THE URINARY BLADDER. 173 



and pouring their contents into an efferent vessel smaller 

 than the afferent one, and emerging close to it. These efferent 

 vessels are likewise to be considered as arteries, for, on their 

 emergence, they break up into capillaries, which supply the 

 whole capillary network of the tubules with blood which 

 has previously passed through the Malpighian corpuscles. 

 The glomerulus, when distended, completely fills the capsule 

 into which it dips, and in which it may be said to hang 

 loose, being covered with only a delicate layer of squamous 

 epithelium. 



The structure of the Malpighian corpuscles leaves little 

 reasonable doubt as to their function ; they are fitted to allow 

 water to drain off from the blood : and it is in keeping with 

 this hypothesis that they are very rudimentary in birds, as 

 in birds the urine is of semi-solid consistence, forming the 

 white part of their mutings. The drain of water from the 

 blood in the glomerulus accounts for the efferent vessel being 

 smaller than the afferent ; and the blood, being freed from 

 superfluous water first, is brought in a more concentrated 

 state into contact with the tubules, whose epithelia remove 

 from it the solid constituents of the urine. 



131. The Urinary Bladder is a hollow viscus, with its outlet 

 at its lower part, and when empty it lies altogether within the 

 pelvis, but when distended it is enlarged in an upward direc- 

 tion, rising considerably above the pelvis, and resting against 

 the abdominal wall, to which it is bound down by peritoneum. 

 In that state it is comparatively unprotected; and when over- 

 distended, it has been known to be ruptured by very slight 

 accidental violence. The ureters open into the bladder 

 behind its orifice or neck, their points of entrance forming 

 with that orifice the angles of an equilateral triangle, with 

 sides about 1J inches long, and called the trigone. The 

 mucous membrane of the bladder has a stratified squamous 

 epithelium, while that of the ureters is intermediate between 

 the squamous and columnar varieties. At the orifices of the 

 ureters, which enter the bladder obliquely, the mucous mem- 

 brane is somewhat redundant, so as to form on each orifice 

 a valve which effectually prevents regurgitation. 



The muscular walls of the bladder have the fibres extend- 

 ing round it in every variety of direction, yet not without 



