574 



URINARY APPARATUS. 



TRANSVERSE SECTION OF THE KIDNEY". 



1, Inferior border ; 2, cortii al tissue ; 3, section of 

 blood-vessels; 4, pelvis; 5, ureter; 6, superior 

 border ; 7, renal artery ; 8, proper capsule. 



little behind the Wolffian bodies. They are then very distinctly lobulated, but 



the lobes gradually become fused, and have entirely disappeared at birth ; the 



small irregularities on the surface are the only indications of their ever having 



existed in Solipeds. 



Functions. — The kidneys are the organs which secrete the urine ; but this 



secretion does not take place to the same extent in all parts of their tissue. The 



abundance of vessels in the cortical 

 Fig. 343. substance, the presence of the Mal- 



pighian corpuscles, and the flexuosi- 

 ties described by the uriniferous 

 tubes, sufficiently indicate that this 

 substance should be the principal, if 

 not the exclusive, seat of the secre- 

 tory function. But in what manner 

 does this secretion take place ? At 

 present it is generally agreed that 

 the urinary secretion is simply an 

 infiltration of the elements of the 

 urine contained in the blood, through 

 the walls of the vessels of the glome- 

 rulus. The difference existing be- 

 tween the diameter of the afferent 

 and efferent vessels of the Malpighian 

 glomerules — a fact the importance 

 of which was pointed out by Ludwig 



— sufficiently explains this filtration of the urine through the tissue of the 



kidneys. 



The nutritive principles of the serum which leave the blood at the same time 



as those of the urine, are taken up by the epithelium of the uriniferous tubes. 



2. The Ureters (Fig. 338). 



Form. — The ureter is a membranous canal, the diameter of a thick goose- 

 quill, which conveys the urine from the pelvis of the kidney into the bladder. 

 Its origin, course, termination, and structure, will be successively considered. 



Orifiin. — It has been already shown that the origin of the ureter is at the 

 infundibulum of the pelvis of the kidney ; it leaves that organ by the internal 

 fissure or hilus, curves outwards, passes along its lower face, and is inflected 

 backwards in quitting the organ. 



Direction. — The course it follows is almost in a straight line towards the pelvic 

 cavity, along with the posterior aorta or posterior vena cava, according to the side 

 to which it belongs ; it is in contact with the psoas parvus, and proceeds above the 

 peritoneum. After passing beyond the terminal branches of the posterior aorta, 

 which it crosses very obliquely, it becomes enveloped in a short peritoneal fold that 

 maintains it against the lateral wall of the pelvis ; it afterwards emerges from 

 this fold, and reaches the posterior and superior part of the bladder. 



Termination. — Having reached that viscus, its termination takes place as 

 follows : instead of opening directly into the bladder by traversing at once, and 

 perpendicularly, the two membranes composing the organ, the ureter at first 

 pierces the muscular coat, between which and the mucous membrane it passes for 

 about an inch, and then opens on the surface of the latter. This arrangement 



