on them. logie, tom.

Anatom.

Compare.

114

In considering the phenomena of irritability, it is ne-' cessary to take notice of the several kinds of stimuli which excite it. These have been reduced by Cuvier to Stimuli exfive orders, viz. volition; external actions operating on citing irri nerves; external actions operating on the fibre itself; tability. mixed actions operating on both the nerves and fibres, and certain diseases or violent emotions.

115

When the animal body is in a state of health, and Volition. awake, the will exercises a prompt and constant influence over the greater part of the muscles, which, on that account, are denominated voluntary muscles. A small number of muscles, viz. those which produce the internal movements necessary to life, and which cannot be interrupted, such as the heart and the alimentary canal, are not subject to the will. It must be observed, however, that some of the muscles, that in man and most other animals are involuntary, are subject to the will in others. This is the case with the stomach in ruminating animals, the movements of which may be exerted at pleasure in two different directions. In some muscles, as in those of respiration, there seems to be a mixed action with respect to the will, as this faculty can interrupt their motion for a time, though, in general, this is continued from habit, without the will, or even consciousness of the animal. Those muscles that are absolutely involuntary, are continually excited by an extraneous irritating cause; for the blood which is brought to the heart on every dilatation, determines that organ to contraction, and the alimentary canal is affected in the same manner by its contents. It seems, therefore, that the will is not essential to the action of these muscles, and that it cannot interrupt their motion (c).

A

Of

A muscle laid bare, and exposed to an irritating cause, Irritability. will contract itself, even in the living subject, without being influenced by the will. It should seem, therefore, that though the muscles which we call voluntary, are usually put in motion by the will, they may yet be excited to action in opposition to that faculty.

116

Nervous energy.

117 External stimuli.

118 Galvanism.

119 Distension.

120

Violent passions.

121

Cause of irritability.

The will itself seems to act only through the medium of the nerves; and it is found that those nerves which supply the voluntary muscles, are generally the largest. The external stimuli that act on the muscular fibre through the medium of the nerves, and on the fibre itself, are chiefly of a mechanical and chemical nature, as concussions, punctures, lacerations, all of which are capable of producing convulsive motions in all the muscular parts to which the nerves extend.

One of the most remarkable of these stimuli is the galvanic influence. It is well known that the experiments by which this influence is made to act on the muscular fibre, consist in establishing between a muscle and the trunk of the nerves which extend to it, an external communication with one, or a series of substances placed close to each other. Metals are not the only means that may be employed in this operation; and in general, the conductors are not the same as those of electricity. Experiments have sometimes been successfully performed, when an interval was left in the series of excitators this circumstance, in the opinion of Cuvier, proves the existence of an atmosphere.

The moment the contact takes place, the muscle suffers violent convulsions. These experiments succeed on the living body, or animals recently dead, and even on parts separated from the body, precisely in the manner of those which Haller accounts for on the principle of irritability. Neither pointed instruments nor acrid liquors are necessary; and the galvanic experiments even succeed when these means have failed.

Distension has been observed to have a powerful effect in exciting irritability.

Violent passions may, to a certain degree, be considered as the acts of the will strongly excited. These, in some cases, have an influence even on the involuntary muscles; for it is no unusual thing for palpitation of the heart, and sometimes even a suspension of its motion, to be the consequence of strong passions. These actions, however, are to be prevented by moderating the excess of sensibility by which they are occasioned. Even in nervous diseases, which appear to be the least connected with those passions whose influence is more immediately felt, the will is often capable of preventing or retarding the approach of nervous symptoms, when the patient is determined to resist the paroxysm.

From what has been said, it appears that, in the superior classes of animals, all the orders of stimuli, either act through the medium of the nerves, or that they are capable of being modified or controuled by the will, the exertion of which depends on nervous influence.

With respect to the immediate cause of irritability,

of nervous

1. We almost always observe the irritability of the Whytt's ar muscular organs of the human body to bear a propor- guments tion to their sensibility. Thus, children, and people of in support delicate nerves and very quick feelings, are most sub-influence. ject to convulsive and spasmodic diseases, while on the other hand old people, and those of less delicate sensibility, have a muscular system that is not so irritable.

2. Whatever increases the sensibility of the muscles, also increases their irritability.

3. Whatever lessens or destroys the sensibility of the muscles, also lessens or destroys their irritability or power of motion.

4.

That the motions of irritated muscles are owing to the sensation excited by the stimulus applied to them, Dr Whytt thinks highly probable, if it be considered that we are in fact conscious of many involuntary motions in our own bodies, proceeding from a particular sensation, either in the organs moved, or in the neighbouring parts*.

* Physiolo

ii.

123

Dr Cullen was so fully convinced of the necessity ofgical Esnervous influence to produce muscular contraction, that ys, Essay he considered the muscular fibre to be only a continuation of the nervous fibre. See Medicine, No 73. Haller, as we have said, strenuously maintained, that Haller's opinion of irritability was quite independent of the nerves, and was the vis inan inherent power or vis insita of the muscular fibre. sita. Indeed there are several circumstances which would induce us to believe that irritability is at least, in some cases, independent of nervous influence. We have seen (N° 111.) that it takes place in those animals in which there is no appearance of nerves; and that it is very remarkable in some species of vegetables, in which none but the most fanciful physiologists have dreamed of finding a nervous system. Nay, it appears that the fibrine of the blood, which we can scarcely suppose to be affected by the nervous power, when taken out of the body, is still susceptible of irritation.

124

on this sub

From a comparison of all these circumstances, we General must either conclude, that the irritability of living conclusion muscles, and of the superior animals, is different from ject. that of the fibrine, of polypes and plants; or, if we admit that nervous influence is essential to irritability, we must also allow that this influence descends to the latter class of organized bodies.

125

doctrines

Before we quit the subject of irritability, we must no- Chemical tice the chemical hypotheses that have been lately pro- of irritabiposed, to explain the immediate cause of this faculty. lity The first of these is that of Girtanner, who considered oxygen as the principle of irritability. The Girtanner's arguments opinions.

126