EXCRETION 



461 



The Blood-vessels of the Kidney. The renal artery enters the kidney 

 at the hilum behind the ureter; it soon divides into several large branches 

 which penetrate the substance of the kidney between the pyramids and pass 

 outward into the cortex. At the base of the pyramids branches 'of the 

 arteries form an anastomosing plexus. From this plexus vessels are given 

 off, some of which follow the straight tubules toward the apex of the pyramids, 

 vasa recta, while others enter the cortex and pass to its surface (Fig. 218). 

 In the course of the latter, small branches are given off, each of which soon 

 divides and subdivides to form a ball of capillary vessels known as the glomer- 

 ulus. These capillaries, however, do not anastomose, but soon reunite to 

 form an efferent vessel the caliber of which is less than that of the afferent 

 artery. In consequence of this, there is a greater resistance to the outflow 

 of blood than to the inflow, and therefore a higher blood-pressure in the 

 glomerulus than in capillaries generally. The relation of the glomerulus to 

 the tubule is important from a physiologic point of view. As stated above, 

 the glomerulus is received into and surrounded by the terminal expansion 

 or capsule of the tubule. This capsule, formed by an invagination of the 

 terminal portion of the tubule, consists of two walls, an outer one consisting 

 of an extremely thin basement membrane, covered by flattened epithelial 

 cells, and an inner one consisting apparently only of flattened epithelium 

 which is reflected over and closely invests the glomerular blood-vessels 

 (Fig. 207). The blood is thus separated from the interior of the capsule 

 by the epithelial wall of the capillary and the epithelium of the reflected wall 

 of the capsule. During the periods 

 of secretor activity the blood-vessels 

 of the glomerulus are filled with blood 

 to such an extent that the sac cavity 

 is almost obliterated. After its exit 

 from the capsule the efferent vessel 

 of the glomerulus soon again divides 

 and subdivides to form an elaborate 

 capillary plexus which surrounds and 

 closely invests the convoluted tubules. 

 From this plexus as well as from the 

 plexus which surrounds the straight 

 tubules veins arise which pass toward 

 and empty into the interlobular veins 

 as ] well as the veins at the base 

 of the pyramids. The renal veins 

 formed by the union of these latter FIG 207> _ ScHEME OF THE RENAL o* MAL- 



Veins emerges from the kidney at the PIG HIAN CORPUSCLE, i. Interlobular artery. 



hilum and finally empties into the 2. Afferent vessel. 3. Efferent vessel. 4. Outer 



. . . J wall. <;. Inner wall. 6. Glomerulus. 7. JNecfc 



vena cava inferior. of tubu i e . (Stohr.) 



The Nerves of the Kidney. . 



The nerves distributed to, and associated with the functional activities of 

 the kidney consist of both pre- and post-ganglionic fibers. The latter have 

 their origin in the cells of small ganglia situated close to the semilunar 

 ganglion. From their origin they pass through the renal plexus and follow 

 the course of the blood-vessels to their termination. The former, the pre- 

 ganglionic nerves, have their origin in the lower portion of the thoracic region 



