The kidneys of Mammalia present one character which is common to the whole tribe, and by which they are distinguished from the other classes of vertebrata. The cha racter alluded to consists in a division of the substance of the gland into two portions, a cortical and a medullary, the former being the secreting part and containing, as will be more fully shown hereafter, tubes which are very tortuous, while in the latter the tubes are straight, forming minute excretory ducts, through which the secreted products are con veyed into the ureter.
In many genera the kidneys are composed of a number of separate lobules or renules, (fig. 142, a a a), each lobe consisting of a cor tical (b) and a medullary substance (c), the latter terminating in a marnillary process (d) which is received into an infundibular offset from the ureter. All the lobules are thus connected with the ureter, forming a clustered mass like a bunch of grapes. The entire kidney is invested by a cellular capsule (e), a deep layer of which (f) passes into the fissures between the lobules, and in the substance of this interlo bular tissue the vessels are_imbedded. There is no anastomosis between the blood-vessels of neighbouring lobules, as shown by the cir cumstance that when the artery in any of them has been obstructed in an injected pre paration they remain uninjected.* This form of kidney is observed in amphibious Carnivora, as the otter and the seal tribes ; it is also found in the bear, and still more retnarkably in the cetaceans. The lobular division of the kidney, which in these animals is a persistent condition, exists in the embryo of all the mammalia. In process of development in the greater number of genera, the lobules coalesce, and thus form a solid glandular organ having a smooth con tinuous surface, and presenting in the normal state no trace of the original lobular divisions.
The kidney of the ox presents a condition intermediate between the lobulated kidney and the solid organ of man and most other rnam miferous genera. In this animal the medullary portion of the kidney has coalesced, while the cortical part is marked out by deep interlobu lar fissures. The coalescence of the lobules appears to have been arrested at a certain period of its progress. The manner in which the tubes open into the pelvis of the solid kidney admits of some variety. In some genera they open on a continuous concave surface, as in the horse and ass ; in others on a continuous ridge, as in the dog. A more common termination is in a conical projection, the apex of which is received into a calyciform cavity in the pelvis of the kidney. In some genera, as in the human subject, there are several of these conical processes in each kidney ; while in other animals, all the tubes of the gland converge to a single cone, as in the lion, the racoon, the kangaroo, the monkey, the squirrel, &c.
The renal artery, derived from the abdominal aorta, enters the hilurn of the kidney. The veins generally follow the arteries, but there are exceptions to this rule. In the lion kind the cat kind, as also in the hyaena and in the seal, perhaps one half of the veins yet on the external surface, over which they pass, enclosed in a doubling of the capsule, and so join the trunks from the inside just as the latter are passing out from the hilurn.t