telodendria. Similar sensory nerve-endings occur in the adventitia of the arteries and veins of the pericardium (Dogiel, 98); and Schemetkin has shown that sensory nerve-endings occur in the adventitia and intima, especially in the latter, of the arch of the aorta and pulmonary arteries. In the heart, under the pericardium on the posterior wall of the auricles and in the sulcus coronarius, are found numerous sympathetic neurones whose cell-bodies are grouped to form sympathetic ganglia. The neuraxes of these sympathetic neurones-varicose, nonmedullated nerve-fibers-form intricate plexuses situated under the pericardium and, penetrating the myocardium, surround the bundles of heart muscle-fibers. From the varicose nerve-fibers constituting these plexuses, fine branches are given off, which terminate on the heart muscle-cells in a manner previously described (see p. 149 and Fig. 127). The cell-bodies of the sympathetic neurones, the neuraxes of which thus terminate on the heart muscle-fibers, are surrounded by end-baskets, the telodendria of small medullated nerve-fibers which reach the heart through the vagi. The slowed and otherwise altered action of the heart-muscle, produced on stimulating directly or indirectly the vagus nerves is therefore due not to a direct action of these nervefibers on the heart muscle-cells, but to an altered functional activity produced by vagus stimuli in at least some of the sympathetic neurones situated in the heart, the neuraxes of which convey the impulse to the heart muscle. The heart receives further nerve supply through sympathetic neurones, the cell-bodies of which are situated in the inferior cervical and stellate ganglia, the neuraxes of which enter the heart as the augmentor or accelerator nerves of the heart. The mode of ending of these nerve-fibers has not as yet been fully determined. It may be suggested as quite probable that they terminate on the dendrites of sympathetic neurones, the cell-bodies of which are not inclosed by end-baskets of nerves reaching the heart through the vagi, as above described. It is also possible that they end directly on the heart muscle-cells. The cell-bodies of the sympathetic neurones, the neuraxes of which form the augmentor nerves, are surrounded by the telodendria of small medullated fibers, forming end-baskets, which leave the spinal cord through the anterior roots of the upper dorsal nerves. Besides the nerves here described, nonmedullated nerves (whether the neuraxes of sympathetic neurones, the cell-bodies of which are situated inside or outside of the heart has not been fully determined), form plexuses in the walls of the coronary vessels, terminating, it would seem, on the muscle-cells of the media (vasomotor nerves).

2. THE BLOOD-VESSELS.

A cross-section of a blood-vessel shows several coats. The inner consists of flattened endothelial cells, and is common to all The second varies greatly in thickness, contains most of

the contractile elements of the arterial wall, and is known as the media, or tunica media. Its elastic fibers have in general a circular arrangement and are fused at the inner and outer surfaces to form fenestrated membranes, the lamina elastica interna and externa. Outside of the media lies the adventitia or tunica externa, consisting in the arteries almost entirely of connective tissue and in the veins principally of contractile elements, smooth muscle-fibers. Between the internal elastic membrane and the endothelial layer is a fibrous stratum which varies in structure in the different vessels of larger caliber. This is the subendothelial layer, or the inner fibrous layer, and forms, together with the endothelium, the intima

Fig. 164.-Cross-section of the human carotid artery; X 150.

or tunica intima. Bonnet (96), as a result of his own investigations, suggests a somewhat different classification of the layers composing the arterial wall. According to him, the endothelium alone constitutes the intima. The elastic membranes, both internal and external, together with the tissue lying between them, and that between the internal elastic membrane and the intima, constitute the media. The tissue layers outside the external elastic membrane form the tunica externa (adventitia).

(a) Arteries. In the great arterial trunks, such as the pulmonalis, carotis, iliaca, etc., the tunica media possesses a very typical structure. It is divided by means of elastic fibers and membranes

(fenestrated membranes) into a large number of concentric layers containing but few muscle-fibers. Here also the tunica media is separated from the intima by an elastic limiting membrane, the fenestrated membrane of Henle, or the lamina elastica interna. the aorta this membrane as such is not recognizable. The intima. presents three distinct layers-the inner composed of flattened endothelial cells, and the other two consisting chiefly of elastic tissue (fibrous layers). Of these latter the inner is the richer in cellular

Endothelium of the

intima.

Intima.

In

Adventitia with
nonstriated mus-
cle-fibers in cross-
section.

Fig. 165.-Section through human artery, one of the smaller of the medium-sized; X 640.

elements and has a longitudinal arrangement of its fibers, while the outer is the looser in structure, possesses few cellular elements, and shows a circular arrangement of its fibers. The adventitia is also made up of fibro-elastic tissue, but in this case with a still looser structure and a longitudinal arrangement of its elastic fibers. In the outer portion of the adventitia the white fibrous tissue is more abundant. The adventitia is rich in blood-vessels.

The medium-sized arteries differ in structure from the larger in that the elastic elements of the intima and media are replaced to a considerable extent by nonstriated muscular fibers. To this type belong the majority of the arterial vessels, ranging in caliber from the brachial, crural, and radial arteries to the supraorbital artery. In these the intima shows, besides its endothelium, only a single connective-tissue layer with numerous longitudinal fibers, the subendothelial layer, which is thin and is

lar tissue.

limited externally by the fenes- 6, precapillary vein possessing no muscu trated membrane of Henle (lamina

elastica interna). The media no longer gives the impression of being laminated, but consists of circularly arranged muscle-fibers separated from each other by elastic fibers and membranes and a small amount of fibrous connective tissue in such a way that the muscle-cells form more or less clearly defined groups. Here also

the media is limited externally by the external elastic membrane. The adventitia, which becomes looser externally, is not so well developed as in the larger vessels, but presents in general the same structure. In certain arteries (renal, splenic, dorsalis penis) it shows in its inner layers scattered longitudinal muscle-cells, which, however, may also occur in other arteries at their points of division.

With regard to the elastic tissues, the arteries of the brain differ to some extent from those of the remainder of the body. The elastica interna is much more prominent, the elastic fibers in the

At the left of the nerve are

Fig. 167.-Cross-section of human internal jugular vein. two large blood-vessels with a smaller one between them (vasa vasorum); X 150.

circular muscular layer are few r, and the longitudinal strands are almost entirely lacking (H. Triepel).

The walls of the smaller arteries consist mainly of the circular muscular layer of the media. The intima is reduced to the endothelium, which rests directly on the elastic internal limiting membrane. Outside of the external limiting membrane is the adventitia, which now consists of a small quantity of connective tissue. The vasa vasorum have disappeared. To this type belong the supraorbital, central artery of the retina, etc.

In the so-called precapillary vessels the intima consists only

of the endothelial layer. The internal elastic membrane is very delicate. The media no longer forms a continuous layer, but is made up of a few circularly disposed muscular fibers. The adventitia is composed of a small quantity of connective tissue, and con

tains no vasa vasorum.

(b) Veins. In the foregoing account of the structure of the arteries we have described the structure of their walls according to the caliber of the vessels. Such a differentiation in the case of the veins would be impossible, since sometimes veins of the same caliber present decided differences in structure in various parts of the body.

Intima.

Media.

For the sake of convenience, we will commence with the description of a vein of medium size. Its intima consists of three layers (1) Of an inner layer of endothelium; (2) of an underlying layer of muscle-cells, interrupted here and there by connective tissue; and (3) of a fibrous connective-tissue layer containing fewer elastic but more white fibrous connective-tissue fibers than is the case in the arteries. Externally, the intima is limited by an internal elastic layer. The media is in general less highly developed than that of a corresponding artery, and contains muscle-cells which have a circular arrangement and in some veins form a continuous layer, although they sometimes occur as isolated fibers. The adventitia shows an inner longitudinal muscular layer, which may be quite prominent and even form the bulk of the muscular tissue in the wall of the vein. Otherwise the adventitia of the veins belonging to this class corresponds in general to that of the arteries of the same size; but here also we have, as in the intima, a preponderance of white fibrous connective-tissue elements.

Adventitia with nonstriated muscle-cells in cross-section.

Fig. 168.-Section of small vein (human); X640.

In the crural, brachial, and subcutaneous veins, the musculature of the media is prominent; while in the jugular, subclavian, and innominate veins, and in those of the dura and pia mater, the muscular tissue of the media is entirely wanting, and, as a consequence, the adventitia with its musculature, if present, is joined directly to the intima.

In the smaller veins the vascular wall is reduced to an endothelial lining, an internal elastic membrane, a media consisting of interrupted circular bands of smooth muscle-fibers (which may be absent), and an adventitia containing a few muscle-fibers. The precapillary veins, which possess in general thinner walls than the corresponding arteries, present a greatly reduced intima and adventitia, while the media has completely disappeared.