Section 41. The mechanism of respiration will be understood by reference to Figure 3, Sheet 2. It will be noted, in dissecting that the lungs have shrunk away from the walls of the thorax; this collapse occurs directly an aperture is made in the thorax wall, and is in part due to their extreme elasticity. In life the cavity of the thorax forms an air-tight box, between which and the lungs is a slight s.p.a.ce, the pleural cavity (pl.c.) lined by a moist membrane, which is also reflected, over the lungs. The thorax wall is muscular and bony, and resists the atmospheric pressure on its outer side, so that the lungs before this is cut through are kept distended to the size of the thoracic cavity by the pressure of the air within them. In inspiration (or breathing-in) the ribs are raised by the external intercostal (Anglice, between-ribs, e.i.c.m.) and other allied muscles, and the diaphragm (dia.) contracts and becomes flatter; the air is consequently sucked, in as the lungs follow the movement of the thorax wall. In expiration the intercostals and diaphragm relax and allow the elastic recoil of the lungs to come into play. The thoracic wall is simultaneously depressed by the muscles of the abdominal area, the diaphragm thrust forwards, as the result of the displacement and compression of the alimentary viscera thus brought about. (r.r.r. in the Figure mark ribs.)

Section 42. The oxygen and carbon dioxide are not carried in exactly the same way by the blood. The student will know from his chemical reading that neither of these gases is very soluble, but carbon dioxide is sufficiently so in an alkaline fluid to be conveyed by the liquid plasma. The oxygen however, needs a special portative mechanism in the colouring matter of the red corpuscles, the haemoglobin, with which it combines weakly to form oxy-haemoglobin of a bright red colour, and decomposing easily in the capillaries (the finest vessels between the arteries and veins), to release the oxygen again. The same compound occurs in all true vertebrata, and in the blood-fluid of the worm; in the crayfish a similar substance, haemocyanin, which when oxygenated is blue, and when deoxydized colourless, discharges the same function.

Section 43. The blood returns from the lungs to the left auricle (l.au.) by the pulmonary veins, hidden in the Figure by the heart, pa.s.ses thence to the thick-walled left ventricle (l.vn.), and on into the aorta (ao.).

Section 44. The beating of the heart is, of course, a succession of contractions and expansions of its muscular wall. The contraction, or systole, commences at the base of the venae cavae and pa.s.ses to the auricles, driving the blood before it into the ventricles, which then contract sharply and drive it on into the aorta or pulmonary artery; a pause and then a dilatation, the diastole follows. The flow of the blood is determined in one direction by the various valves of the heart. No valves occur in the opening of the superior cavae but an imperfect one, the Eustachian valve, protects the inferior cava; the direction of the heart's contraction prevents any excessive back-flow into the veins, and the onward, tendency is encouraged by the suck of the diastole of the ventricles. Between the left ventricle and auricle is a valve made up of two flaps of skin, the mitral valve, the edges of the flaps being connected with the walls of the ventricle through the intermediation of small muscular threads, the chordae tendinae, which stretch across its cavity to little muscular pillars, the papillary muscles; these attachments prevent the mitral valve from flapping back into the auricle, and as the blood flows into and acc.u.mulates in the ventricle it gets behind the flaps of the valve and presses its edges together. When the systole of the ventricle occurs, the increased, tension of the blood only closes the aperture the tighter, and the current pa.s.ses on into the aorta, where we find three watch-pocket valves, with the pocket turned away from the heart, which are also closed and tightened by any attempt at regurgitation (back-flow). A similar process occurs on the right side of the heart, but here, instead of a mitral valve of two flaps between auricle and ventricle, we have a tricuspid valve with three. The thickness of the muscular walls, in view of the lesser distance through which it has to force the blood, -are- [is] less for the right ventricle than the left.

Section 45. The following are the chief branches of the aorta. The student should be able to follow them with certainty in dissection; they are all displayed in the Figure; but it must not be imagined for a moment that familiarity with this diagram will obviate the necessity for the practical work; (in.) is the innominate artery; it forks into (s.cl.a.) the right subclavian, and (r.c.c.) the right common carotid.

Each carotid splits at the angle of the jaw into an internal and an external branch. The left common carotid, (l.c.c.) arises from the base of the innominate,* (l.s.cl.a.) the left subclavian, directly from the aorta. The aorta now curves round to the dorsal middle line, and runs down as seen in Figure 1, Sheet 1 (d.ao.) and Figure 1, Sheet 2 (d.ao.). Small branches are given off to the ribs, and then comes the median coeliac (coe.a.) to the stomach and spleen, the median superior mesenteric (s.mes.a.) to the main portion of the intestine, and the inferior mesenteric (p.m.a.) to the r.e.c.t.u.m. Note that no veins to the inferior vena cava correspond to these arteries-- the blood they supply going back by the portal vein (p.v.). The paired renal arteries (r.a.) supply the kidneys, and the common iliacs (c.il.a.) the hind legs, splitting into the internal iliacs (i.il.a.) and the femoral (f.).

{Lines from Second Edition only.} [The student should note that the only arteries in the middle line are those supplying the alimentary ca.n.a.l.]

{Lines from First Edition only.} * -The figure is inaccurate, and represents the left common carotid as arising from the aortic arch.-

Section 46. The distribution of the veins of the rabbit has only a superficial parallelism with arteries. The chief factors of vena cava inferior are the hepatic vein (h.v.), which receives the liver blood, the renal veins (r.v.), from the kidneys, the ilaeo-lumbar, from the abdominal wall, and the external (e.il.v.) and internal ilias (i.il.v.); with the exception of the renal veins none of these run side by side with arteries. The superior cavae (r. and l.v.c.s.) are formed by the union of internal (i.j.) and external jugular (e.j.) veins with a subclavian (s.cl.v.) from the fore limb. The term pre-caval vein is sometimes used for superior cava. The attention, of the student is called to the small azygos vein (az.) running into the right vena cava superior, and forming the only asymmetrical (not-balancing) feature of the veins in front of the heart; it brings blood back from the ribs of the thorax wall, and is of interest mainly because it answers to an enormous main vessel, the right post-cardinal sinus, in fishes. There are spermatic arteries and veins (s.v. and a.) to the genital organs. All these vessels should be patiently dissected out by the student, and drawn.

Section 47. Between the final branches of the arteries and the first fine factors of the veins, and joining them, come the systemic capillaries. These smallest and ultimate ramifications of the circulation penetrate every living part of the animal, so that if we could isolate the vascular system we should have the complete form of the rabbit in a closely-meshed network. It is in the capillaries that the exchange of gases occurs and that nutritive material pa.s.ses out to the tissues and katastases in from them; they are the essential factor in the circulatory system of the mammal-- veins, arteries, and heart simply exist to remove and replace their contents. The details of the branching of the pulmonary artery and the pulmonary veins need not detain us now.

Section 48. Summarising the course of the circulation, starting from the right ventricle, we have-- pulmonary artery, pulmonary capillaries, pulmonary vein, left auricle, left ventricle, aorta, arteries, and systemic capillaries. After this, from all parts except the spleen and alimentary ca.n.a.l, the blood returns to systemic veins, superior or inferior cavae, right auricle, and right ventricle. The blood from the stomach spleen, and intestines however, pa.s.ses via {through} the portal vein to the liver capillaries and then through the hepatic vein to inferior cava, and so on. Material leaves the blood to be excreted in lungs, kidneys, by the skin (as perspiration), and elsewhere. New material enters most conspicuously;

(a) by the portal veins portal veins and

(b) by the thoracic duct and left superior cava.

Section 49. The following table summarises what we have learnt up to the present of the physiology of the Rabbit, considered as a mechanism using up food and oxygen and disengaging energy:--

-Air_ {Nitrogen... returned unchanged.} {Oxygen... through Pulmonary Vein to--} {see 3.}

-Food_ {Carbo-Hydrates (Starch, Sugar, Cellulose.)} Sugar.

{Protein.} {Peptones.} {Fat (little in Rabbit.)} {Glycerine, and fatty acids in soups.} {Rejected matter got rid of in Defaecation.}

1a. {Chyle in Lacteals going via {through} Thoracic Duct and Left Superior Cava to--} {see 2.}

1b. {Veins of Villi--} {Portal Vein--} {Liver--} {Hepatic Vein and Inferior Cava to--} {see 2.}

2. {Right side of heart; then to lungs, and then to--} {see 3.}

3. {Left side of heart; whence to Systemic Arteries and Capillaries.}

4. {The tissues and -Kataboly_.}

5. {Urea (?Liver) Kidney and Sweat Glands} {CO2} {Lungs} {H2O} {Lungs, Kidney, Sweat Glands} {Other Substances} {Mainly by [Kidney,] Liver and Alimentary Ca.n.a.l}

4. _The Amoeba. Cells, and Tissue_

Section 50. We have thus seen how the nutritive material is taken into the animal's system and distributed over its body, and incidentally, we have noted how the resultant products of the creature's activity are removed. The essence of the whole process, as we have already stated, is the decomposition and partial oxydation of certain complex chemical compounds to water, carbon dioxide, a low nitrogenous body, which finally takes the form of urea, and other substances. We may now go on to a more detailed study, the microscopic study, or histology, of the tissues in which metaboly and kataboly occur, but before we do this it will be convenient to glance for a moment at another of our animal types-- the Amoeba, the lowest as the rabbit is the highest, in our series.

Section 51. This is shown in Figure III., Sheet 3, as it would appear under the low power of the microscope. We have a ma.s.s of a clear, transparent, greyish substance called protoplasm, granular in places, and with a clearer border; within this is a denser portion called the nucleus, or endoplast (n.), which, under the microscope, by transmitted light, appear brighter, and within that a still denser spot, the nucleolus (ns.) or endoplastule. The protoplasm is more or less extensively excavated by fluid s.p.a.ces, vacuoles; one clearer circular s.p.a.ce or vacuole, which is invariably present, appears at intervals, enlarges gradually, and then vanishes abruptly, to reappear after a brief interval; this is called the contractile vacuole (c.v.). The amoeba is constantly changing its shape, whence its older name of the Proteus animalcule, thrusting out ma.s.ses of its substance in one direction, and withdrawing from another, and hence slowly creeping about. These thrust-out parts, in its outline, are called pseudopodia (ps.). By means of them it gradually creeps round and encloses its food. Little particles of nutritive matter are usually to be detected in the h.o.m.ogeneous protoplasm of its body; commonly these are surrounded by a drop of water taken in with them, and the drop of water is then called a food vacuole. The process of taking in food is called ingestion. The amoeba, in all probability, performs essentially the same chemical process as we have summarised in Sections 10, 11, 12; it ingests food, digests it in the food vacuoles and builds it up into its body protoplasm, to undergo kataboly and furnish the force of its motion-- the contractile vacuole, is probably respiratory and perhaps excretory, acc.u.mulating and then, by its "systole" (compare Section 44), forcing out of its body, the water, carbon dioxide, urea, and other katastases, which are formed concomitantly with its activity. The amoeba reproduces itself in the simplest way; the nucleus occasionally divides into two portions and a widening fissure in the protoplasm of the animal's body separates one from the other. It is impossible to say that one is the parent cell, and the other the offspring; the amoeba we merely perceive, was one and is now two. It is curious to note, therefore, that the amoeba is, in a sense, immortal-- that the living nucleus of one of these minute creatures that we examine to-day under a microscope may have conceivably drawn, out an unbroken thread of life since the remotest epochs of the world's history. Although no s.e.xual intercourse can be observed, there is reason to believe that a process of supposed "cannabalism," in which a larger amoeba may occasionally engulph a smaller one, is really a conjugative reproductive process, and followed by increased vitality and division.

Section 52. Now if the student will compare Section 35, he will see that in the white blood corpuscles we have a very remarkable resemblance to the amoeba; the contractile vacuole is absent, but we have the protoplasmic body, the nucleus and nucleolus, and those creeping fluctuations of shape through the thrusting out and withdrawal of pseudopodia, which const.i.tute "amoeboid" motion. They also multiply, in the same way, by division.

Section 53. It is not only in the white corpuscle of the blood that we find this resemblance; in all the firmer parts of the body we find, on microscopic examination, similar little blebs of protoplasm, and at an early stage of development the young rabbit is simply one ma.s.s of these protoplasmic bodies. Their division and multiplication is an essential condition, of growth. Through an unfortunate accident, these protoplasmic blebs, which const.i.tute the living basis of the animal body, have come to be styled "cells," though the term "corpuscles" is far more appropriate.

Section 54. The word is "cell" suggests something enclosed by firm and definite walls, and it was first employed in vegetable histology.

Unlike the typical cells of animals, the cells of most plants are not naked protoplasm, but protoplasm enclosed in a wall of substance (cell wall) called cellulose. The presence of this cellulose cell wall, and the consequent necessity of feeding entirely upon liquids and gases that soak through it instead of being able to ingest a portion of solid food is indeed, the primary distinction between the vegetable and the animal kingdoms, as ordinarily considered.

Section 55. Throughout life, millions of these cells retain their primary characters, and const.i.tute the white corpuscles of blood, "phagocytes," and connective tissue corpuscles; others again, engage in the formation of material round themselves, and lie, in such cases, as gristle and bone, embedded in the substance they have formed; others again, undergo great changes in form and internal structure, and become permanently modified into, for instance, nerve fibres and muscle substance. The various substances arising in this way through the activity of cells are called tissues, the building materials of that complex thing, the animal body. Since such a creature as the rabbit is formed through the co-operation of a vast mult.i.tude of cells, it is called multicellular; the amoeba, on the other hand, is unicellular. The rabbit may be thus regarded as a vast community of amoeboid creatures and their products.

Section 56. Figure IV., Sheet 3 represents, diagrammatically, embryonic tissue, of which, to begin with, the whole animal consists. The cells are all living, capable of dividing and similar, but as development proceeds, they differentiate, some take on one kind of duty (function), and some another, like boys taking to different trades on leaving school, and wide differences in structure and interdependence become apparent.

Section 57. It is convenient to divide tissues into three cla.s.ses, though the divisions are by no means clearly marked, nor have they any scientific value. The first of these comprises tissues composed wholly, or with the exception of an almost imperceptible cementing substance, of cells; the second division includes the skeletal tissues, the tissue of mesentery, and the connective and bas.e.m.e.nt tissue of most of the organs, tissues which, generally speaking, consist of a matrix or embedding substance, formed by the cells and outside of them, as well as the cells themselves; and, thirdly, muscular and nervous tissue. We shall study the former two in this chapter, and defer the third division until later.

Section 58. The outer layer of the skin (the epidermis), the inmost lining of the alimentary ca.n.a.l, the lining of the body cavity, and the inner linings of blood-vessels, glands, and various ducts const.i.tute our first division. The general name for such tissues is epithelium. When the cells are more or less flattened, they form squamous epithelium (Figure VI.) such as we find lining the inside of a man's cheek (from which the cells sq.ep. were taken) or covering the mesentery of various types-- sq.end. are from the mesentery (Section 16) of a frog.

A short cylindroidal form of cell makes up columnar epithelium, seen typically in the cells covering the villi of the duodenum (Figure V.).

This epithelium of the villi has the outer border curiously striated, and this is usually spoken of as leading towards "ciliated" epithelium, to be described immediately. The epithelium of the epididermis is stratified-- that is to say, has many thicknesses of cells; the deeper layers are alive and dividing (stratum mucosum), while the more superficial are increasingly flattened and drier as the surface is approached (stratum corneum) and are continually being rubbed off and replaced from below.

Section 59. In the branching air-tubes of the lung, the central ca.n.a.l of the spinal cord, and in the ureters of the rabbit, and in most other types, in various organs, we find ciliated epithelium (Figure VII.).

This is columnar or cubical in form, and with the free edge curiously modified and beset with a number of hair-like processes, the cilia, by which, during the life of the cell, a waving motion is sustained in one direction. This motion a.s.sists in maintaining a current in the contents of ducts which are lined with this tissue. The motion is independent of the general life of the animal, so long as the const.i.tuent cell still lives, and so it is easy for the student to witness it himself with a microscope having a 1/4-inch or 1/6-inch objective. Very fine cilia may be seen by gently sc.r.a.ping the roof of a frog's mouth (the cells figured are from this source), or the gill of a recently killed mussel, and mounting at once in water, or, better, in a very weak solution of common salt.

Section 60. The lining of glands is secretory epithelium; the cells are usually cubical or polygonal (8, g.ep.), and they display in the most characteristic form what is called metabolism. Anaboly (see Section 14) we have defined, as a chemical change in an upward direction-- less stable and more complex compounds are built up in the processes of vegetable and animal activity towards protoplasm; kataboly is a chemical running down; metaboly is a more general term, covering all vital chemical changes. The products of the action of a glandular epithelium are metabolic products, material derived from the blood is worked, up within the cell, not necessarily with conspicuous gain or loss of energy, and discharged into the gland s.p.a.ce. The most striking case of this action is in the "goblet cells"

that are found among the villi; these are simply glands of one cell, unicellular glands, and in Figure V. we see three stages in their action: at g.c.1 material (secretion) is seen forming in the cell, at g.c.2 it approaches the outer border, and at g.c.3 it has been discharged, leaving a hollowed cell. Usually however, the escape of secreted matter is not so conspicuous, and the gland-cells are collected as the lining of pits, simple, as in the gastric, pyloric, and Lieberkuhnian glands (Figure VIII., Sections 23, 29), or branching like a tree or a bunch of grapes (Figure r.g.), as in Brunner's glands (Section 29) the pancreas, and the salivary glands. The salivary glands, we may mention, are a pair internal to the posterior ventral angle of the jaw, the sub-maxillary; a pair anterior to these, the sub-lingual; a pair posterior to the jaw beneath the ear, the parotid, and a pair beneath the eye, the infra orbital.

Section 61. The liver is the most complicated gland in the body (Figure X.). The bile duct (b.d.) branches again and again, and ends at last in the final pits, the lobuli (lb.), which are lined with secretory epithelium, and tightly packed, and squeeze each other into polygonal forms. The blood supply from which the bile would appear to be mainly extracted, is brought by the portal vein, but this blood is altogether unfit for the nutrition of the liver tissue; for this latter purpose a branch of the coeliac artery, the hepatic serves. Hence in the tissue of the liver we have, branching and interweaving among the lobuli, the small branches of the bile duct (b.d.), which carries away the bile formed, the portal vein (p.v.), the hepatic artery (h.a.), and the hepatic vein (h.v.). (Compare Section 45.) Figure X.b shows a lobule; the portal vein and the artery ramify round the lobules-- are inter-lobular, that is (inter, between); the hepatic vein begins in the middle of the lobules (intra-lobular), and receives their blood.

(Compare X.a.) Besides its function in the manufacture of the excretory, digestive, and auxiliary bile, the liver performs other duties. It appears to act as an inspector of the a.s.similation material brought in by the portal vein. The villi, for instance, will absorb a.r.s.enic, but this is arrested and thrown down in the liver. A third function is the formation of what would seem to be a store of carbo-hydrate, glycogen, mainly it would appear, from the sugar in the portal vein, though also, very probably, from nitrogenous material, though this may occur only under exceptional conditions. Finally, the nitrogenous katastates, formed in the working of muscle and nerve, and returned by them to the blood for excretion, are not at that stage in the form of urea. Whatever form they a.s.sume, they undergo a further metabolism into urea before leaving the body, and the presence of considerable quant.i.ties of this latter substance in the liver suggests this as a fourth function of this organ-- the elaboration of urea.

Section 62. Similar from a physiological point of view, to the secretory glands which form the digestive fluids are those which furnish lubricating fluids, the lachrymal gland, and Harderian glands in the orbit internally to the eye, and posterior and anterior to it respectively, the sebaceous glands (oil glands) connected with the hair, and the a.n.a.l and perineal glands. The secretions of excretory glands are removed from the body; chief among them are the sweat glands and kidneys. The sweat glands are microscopic tubular glands, terminating internally in a small coil (Figure VIII. s.g.) and are scattered thickly over the body, the water of their secretion being constantly removed by evaporation, and the small percentage of salt and urea remaining to acc.u.mulate as dirt, and the chief reasonable excuse for washing. The kidney structure is shown diagrammatically in Figure 5, Sheet 7. A great number of branching and straight looped, tubuli (little tubes) converge on an open s.p.a.ce, the pelvis. Towards the outer layers (cortex) of the kidney, these tubuli terminate in little dilatations into which tangled knots of blood-vessels project: the dilatations are called Bowman's capsules (B.c.), and each coil of bloodvessel a glomerulus (gl.). In the capsules, water is drained from the blood; in the tubuli, urea and other salts in the urine are secreted from a branching network of vessels.