734 THE IJEINAEY TRACT. 



in the stomach. Many an eminent observer has contributed to 

 our stock of knowledge regarding the minute anatomy of the 

 kidneys, while the physiology of the excretion of urine is still 

 unexplained. 



(1) The Kidneys. The kidney consists of a cortical and a 

 pyramidal portion, and in man is made up of from six to twelve 

 united parts, each of which has a cortical and a pyramidal 

 substance. The boundary between the two main divisions is 

 irregular, and in parts illy denned ; each subdivision terminates 

 at the concave border in the center of the organ in the so-called 

 papilla, which is surrounded by a membranous sac the calyx. 

 The loose connective tissue between the pyramids carries the 

 larger blood- and lymph- vessels and the principal nerves. The 

 surface of the kidney of the adult is smooth, being ensheathed 

 by a dense, fibrous connective-tissue capsule; while the foetal 

 kidney exhibits a distinctly lobate surface, indicative of its gen- 

 eral construction of lobes or segments. The main constituents 

 of the kidney are the blood-vessels and the uriniferous tubules, 

 which are held together by a delicate, fibrous connective tissue, 

 rich in elastic substance, giving the whole structure a high 

 degree of consistency. From the arrangement of the tubules the 

 kidney is classified among the compound tubular glands. 



The renal artery enters the organ in two main branches 

 (Hyrtl), each of which, by a number of bifurcations dividing 

 into smaller ramules, supplies an independent half. These ves- 

 sels, upon reaching the boiindary zone between the cortical and 

 pyramidal substance, deviate in an oblique direction, and produce 

 the so-called arterial bows or arches, the convexities of which, if 

 marked at all, look toward the cortical substance. Each artery 

 in this situation is accompanied by a vein, the veins being con- 

 nected by lateral branches, and producing a sort of a venous 

 plexus. 



From the arches arise at short intervals straight arterial 

 branches, which penetrate the cortical substance in a straight 

 direction and divide at very acute angles ; these arteries produce 

 transverse ramules, the afferent vessels which go to form the tuft. 

 A successful injection of the blood-vessels of the human kidney is 

 quite exceptional, while the dog's kidney, which can be had in a 

 perfectly fresh condition, allows a plain demonstration of the 

 vascular supply. As the relations in the latter are very similar 

 to those of human kidneys, they may with preference be used for 

 demonstration. (See Fig. 329.) 



