490 THE URINARY APPARATUS. 



small irregularities on the surface being the only indications of their 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 then- 

 tissue. The abundance of vessels in the cortical substance, the presence of 

 the Malpighian corpuscles, and the flexuosities described by the uriniferous 

 tubes, sufficiently indicate that this substance should be the principal, if not 

 the exclusive, seat of the secretory 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 and the uriniferous tubes. A 

 knowledge of the phenomenon of dialysis, discdvered by Graham ; and the 

 difference existing between the diameter of the afferent and efferent vessels of 

 the Malpighian glomerules a fact whose importance was pointed out by 

 Ludwig sufficiently explains this nitration of the urine through the tissue 

 of the kidneys. 



2. The Ureters. (Fig. 247.) 



Form. The ureter is a membranous canal, having 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. 



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

 infundibulum of the pelvis ; it leaves the kidney 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 afterwards follows is almost in a straight line 

 towards the pelvic cavity, along with the aorta or posterior vena cava, 

 according to the side to which it belongs ; it is in contact with the small 

 psoas muscles, and proceeds above the peritoneum. After passing beyond 

 the terminal branches of the 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 

 olie 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 prevents the flowing back of the urine into the ureter 

 during its expulsion, the intermembranous portion of that canal being 

 strongly compressed by the external pressure then exerted by the muscular 

 coat, and by the internal resistance which the accumulation of urine in the 

 bladder opposes to this pressure. So well ar^ Nature's intentions fulfilled 

 in this respect, that we may inflate the bladder by the ureter, after tying the 

 canal of the urethra, and press vigorously on the distended organ, without 

 being able to make a single bubble of air pass through the perfectly pervious 

 canal. 



STRUCTURE. The excretory canal of the kidney is composed of three tunics: 



1. An internal mucous tunic, continuous, in front, with that lining the 

 pelvis of the kidney, and behind, with that of the bladder. It is very thin, 

 pale, ridged longitudinally, and has a stratified tesselated epithelium. (It 

 has some mucous follicles, but no villi.) 



