tension and the slow beat give rise to distention of the ventricle, which, when a certain point is reached, impedes the working of the heart, and its muscle begins to beat more and more feebly, so that in the third stage the pulse can hardly be felt. The muscular arterioles then become exhausted and relax, the blood pressure falls rapidly, and with the death of the animal it reaches the level of atmospheric pressure. Both sides of the heart and great veins are engorged with blood in the last stage of asphyxia ; the cardiac muscle being exhausted, from want of oxygen, is unable to pump the blood out of the veins or empty its cavities.

Owing to the force of the rigor mortis of the left ventricle, and the greater capacity of the systemic veins, the left side is found. comparatively empty some time after death, and at post-mortem examination the right side alone is found over-filled.

CHAPTER XX.

BLOOD-ELABORATING GLANDS.

In the preceding chapters we have seen that the blood undergoes important changes as it courses through the different parts of its circuit. Where it comes in contact with the tissues it yields to them nutrient material for assimilation, and oxygen for their metabolism, and carries away from them some waste products. In the lungs it receives oxygen and gives off carbonic acid. While it flows through the minute vessels of the alimentary tract, some of the materials elaborated by the digestion of food are absorbed, and directly added to the blood; at the confluence of the great veins in the neck the stream, composed of lymph and chyle, is poured into the blood before it enters the heart, so as to be thoroughly mingled with it on its return from the general circulation. Moreover, in various glands, different substances are used in the manufacture of their secretions.

Thus there is a kind of material circulation, a constant income and output going on in the blood itself as it passes through the different parts of the body. The investigation of the exact changes which take place in the blood in each organ or part is surrounded with difficulty, and in many cases it is quite impossible to ascertain what changes occur. In some parts it may be made out by noting the results produced, or the substances given off or taken up by the blood, as seen in the changes found in the air after its exposure to the blood in the lungs, where we can definitely state that the blood has lost or gained certain materials, and is so far altered. In other parts, such as the muscles or the ductless glands, where, no doubt, profound changes in the blood occur, we have no separate outcome which we can analyze, and we must therefore trust altogether for the elucidation of the change going on in them to the differences which may be found to exist in the blood flowing to, and that flowing from, such an organ. For this purpose one can either examine samples of the

blood from the artery and vein of the organ, while the ordinary circulation is going on, or, immediately after the removal of the

FIG. 156.

organ, by causing the artificial stream of blood to flow through it; then the changes brought about in the blood in its passage through the organ will give the required information. It can be seen, from the foregoing enumeration of processes, that some organs have a double function as regards the blood. Thus, in the lung there is both renovation by taking in oxygen, and purification by getting rid of carbon dioxide. textures in their internal respiration take the nutriment and oxygen, and give the blood CO, and various other waste products of tissue change.

The

DUCTLESS GLANDS.

There is a certain set of organs which have but slight traits of resemblance to one

Vertical section of the Supra-renal Capsule. another, and in consequence of

(Eberth.)

1. Cortex. 2. Medulla. a. Fibrous capsule. b. External cell masses. c. Columnal layer. d. Internal cell masses. e. Medullary substance, in which lies a large vein, partly seen in section f.

the want of more accurate knowledge as to their exact

function, and the fact that they do not pour their products into

ducts, but probably into the blood current, are commonly grouped together as ductless or blood glands.

It has been shown that a great part of the absorbed nutrient material passes through a special set of vessels called the lacteals or lymphatics, and in so doing has to traverse peculiar organs

called lymphatic glands, where it is no doubt modified, and has added to it a number of cells (lymph corpuscles) which subsequently are poured into the large veins with the lymph and become important constituents of the blood.

Some of these blood glands are doubtless nearly akin to the lymphatic glands already described (Fig. 151), their duty being the further elaboration and perfection of the blood. In this group are commonly placed the supra-renal capsules, the thyroid, the thymus, and the spleen.

Section of the Thyroid Gland of a child, showing two complete sacs and portions of others. The homogeneous colloid substance is represented as occupying the central part of the cavity of the vesicles, which are lined by even cubical epithelium. (Schäfer.)

SUPRA RENAL CAPSULE.

With regard to the function of the supra-renal capsule we may say that nothing definite is known. The cortical part is said to resemble the lymph follicles in structure, while the central part, on account of its numerous peculiar, large cells and great richness in nerves, has been explained as belonging to the nervous system.

THYROID BODY.

The thyroid is made up of groups of minute closed sacs embedded in a stroma of connective tissue, lined with a single

row of epithelium cells, and filled with a clear fluid containing mucin. In the adult the sacs are commonly much distended with a colloid substance and peculiar crystals, and the epithelium has disappeared from their walls. Although said to be rich in lymphatics and to contain follicular tissue, positive proof of the

Magnified section of a portion of injected Thymus, showing one complete lobule, with soft central part (cavity) (b), and parts of other lobules. (Cadiat.)

(a) Lymphoid tissue. (c) Blood vessels. (d) Fibrous tissue.

FIG. 160.

Portion of Thymus removed from its envelope and unraveled SO as to show the lobules (6, b) attached to a central band of connective tissue (a).

Elements of Thymus (high power). (Cadiat.) (a) Lymph corpuscles. (6) Epithelioid nests of Hassall.

relation of the thyroid body to the lymphatic system is still wanting.

THYMUS GLAND.

The functional activity of the thymus is restricted to that period of life when growth takes place most rapidly. It is well