570 VETERINARY PHYSIOLOGY 



1st. The chief differences between the urine and the blood 



plasma. 1. The blood plasma contains some 7 to 8 per cent, of 



native proteins ; in normal urine these are absent. 



2. The blood plasma is almost neutral, i.e. it has a Ch 

 of 10"'^ or pH 7'4. The urine is generally markedly acid 

 with a pH of about 6 (Appendix III.). 



3. The molecular concentration of the plasma, as determined 

 bv the reduction of the freezing-point, corresponds to about 

 aO'56'' C, corresponding with about 0-9 per cent, of NaCl. 

 The urine has a molecular concentration of from A 1 to 25 or 

 even higher. 



4-. The urea in the plasma amounts to about 0"03 per cent. 

 In the urine it is about 2 per cent., i.e. it is concentrated 60 



times. 



5. Uric acid is concentrated 2-5 times. 

 Ammonia „ 40 ,, 



PC, „ 30 „ 



SO, „ 60 „ 



27id. The apparatus which brings about these changes. — 

 The kidnev, the apparatus which has to effect this change, is a 

 structure evolved from the nephridium of the annelid. 



This nephridium consists of a tube, opening by a funnel- 

 shaped end into the coelomic cavity and by a narrower orifice 

 on the surface. The tube is lined by a syncytial secreting 

 epithelium, in which may be seen the waste particles which 

 occur in the ccElomic fluid, or which have been injected into the 

 coelome, e.g. Indian ink. It thus allows an escape of fluid 

 from the ccelome and passes out waste particles through the 

 epithelium. Id. the vertebrate the coelomic orifice of each 

 tubule is invaded by a tuft of capillary vessels, forming the 

 glomerular tuft round which the remains of the funnel-shaped 

 opening persists as the capsule of Bowman. The whole 

 structure is often called a Malpighian body. The tubule in the 

 vertebrate becomes enormously lengthened and differentiated 

 into distinct segments. Myriads of these Malpighian bodies 

 and tubules are massed together to form the kidney. 



This may be briefly described as follows : — 



