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The great function performed by the internal surface of the stomach is the secretion of a peculiar fluid, by means of which the chief part of the process of digestion is effected. This fluid, termed the gastric juice, is one of the most singular in the whole animal economy. In its external properties it is without colour, without odour, without taste, yet it is the most powerful solvent known. It speedily reduces the food which is brought into contact with it, into a pulpy and homogeneous mass, and the hardest textures yield to it; not only the tough fibre of the vegetable substance, not only the muscular and membranous fibre of the animal solid, but even hair, shell, and bone itself. It is a universal solvent. Thus the same sac which forms the receptacle of the food, furnishes the menstruum by which the solution, the chief part of the digestion of it, is effected.
By the contractile power of the stomach, its contents are propelled into the first intestine, called the duodenum. There the aliment is mixed with a fluid secreted by the liver, termed the bile ; with another fluid secreted by the pancreas, termed the pancreatic juice, and with the secretion of the surface of the intestine itself. In its progress through the other intestines, mixed in like manner with the secretion of their surface, the aliment becomes as completely digested as is possible, by means of this system of organs. The mass is now separated into two parts; that which is fit for nourishment is absorbed by a system of vessels termed the lacteals; the rest, the excrementitious portion, is discharged from the body.
In numerous tribes of animals there is no proper circulation. In the more perfect animals, the system of the circulation consists of two distinct sets of organs, namely, the blood vessels, which contain the nutritive fluidand the heart, the great centre of the force by which chiefly it is propelled into the different parts of the body. The blood is conveyed from the heart by vessels termod arteries. This system begins by one great vessel attached to the heart, called the aorta. The blood is returned to the heart by vessels termed the veins. In their structure and properties there are important differences between these two systems. The coats of arteries are more thick; and so elastic, that the tubes retain their round shape when empty. The coats of veins are thin, and so pliable, that they collapse as soon as their contents are discharged.
The arteries divide and subdivide into ramifications of extreme minuteness ; ultimately they become so small, as entirely to escape our senses. The minute branches are termed the capillaries. The capillaries, gradually joining each other, and becoming larger and larger as they unite, terminate in the roots of the other systems of vessels, that is, in the minute branches of veins. The veins uniting with each other, and returning the blood from all parts of the body, at length terminate in two great vessels, called the VENÆ CAVÆ. These
the blood directly into the heart. In all parts of the body except the lungs, the veins exceed the arteries, both in number and size; consequently, in this division of the system, the motion of the blood is proportionally slower.
The lecturer then described the structure of the heart in the different classes of animals, from the lowest to the highest, and showed the simplicity of its structure in the former, and its more complex organization in the latter. From the structure of the heart, and the position of its valves, he showed what the course of the circulation must necessarily be, and described that course at length. As this part of the lecture was wholly demonstrative, it does not admit of being reported. The proofs that the blood really circulates, and that it flows in the course described, were stated to be threefold. 1. It is established by the structure and disposition of the valves. The blood can flow in one directioli, but in no other; the position and action of the valves prevent the possibility of its taking any course but one. 2. The effect of ligatures shows the direction in which the blood flows. If a ligature be placed around an artery, the portion of the artery between the heart and the ligature becomes tumid ; that portion of it which is below the ligature becomes empty. On the contrary, if a ligature be placed around a vein, the portion of the vein which is between the heart and the ligature becomes collapsed ; that portion of it which is between the ligature and the extremity becomes swollen. The inference can be but one ; that in the artery the blood flows from the heart; that in the vein it Aows to the heart. 3. But the circulation and the course of it can be seen. There are parts of certain animals so transparent that the blood vessels and the fluid they contain are visible. This is the case in the tail of certain fish, and in the web of the frog's foot. If either be examined with a microscope of moderate power, the circulation can be seen perfectly and beautifully. This was accordingly well shown in the web of the frog's foot.
The lecturer next proceeded to speak of the composition of the blood, and to exhibit with the microscope the red particles on which its colour depends, and which was distinctly and beautifully shown. Without following Dr. Smith into the details into which he entered, it must suffice to state the result to which he arrived ; namely, that the blood, as it is found in the circulating vessels, contaius every element of which the animal body is composed ; namely, carbon, hydrogen, oxygen, and azote, together with the different compounds formed by the various combination of these elements, such as fibrine, gelatine, albumen, and so on; and almost every chemical substance which is found in the body, as phosphorus, lime, iron, and so on.
In this heterogeneous composition of the blood, it was observed, we see the material out of which it is possible for the different animal solids and fluids, numerous and varied as they are, to be elaborated ; and in the varied disposition of the vessels which contain the fluid, we perceive the first steps of the preparation which is made for the operation of this subtle and mysterious chemistry.
The absorbent vessels establishing the communication between the digestive organs and the circulating system, or, in other words, forming the channels by which the nutritive matter destined to renovate the blood is conveyed into this fluid ; it was stated, that in the more perfect animals these vessels consist of two distinct sets. The first, on account of the colour of the fluid they contain, which is similar to that of milk, are termed lacteal vessels. These receive the nutriment vessels directly from the intestines. Gradually becoming larger and larger, by constantly uniting together in their progress to the venous system, they at length form one trunk, which, from its passing through the thorax, is called the thoracic duct. The other set, also named from the colour of the fluid they contain, which is pellucid like lymph, are therefore termed lymphatic vessels. These arise from every part of the body, and reconvey into the blood the remnant of the nutrient particles which have not been expended in the reparation of the system, together with the new substances which have been absorbed from the different surfaces of the body. These also terminate in the thoracic duct. The thoracic duct itself opens into a large vein near the right side of the heart, that side by which the blood flows into the lungs, consequently the new matter furnished to the blood by the process of digestion passes, with this fluid, directly through the lungs: in this organ it is assimilated, that is, converted into perfect blood ; and from this great laboratory, in which its complete preparation is effected, the nutritive fluid is returned to the left side of the heart, thence to be sent out to the system in general.
The blood when it leaves the left side of the heart is of a bright red colour. A remarkable change takes place in its appearance during its circulation through the body. When it returns to the right side of the heart, it is of a dark mulberry colour. Hence the blood contained in the artery, and therefore termed arterial, is said to be red; that contained in the vein, or venous, is called black. Black or venous blood is found to be unfit for the purposes of life; its power to repair the waste of the system, and to afford the necessary stimulus to the action of the different organs, is exhausted. To accomplish its renovation, it is necessary that a particular process should be established; that process is denominated respiration. It seems to be essential to the life of every organized body that its nutrient matter, whatever it be, should be brought into contact with the air. Even in the vegetable, this is indispensable. In the lower tribes of animals, the mode in which this object is effected is very similar to that in which it is accomplished in the vegetable. The entire surface of the tissue of which the animal is composed, appears to be a respiratory organ. As we ascend in the scale, special means are provided for the conveyance of air, either to particular parts of the body, or throughout its whole surface. The lower the animal, the more diffused is its organ of respiration; the higher, the more concentrated. In animals in which there is no proper circulation, a particular system of vessels is pro-.. vided for respiration, termed trachea. By these tubes, which are distributed to all parts of the body, the air is made to pervade every portion of the animal, and to act upon the nutritive fluid, at the instant it is expended in performing the various functions of its economy.
In animals which possess a circulating system, a distinct organ is invariably provided for the performance of this important function. In every case the object of the function is the same, namely, to expose an immense surface of the venous blood to the influence of the air. In every case the oliject is effected by the same means, namely, hy a minute ramification of the venous blood vessels upou an extremely delicate membrane. The mechanical arrangement of the apparatus differs exceedingly in different animals, but its structure and action is always essentially the same, and therefore, considered physiologically, it is the same organ. The principal deviations, however, are only two, both of which have a peculiar relation to the element in which the animal lives, and which are evidently mere adaptations of one and the same
organ, in animals constructed upon the same general plan, to a mode of life essentially different. If the animal reside in water, a fine membrane is arranged in the form of lamina, which generally consist of several series ; upon these laminæ the venous vessels are expanded to an extreme degree of minuteness, and with these the water is brought into continual contact. In this case it is the air contained in the water that acts upon the blood. Organs constructed in this manner are termed branchiæ, or gills. In general the branchiæ are protected by a covering, which is partly ossious, or cartilaginous, and partly membranous, termed the opeaculum.
If, on the contrary, the animal inhabit the land, an exceedingly delicate membrane is folded into numerous and minute cells. The venous blood-vessels, divided to an extreme degree of tenuity, are spread out upon the walls of those cells. A tube, termed the windpipe, opening externally by one extremity, terminates at the other in these cells ; ihe atmospheric air passes by this tube into all these delicate and minute cavities, and at certain periods, during the process of respiration, completely fills them.
The object and the result of these different arrangements are precisely the same; by both, the blood and the air are brought, not into immediate contact, but so near to each other, that nothing intervenes between them, excepting an exceedingly thin membrane, which presents no obstacle to their reciprocal action. The blood when it entered the lungs was of a black or venous colour; it had distributed to the system its nutrient particles ; it was no longer capable of affording it the requisite degree of nourishment. After having been subjected to the action of respiration in the lungs, it flows into the vessels destined to receive it, of a bright red colour : it is now arterial blood; it is renovated; it is refitted to supply the wants of the system.
Did the animal possess no other organs, and exercise no other functions than those which have been described, its existence would still be merely vegetative ; but it is further endowed with the faculties of perception and of voluntary motion. The lecturer then proceeded to exhibit a general view of the nervous and muscular systems, the organs by which those powers are exercised.
The only function which remains to be considered is that of reproduction: its organs consist of two classes; first, that which prepares the fluid, which is necessary to excite the pre-existing germ, and to apply it to that germ; and, secondly, those which contain and protect the germ during the first stages of its development. The former constitute the male, the latter the female organs. The vagina receives the fecundating fluid, and conducts it to the uterus. From the uterus a duct called the fallopian tube passes to the ovarium, which it embraces. The ovarium is the organ which contains the ova, that is, the pre-existing germ. At the instant of impregnation, one or more of the ova are separated from the ovarium, and received into the fallopian tube. By this tube it is conveyed out of the body, if the animal be oviparous; into the uterus, if it be viviparous. The ovum has now become an embryo; it has received an impulse by which the principles that were inherent in it, but which until now had remained dormant, are called into action. It begins to be evolved ; gradually as its development goes on, it derives its nourishment, either from the matter,
by means of a spungy mass of vessels connected with the maternal system ; or if it be at once separated from the parent, from an organized mass called the egg. After it has arrived at the full term, it is forcibly expelled by the uterus, or it bursts the shell within which it had been enclosed.
Dr. Smith concluded this branch of the subject by stating, that in the general view which he had thus exhibited of the organs and functions, the organs have been considered as complex bodies, composed of a variety of different textures, and the functions as general powers, subservient to particular cases in the economy: that there are few branches of human knowledge which exceed in interest the study of this part of the science; but that there is another part, which, though not so fascinating, is more strictly scientific, and which has been too generally neglected; namely, the examination of the physical, in contradistinction to the final causes by which the phenomena peculiar to the living body are produced. He observed, that when we consider the relation between an organ and its functions, we engage in the investigation of what is termed the final cause; that is, we inquire into the special use which the organ serves in the economy. When we examine the physical means by which any particular object is effected in the system, we investigate what is denominated the physical cause. The circulation of the blood, considered in relation to its function, discloses a complicated system, all the parts of which are adjusted with exquisite and wonderful skill to the completion of its object; considered in relation to the physical agents by which the phenomena are produced, they are resolvable into a few general powers, such as muscular contractility, membranous elasticity, the hydraulic properties of the blood, &c. It was observed, in conclusion, that such analytical investigations are eminently scientific, and are, indeed, the only means by which we can arrive at any truly philosophical induction; and that, after the general view which has been taken of the organs and functions, such an analysis of the textures of which the organs are composed, and of the powers by which the functions are accomplished, cannot but conduce to the clearness and exactness of our conception, both of structure and function.
TALES OF THE O'HARA FAMILY.-SECOND SERIES.* We have read these Tales with an interest often excited even to a painful degree of intensity, and with frequent admiration of the author's powers; and yet they are productions wild in fable, clumsy in their machinery, and generally defective in the portraiture of character. The author's genius, indeed, seems to be somewhat of a mongrel breed. In extravagance and passion he resembles Maturin; in incident, Scott; in accurate description of manners, Edgeworth ; in prosing dialogue he is alone comparable with Galt. His palpable deficiencies
• Tales of the O'Hara Family.-Second Series. Comprising The Nowlans and Petor of the Castle. In three volumes. London. Colburn. . 1826.
† An anthor who ought never to be trusted beyond a single volume. His Provost and Annals of the Parish were perfect in their way ; but in his other and larger works he has given full scope to bis turn for prosing, and incessantly reminds us of Mr. Ma. thew's old Scotch lady, with her interminable story about nothing.