as through a sponge; the venous blood flows in steady circuit. towards the heart.

The heart, the central driving organ of the circulation, we should find to be,--if we carried our inquiry so far,-made up of muscular fibres, in fact, a strong muscular organ. We should discover that it possesses an independent arterial and venous cirenlation of blood, and that it is fed by the first supply of blood which it, itself, pours forth. We should learn that it has two lines of nervous supply, one stimulating it to motion, the other controlling its motion. On its outside, the heart would be disclosed as coated with a strong fibrous membrane which can be stripped off its surface, and this membrane, reflected on and over the heart, would be observed as enclosing the organ in a membranous bag, -the pericardium,-containing a small quantity of serous or pericardial fluid. Inside the muscular organ we should find another fine strong serous membrane lining the walls and the four sets of valves of both cavities of the heart,-the endocardium.

The arteries, if we looked into their structure, would exhibit a smooth internal lining or coat,-epithelial,--surrounded by a coat of elastic fibres. Next to the elastic fibres would be detected a middle coat of circular muscular fibres with some interwoven elastic tissue; and outside all a firm strong coat, made up of connective tissue containing some more elastic fibres. Thus an artery is elastic under internal pressure or dilatation; contractile by its own muscular power; and strong and resistant by its external investment.

If, following up the arteries, we paused to consider next the minute circulating or capillary system, we should come upon a simpler but much more extended vascular arrangement. We should find a web or net-work of vessels, springing from divisions of the arteries, and themselves now of nearly equal dimensions, onethree-thousandth of an inch, throughout. A lining and surrounding membrane is all that would be found in the finer structure of these vessels; but there would be exposed a free supply of nerves, under the influence of which the small vessels would either contract or dilate, as directed by the nervous current. The contents would still be the blood, the corpuscles of which would be seen to pass in single file, through the capillaries. The inner lining of the capillaries, derived or continuous from the arteries, would be traceable into the veins.

The veins rising, as it would appear, from the capillaries and passing on towards the heart, increasing in size as they progress, would be disclosed to us next as series of tubes closely resembling the arteries but less strong in structure. Internally we should detect in them a similar epithelial coat or lining; around that a lining or coat of circular muscular fibres with some elastic fibres; and outside all a covering or coat of connective tissue. In addition, the veins would present interiorly a series of small valves, derived from their inner coat, which open upwards and direct the blood onwards in its course to the heart.

Shall any local disease visit this circulating system the result must be some disturbance of the balance of life. Let the blood be changed in physical property; let the heart overact or fail in its beat; let the valves of the heart get wrong; let the arteries or veins give way; let the vital arena of the minute circulation be the seat of change, let its minute vessels distend or break, its fire glow too fiercely, or its fire die out in whole or in part; and the result must be disturbance of the balance of life.

THE RESPIRATORY SYSTEM.

From the circulation and the parts of which it is, as a system, composed, we would next, naturally, turn to the respiratory system at which we did already briefly glance when our attention was directed to the course of the blood from the right to the left side of the heart. In the respiratory mechanism we should observe the mode in which the blood is changed from the dark venous to the red or arterial hue. We should see in the two spongy organs, the lungs, one on the right, the other on the left side of the chest, the vessel which comes into them from the right ventricle of the heart dividing into two large branches, a branch for each lung. We should observe, further, that the vessel belonging to each lung divides again into smaller branches, and that these smaller branches, after numerous divisions and subdivisions, become, at length, a vast web of very fine vessels, like to the minute circulation of the body at large. We should, in this instance, however, detect a difference, owing to the introduction of a new element into the refined circulation of the lung. We should discover that in the net-work of the minute vessels of the lung as the vessels pass on in their course to form the commencement of veins, there are interposed mill

ions of little vesicles or bladders, over the outer surfaces of which the web of vessels spreads. We should find that with each breath drawn the little vesicles expand, while with each expiration of breath they contract, so that the whole mass of the lung expands and contracts by the expansion and contraction of the whole mass of the vesicles. Tracing out the meaning of this we should learn that the tiny vesicles are the terminal pouches or cysts of small tubes from which they cluster, something like as grapes cluster from their stems. Following the tubes from the vesicles we should trace them becoming larger and firmer, until at last they became large strong tubes lined with mucous membrane, coated with mucous secretion, and filled with air. Finally we should see that the tubes, as they increase, become firmer still,--constructed now of rings of cartilage,-and that at last they emerge, by a single tube, from the right lung and the left, to form one common large tube which runs up to the throat, and opens into the throat by a chink from which the sound issues when the voice is heard. The anatomist would tell us that the tubes springing from the little vesicles are the minute ramifications of what he calls the bronchial tubes; that the larger tubes are the bronchial tubes themselves; and that the final large tube, ending in the larynx or voice-box, is the trachea or windpipe. The anatomist would also explain to us that all the parts named as belonging to the lungs, the large blood-vessels, the minute blood-vessels, the air vesicles, the bronchial ramifications, and the bronchial tubes, are united together, in each lung, by connective tissue and by abundance of elastic tissue; and that each lung, made up of a great number of little lobules, all constructed on the one plan, is finally included in a delicate, semitransparent sensitive membrane. This membrane surrounding the whole spongy mass, is reflected over the walls of the chest, enclosing the lung in a double bag,--the pleural membrane, or pleura, the bag or space of which is the pleural cavity.

If, now, knowing these general facts of construction, we looked at the intention of so much fine and beautiful mechanism, we should detect that in each act of breathing the chest, by its expansion, draws, like a bellows that is being expanded, a quantity of air, from three to four cubic inches by volume, into the lungs through the windpipe; that under this the small air vesicles are filled with air; that during the expansion of the vesicles the

blood, which courses over them in the minute circulation of the lung, changes in color from dark to red. We should further detect that, in expiration, or emptying of the chest, the vesicles contract, and give back into the bronchial tubes and into the windpipe, and so into the outer air, the same or nearly the same quantity of air as that which they took in during inspiration.

One change more would demand our attention. The air which the tiny vesicles receive in the act of drawing in the breath is common atmospheric air. It is a mixture of two gases in unequal quantities. Nearly four parts out of five are made up of a gas called nitrogen; rather more than one part is made up of a gas called oxygen, in which gas substances capable of going into flame or of being more slowly consumed by slow combustion, burn, with evolution of heat. From this atmospheric air then we should see, as it passes over into the air vesicles, a certain portion of oxygen extracted by the blood. The myriads of small disks of red color floating in the blood stream would be observed to seize upon the oxygen till saturated with it, while the blood would give up, in return, into the bronchial tubes, the windpipe, the mouth and nostrils, and the outer air another gas, a mixture of oxygen and carbon, carbonic acid,-with some returned nitrogen, and with some unused oxygen.

Presuming that we were not acquainted with the nature of this curious process, we might think that by these interchanges of air and blood some heat was produced in the lung when the dark blood began to glow. The industrious scholars who first noticed these changes thought so, naturally enough. We know better now. The process is all for this primary end, that the blood may be charged with oxygen and may carry oxygen into that minute circulation of the body at large which we have already seen to be the vital arena. There the oxygen is to be given up; there the animal fire is to burn; there the carbon supplied by the food is to consume; and there the blood, losing its oxygen, is to exchange it for the newly-formed carbonic acid, and to convey that product of combustion to the lungs, to be cast off by them.

To render the physical mechanism of the lungs more perfect, there is planted,—as the microscope would next reveal to us,—all along the mucous membrane covering the bronchial surface, not only bronchial glands to afford secretion, but an exquisite apparatus called the ciliary. This, if we got to view it, would be

found to consist of an immense number of fine processes,-cilia, -which pave the mucous membrane round the tubes and keep ever in motion, like the waving motion of corn in the breeze, towards the outlets of the tubes. Thus fine particles of offending dust drawn into the depths of the lungs, in inspiration, are wafted back by the cilia, and brought into the throat by the windpipe, are removed from the lungs by being expectorated or swallowed.

Shall any local disease visit this respiratory system, the result must be some disturbance of the body at large. Let the bronchial channels be obstructed, then the animal fire, unfed by vital air, must decline in proportion to the obstruction, even to the extent of going out altogether. Let the air contain some gas or vapor that modifies the character of the respiratory act or carries new matter into the blood, and the whole body must suffer. Let the minute air vesicles be charged with fluid, or be congested with blood, or be dilated, or broken, and the whole body must feel the disturbance. Let the structure of the lungs be the seat of a new growth, and the whole body must suffer something from the mischief. Let the heart fail to send arterialized blood to the minute circulation, or blood that cannot sustain combustion, and the whole body must needs collapse. Let the sensitive pleural membrane be injected with blood, or rendered dry and inflamed so that its surfaces rub one against the other, and pain will follow, from the lowering effects of which all the body will sympathize. Let the surfaces of the pleural membrane be fixed by adhering one to the other, or be compressed by an excess of fluid secreted between them, and so much breathing power must be lost, to the comparative detriment of all the organism. Lastly, let the elasticity of the elastic fibre of the lung tissue become impaired, and the vital capacity, as a whole, must suffer relative impairment.

THE NERVOUS SYSTEMS.

Keeping our attention still directed on the construction of the living man, looking through him still, we should remark that in parts of his organism, on the terminals of his vascular system, there are certain solid organs from which spring an infinite number of cords and threads, to traverse the body in as many directions as the blood-vessels themselves, and to enter freely into the fleshy structures, to which, at times, they seem to impart motion. At the uppermost part of the body, crowning the whole, and