EXCRETION OF MATTER FROM THE BODY 403 



YIII. Carbonic and Oxalic Acids. 



1. Carbonic Acid. Small amounts of this are present 

 in urine, and after the administration of citrates, malates, 

 or tartrates, the amount may be considerably increased, 

 and the urine may then effervesce strongly when an acid 

 is added. 



2. Oxalic acid is a substance in a 

 stage of oxidation just above that of 



u 



TT Q Q Q Q jj carbonic acid. It is frequently pre- 

 sent in the urine linked with lime, 



and the lime salt tends to crystallise out in characteristic 

 octohedra, looking like small square envelopes under the 

 microscope. Under certain conditions these crystals assume 

 other shapes. The oxalic acid of the urine is chiefly derived 

 from oxalates in vegetable foods, but it has been detected in 

 the urine of animals on a purely flesh diet. 



SECRETION OF URINE. 



Structure of the Kidney. 



(This must be studied practically.} The kidney (Fig. 156) 

 is a compound tubular gland, consisting of innumerable 

 tubules, each made up of 



A closed extremity or Malpighian body (M.B.), consisting 

 of an expansion at the end of the tubule Bowman's capsule 

 into which a tuft of capillary vessels the glomerulus 

 projects. Extending away from this is 



A proximal convoluted tubule (P.C.T.) lined by pyramidal 

 and granular epithelial cells. This dives into the medulla 

 and again ascends to the cortex, forming Henle's Loop 

 {H.L.), which terminates in the distal convoluted tubule. 

 This exactly resembles the proximal (D.C.T.). It opens 

 into a collecting tubule (C.T.) lined by a low transparent 

 epithelium, which conducts the urine to the pelvis of the 

 kidney. 



The renal artery breaks up and gives off a series of straight 

 branches the interlobular arteries (IL.A.) which, as they 

 run towards the surface, give off short side branches which 

 terminate in the glomeruli. The efferent vein passing from 



