CHAP. Yiii.] OliCfANS OF liESFIBATION AND SECRETION. 235 



of purity and oxygenation, and is as briglit and scarlet as when it 

 entered it. 



The watery substance of the urine appears to drain from the 

 vessels of the glomerulus into the cavities of the Malpighian cor- 

 puscles, and its more solid constituents to bo secreted by the thick 

 epithelium of the tubules. 



Some of the constituents of the urine — such as urea, uric acid, 

 and a substance called kreatine — already exist in the blood which 

 comes to the kidneys, and their elimination therefore is rather an 

 excretion than a true secretion ; not but what it may be doubted 

 whether any such a purely passive and physical process as a mere 

 strainiug-oif action, really takes place in the living organism at 

 all.* 



The action of the kidney is constant, and small quantities of 

 urine are continually passing from the mammilla of each kidney to 

 the pelvis, and thence, down the ureters, into the bladder. 



§ 11. The iTRETEiis are, as has been already explained, two ducts 

 or tubes which proceed one from the pelvis of each kidney. They 

 lie on the dorsal side of the peritoneum, and proceed inwards and 

 backwards to the bladder (Fig. 105), being connected by loose 

 areolar tissue to the parts adjacent to them. 



They enter the wall of the bladder very obliquely, opening within 

 it each by a narrow, slit-like aperture, the margins of which are 

 somewhat thickened so as to have a valvular action and check the 

 reflux of urine from the bladder into the ureters. 



Each tube consists of a canal of mucous membrane, lined with 

 epithelium ; the mucous tube being invested with organic muscular 

 fibres, which are again invested externally by connective and elastic 

 tissue. 



§ 13. The BLADDER (Figs. 105 b and 115), is a hollow, rounded 

 vessel with three openings, two (those of the ureters,) by which the 

 urine is received, and one (that of the urethra,) by which it is 

 discharged. These three apertures define a triangular portion of the 

 bladder called the trigone. 



The bladder is connected with the anterior wall of the abdomen, 

 at the umbilicus, by a fibrous cord called the urac/iiis, which is the 

 remnant of a foetal structure, and at least occasionally includes 

 a small irregular internal cavity lined with ej)ithelium. 



The bladder is also attached to adjacent parts by folds of peri- 

 toneum and of fascia. The inside of the bladder is lined with 

 mucous membrane invested with squamous epithelium. The mucous 



* Tlie pungent, and to most persons, 

 ilisagreeaLle odour of tire male cat is 

 notorious. Yet it must be due to causes 

 other than those which determine tlie 

 essential function of renal secretion, siiic(! 

 it is absent from the urine of the female 

 cat, and also from tliat of the castrated 



male. Tlic urine of all the species of tlie 

 cat family has (as for as known) a more 

 or less powerful and disngreeal)Ie smell, 

 but its odorous qualities dilier much in 

 dill'erent species, as the author has been 

 assured by Air. A. D. liartlctt. 



