ABSORPTION. 193 



Mucin is abundant. The urea is dependent on the amount of 

 oats, grain and roots ingested, the hippuric acid (CaHyNO.), to 

 which and phenol the odor is due, on the amount of green stuff, 

 hay and straw. 



The amount in twenty-four hours is five or six litres, varying 

 with the amount of fluid excreted by the sweat glands. 



The secretion of urine takes place — 



1. ^y filtration, the water and soluble salts passing out into the 

 interior of the glomerulus. 



2. By active secretion, urea being secreted by the epithelial cells 

 of the convoluted tubules. The latter process has been proved 

 by injecting i?idigo caj-mine into the blood after section of the 

 spinal cord to prevent filtration. The carmine can be found in 

 the cells of the convoluted tubes, and also in the urine, none be- 

 ing found in the glomerulus. 



Chine is decreased in summer, due to the increased amount of 

 secretion by the skin, by slowing the heart, by sleep and rest. 



It is increased by cold weather, stimulation of the heart, after 

 eating, and also by stimulation of the medulla. 



After secretion, the urine passes into the pelvis of the kidney, 

 through the ureter, drop by drop, into the bladder, where it is 

 accumulated till discharged, the bladder being emptied three or 

 four times in twenty-four hours. 



Nervous 77iechanism of micturitio7i. 



The centre for micturition is located in the lumbar enlargement 

 of the cord. The sensation of fullness is carried to this centre, 

 which sends back an impulse, relaxing the contracted sphincter, 

 and causing a contraction of the longitudinal (or detruso?-) fibres. 

 This forces the contents into the urethra, the contraction of its 

 fibres terminating the act. 



After section of the spinal cord, a condition known as I'etention 

 with overflow takes place, the bladder is distended, and soon also 

 the sphincter vesicce. by mechanical means, the urine escaping 

 drop by drop and soon becoming ammoniacal. 



THE DUCTLESS GLANDS. 



These are the spleen, see Abdomen ; the thyroid and thymus, see 

 Thorax; and the supra-renal capsules, see Kidney. 



The Supra-renal capsules are relatively larger before birth, 

 can be extirpated with no apparent result, are probably connected 



