M. Heidenhain (94) has made a particular study of the giant cells. According to him the nuclei of these cells take the form of perforated hollow spheres whose thick walls contain "endoplasm." The latter is continuous with the remaining protoplasm of the cell, the " exoplasm" through the "perforating canals" of the nuclear wall. The exoplasm is arranged in three concentric layers, separated from each other by membranes, the external membrane of the outer zone being the membrane of the cell. The outer layer or marginal zone is of a transient nature, but is always renewed by the cell. Thus, the cell-membrane is replaced by the secondary membrane situated between the second and third zone. According to the same author the functions of the giant cells appear to consist in "the selection and elaboration of certain albuminoid substances of the lymph and blood currents, which are later returned to the circulation." The number of centrosomes occurring in the mononuclear giant cells of the bone-marrow is very large, and in some cases, as in a pluripolar mitosis, may even exceed one hundred in number.

The distribution of the blood-vessels in the bone-marrow is as follows: On entering the bone the nutrient arteries divide into a large number of small branches, which then break up into small arterial capillaries. The latter pass over into relatively large venous capillaries, whose walls either finally disappear entirely or are broken through in many places so that the venous blood pours into the spaces of the red bone-marrow where the current is very slow. The blood passes out by means of smaller veins formed by the confluence of the capillaries which collect the blood from the marrow. It is worth mentioning that the venous vessels, while inside of the bone-marrow, possess no valves; but, on the other hand, they have an unusually large number of valves immediately after leaving the bone.

Yellow bone-marrow is derived from red bone-marrow by a change of the marrow-cells into fat-cells. The gelatinous marrow, on the contrary, is characterized by the small quantity of fat which it contains. Neither the yellow nor the gelatinous bone-marrow is a blood-forming organ (compare Neumann, 90; Bizzozero, 91; H. F. Müller, 91; van der Stricht, 92).

E. THE THYMUS GLAND.

The thymus gland is usually considered as belonging to the lymphoid organs, although in its earliest development it resembles an epithelial, glandular structure. In the epithelial stage, this gland develops from the entoderm of the second and third gill clefts, Mesodermic cells grow into this epithelial structure, proliferate and then differentiate into a tissue resembling adenoid tissue. It retains this structure until about the end of the second year after birth, when it slowly begins to retrograde into a mass of fibrous tissue, adipose tissue, and cellular débris, which structure it presents in adult life.

By means of connective-tissue septa, the thymus is divided into larger lobes, and these again into smaller lobes, until finally a number of very small, almost spheric structures are formed-the lobules of the gland. These consist of a reticular connective tissue much more delicate at the periphery than at the center of the

Fig. 162.-A small lobule from the thymus of child, with well-developed cortex, presenting a structure similar to that of the cortex of a lymph gland; Hilus; b, medullary substance; c, cortical substance; d, trabecula.

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a,

lobule. In the meshes of the reticular tissue are cellular elements, in structure similar to the lymphocytes, which are more numerous at the periphery of the lobule than at its center, so that we may here speak of the lobule as divided into a cortical and a medullary portion. The latter is usually entirely surrounded by the cor

tical substance, but may penetrate to the periphery of the lobule, allowing the blood-vessels to enter and leave at this point. In the cortical substance occur changes which result in the formation of structures closely resembling the cortical nodules of lymphglands.

Fig. 163.-Hassal's corpuscle and a small portion of medullary substance, showing reticulum and cells, from thymus of a child ten days old.

Until recently, little was known of the significance of this organ. A careful study revealed a similarity between certain cellular elements of the thymus and the constituents of the blood-forming organs,a similarity still more striking from the presence of nucleated red blood-cells in the thymus. Logically, then, the embryonal thymus is to be regarded as one of the blood-forming organs (Schaffer, 93, I). The meshes of the capillary network are much wider in the medullary than in the cortical substance of the

lobules, but small arteries also penetrate directly into the cortex. The lymphatics, concerning the origin of which nothing certain is known, pass out side by side with the arteries. It is probable that the peripheral looser portion of the cortex represents a lymph-sinus.

During embryonic life from the fourth month on and for some time after birth, there are found in the thymus peculiar epithelial bodies, known as the corpuscles of Hassal. They are spheric structures, about 0.1 mm. in diameter, whose periphery shows a concentric arrangement of the epithelial cells. In their central portions are found a few nuclear and cellular fragments. These bodies occur only in the thymus gland. They are remnants of the primary epithelial, glandular structure of the thymus, and are formed by an ingrowth of mesoderm which breaks down the epithelium into small irregular masses, mechanically compressed by the proliferating mesoderm.

II. THE CIRCULATORY SYSTEM.

THE walls of the blood-vessels vary in structure in the different divisions of the vascular system. All the vessels, including the heart, possess an inner endothelial lining. In addition to this, the larger vessels are provided with other layers, which consist, on the one hand, of connective and elastic tissue and, on the other, of nonstriated muscle-fibers. The vessels are also richly supplied with nerves, that form plexuses in which ganglion cells are sometimes found, and in the larger vessels the outer layer is honeycombed by nutrient blood-vessels, called vasa vasorum. In the heart, the muscular tissue is especially well developed. According to the structure of the vessels, we distinguish, in both arteries and veins, large, medium-sized, small, and precapillary vessels, and finally, the capillaries themselves. The latter connect the arterial and venous precapillary vessels. In the lymphatic system we must further distinguish between the larger lymph-vessels, the sinuses, and the capillaries.

A. THE VASCULAR SYSTEM.

1. THE HEART.

In the heart there are recognized three main coats-the endocardium, the myocardium, and the pericardium or epicardium.

The endocardium consists of plate-like endothelial cells, with very irregular outlines. Beneath this endothelial layer is a thin membrane composed of unstriped muscle-cells, together with a small number of connective-tissue and elastic fibers. Below this is a somewhat thicker and looser layer of elastic tissue connected externally with the myocardium. Between the two layers are found, here and there, traces of a layer of Purkinje's fibers (compare p.

132). Purkinje's fibers are found in the heart of many mammalia, although absent in the heart of the human adult.

The auriculoventricular valves of the heart represent, in general, a duplication of the endocardium. The layer of smooth musclefibers found in the latter is better developed on the auricular surface, while the elastic tissue is not more prominent on the ventricular surface. At the points of insertion of the chordæ tendineæ the connective-tissue layer is strongly developed and assumes a tendon-like consistency. The semilunar valves of the aorta and pulmonary artery have a similar structure. In the nodules of these valves the elastic fibers are especially dense in their arrangement.

The myocardium is made up of the heart muscle-cells already described (vid. p 132). Between the heart muscle-fibers and bundles of such fibers are thin layers of fibrous connective tissue containing a network of capillaries. The myocardium of the auricles may be divided into two layers, of which the outer is common to both auricles; the heart muscle-fibers of this layer have a nearly circular arrangement. Three layers of muscle-fibers are met with in a longitudinal section through the ventricular wall, the outer and inner being chiefly longitudinal in direction, although not exactly parallel. In the left ventricle the outer layer is very strongly developed. The musculature of the auricles is almost completely separated from that of the ventricles by means of the annulus fibrosus atrioventricularis, which consists in the adult of connective tissue containing numerous delicate and densely interwoven elastic fibers.

The pericardium consists of a visceral layer, the epicardium, adhering closely to the myocardium, and a parietal layer (pericardium), loosely surrounding the heart and continuous at the upper portion of the heart with the visceral layer. Between the two layers is the pericardial cavity, containing a small quantity of a serous fluid— the pericardial fluid. In the visceral layer (the epicardium) we find a connective-tissue stroma covered by flattened endothelial cells. A similar structure occurs also in the parietal layer, although here the connective-tissue stroma is considerably reinforced. Deposits of fat, in most cases in the neighborhood of the blood-vessels, are sometimes seen between the myocardium and the visceral layer of the pericardium.

According to Seipp, the distribution of the elastic tissue in the heart is as follows: The endocardium of the ventricles contains far more elastic tissue than that of the auricles, especially in the left ventricle, where even fenestrated membranes may be present. In the myocardium of the ventricles there are no elastic fibers aside from those which are found in the adventitia of the contained bloodvessels. In the myocardium of the auricles, on the contrary, such fibers are very numerous and are continuous with the elastic elements in the walls of the great veins. The epicardium also presents elastic fibers in the auricles continuous with those of the great veins emptying into the heart, and in the ventricles continuous with

those in the adventitia of the conus arteriosus.

In those portions of the heart-wall containing no muscular tissue the elastic elements of the epicardium are continuous with those of the endocardium. In the new-born the cardiac valves possess no elastic fibers, although they are present in the adult. They are developed on that side of each valve, which, on closing, is the more stretched-for instance, on the auricular side of the auriculoventricular valves.

The heart has a rich blood supply. The capillaries of the myocardium are very numerous, and so closely placed around the muscle bundles that each muscular fiber comes in contact with one or more capillaries. In the endocardium the vessels are confined to the connective tissue. The auriculoventricular valves contain blood-vessels, in contradistinction to the semilunar valves, which are non-vascular, while the chorda tendineæ are at best very poorly supplied with capillaries.

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The coronary arteries, which terminate in the capillaries above mentioned, are terminal arteries in the sense that the resistance in the anastomosing branches is greater than the blood pressure in the arteries leading to those branches (Pratt, 98). This observer has further shown that the vessels of Thebesius (small veins which open on the endocardial surfaces of the ventricles and auricles and communicate directly with all the chambers of the heart) "open from the ventricles and auricles into a system of fine branches that communicate with the coronary arteries and veins by means of capillaries, and with the veins, but not with the arteries, by passages of somewhat larger size”; so that, although the blood supply through the coronary arteries for a given area of the myocardium is cut off, the heart muscle of this area may receive blood through the vessels of Thebesius.

Lymphatic networks have been shown to exist in the endocardium, and their presence in the pericardium is not difficult to demonstrate. Little is known with regard to the lymph-channels of the myocardium.

The nerve supply of the heart includes numerous medullated nerve-fibers, the dendrites of sensory neurones, and numerous nonmedullated fibers, the neuraxes of sympathetic neurones. Smirnow (95) described sensory nerve-endings in the endocardium of amphibia and mammalia, which he suggests may be the terminations of the depressor nerve. Dogiel (98) has corroborated and extended these observations, and has described complicated sensory telodendria situated both in the endo- and pericardium. The latter states that, after forming plexuses and undergoing repeated division, the medullated sensory nerves lose their medullary sheaths, the neuraxes further dividing in numerous varicose fibers, variously interwoven and terminating in telodendria, which vary greatly in shape and configuration. These telodendria are surrounded by a granular substance containing branched cells, probably connective-tissue cells, the interlacing branches of which form a framework for the