The nerves and lymphatic vessels of the in terior of the liver, though but little known, are too inconsiderable in point of size to affect the general accuracy of this description. Hence it evidently appears, on what modifications of the elements of the mucous tissue and of those appended to it, the peculiar friability, colour, and other properties of this organ depend. If the " parenchymatous" areolar tissue abounded in this gland to the extent implied in the de scriptions of Bichat and some more recent authors, no doubt its toughness would be far greater than it really is. But where an organ is sufficiently screened from injury by its po sition, where its different parts are so well connected by the continuity of a close network of capillary vessels, and are not required to move on one another, it would be difficult to imagine what purpose a greater development of areolar tissue would serve.
In the kidney, the epithelial and vascular elements are in corresponding abundance, the areolar tissue in very small quantity. The general texture, however, is more tough than in the liver, from the universal presence of the basement membrane on the tubes. In the me dullary portion, the tubes radiate from the apex towards the base of the cones, and are imbedded in a firm, granular substance, not hitherto described, but which resembles a blastema, and is probably composed of cells. In this substance is also imbedded the capil lary plexus surrounding the tubes, as well as the vessels, that convey blood to and from this plexus, and take the same direction as the tubes. Hence the firmness and close texture of this part of the kidney as compared with the other, and the facility with which it tears from the apex to the base of the cones. At the base of the cones, the tubes enter the cortical substance and take a course, in sets, towards the surface. The central tubes of each set reach the surface and then recline inwards and become convoluted. But the others bend down one after another and become convoluted before reaching the surface. All at length terminate in the Malpighian bodies, which lie among the convolutions. The arteries and veins also take a general course from the hilus towards the surface. Hence, on tearing the cortical part of the organ, there is a disposition for the lacera tion to occur in lines continuous with the radii of the medullary cones, and this disposition is less evident as we approach the surface; but between these lines the torn surface is very uneven, where it is formed by the contorted tubes. The cortical part has less of the inter tubular matrix than is met with in the medul lazy cones.
In the kidney there is a peculiarity of the highest interest in the relative situation of the vascular and mucous tissues, which seems to have reference to the peculiar function of the gland. There are two systems of capillary vessels, the former of which, or that in con nection with the renal artery, perforates the mu cous membrane at the extremity of each tube, and lies on the outer surface of the membrane, that is, bare and loose within the dilated ex tremities, which thus form the capsules of the Malpighian bodies!' (See IttN.)
The common submucous areolar membrane of the kidney, or that forming its capsule, is in most animals chiefly composed of ordinary areolar tissue with close meshes. But where a more resisting covering is required, as in the lion, this areolar tissue is modified ; the white fibrous element predominates so much as to give the capsule the glistening aspect of an aponeurosis. This is an admirable example of the transition from areolar tissue into white fibrous tissue, and helps to show the true nature and relations of the tunica albuginea of the testis.
The testis, compared with the liver and kid ney, presents several modifications of the ele mentary tissues. The basement membrane is much stouter than in the latter gland, the tubes are larger and their convolutions more loosely joined by any intervening substance. There is no appearance of an intertubular substance except towards the corpus Ilighmorianum, and the principal connecting medium between the tubes seems to be the vessels, which are less nu merous than in the glands already mentioned, and form a looser network. The secreting tu bules for these reasons admit of being very easily separated from one another, and un ravelled to great lengths. The epithelial ele ment of the testis constitutes a lining of con siderable thickness, and is highly remarkable (see fig. 274). Though no seminal animalcules have been hitherto seen in the interior of the particles while still attached to the basement membrane of the tubes, yet from recent searches, and especially from those of Wagner, on the phases of their development, it is ren dered highly probable that these singular moving bodies originate in the epithelial particles, as one of the results of their natural evolution. The loose aggregation of the tu bules of the testis makes a firm external cap sule necessary, and where, as in man, this gland is much exposed to injury by its situ, ation, a further protection of this kind is made requisite. Hence the firm and unyielding character of the tunica albuginea in main the contrast of which with the thin covering of the large but wellprotected testicle of the por poise (for example), is well worthy of attention. In many large animals, the tunica albuginea, like the aponeurotic capsule of the lion's kid, ney, is traversed more or less completely by large veins which it thus serves to support. The tunica albuginea consists almost solely o white fibrous tissue, and represents the sub mucous areolar tissue of the mucous system. The peculiarities of the salivary glands re-jh snit from die predominance of their epithelial element over the others. The ducts terminate in vesicles, very similar in the figure they assume to those of the lungs, but nearly filled up with epithelial particles. The basement mem brane is very delicate. The capillary vessels encircle the vesicles, and are comparatively few in number, whence the pale colour of these glands. The areolar tissue forms capsules for those aggregations of vesicles, termed lobules, but does not penetrate between the individual vesicles.