lo Nov., 1908.] Elements of Annual Physiology. 675 



irritation of the mucous membrane. It can be started or stopped by 

 voluntary effort. The nerve centre for this reflex lies in the lower spinal 

 cord. 



THE URINE. 

 The amount passed per diem varies with the age and species and in 

 each animal with the quantity of fluid drunk and inversely as the amount 

 of evaporation from the skin and lungs. The following table gives an 

 idea of the amount passed in twenty-four hours : — 



Ox ... . . ... .. 10 to 25 litres* 



Horse ... ... ... ... S to 8 ,, 



Pig ... ... ... ... 1-5 to 8 ,, 



Small riiniinants ... ... ... 1 to 5 ,, 



Man .. ... ... ... about To ,, 



Dog ... ... ... ... 0-5 to 3 ,, 



Cat ... ... ... ... about O".") ,, 



Certain chemical substances are important constituents of the urine of 

 all mammals : — 



1. Urea, a nitrogenous body containing also carbon, hydrogen and 

 oxygen. If urine be concentrated by evaporation, cooled and treated with 

 nitric acid, a rich deposit of flaky crystals of urea nitrate is formed. Urea 

 is readily attacked by bacteria if urine be exposed to air, and is changed 

 into ammonium carbonate — hence the ammoniacal odour of stables, &c. 

 Urea is derived from protein and the amount excreted in a day varies 

 parallel with the amount of protein eaten. It represents the protein used 

 as energy supply rather than that used for repair. 



2. Purin bodies including Uric Acid. The.se are substances containing 

 the same elements as urea but they are more complex in constitution. They 

 are derived partly from the nucleo-proteins of food and partly from the 

 activity of the nuclei of not only the fixed tissues but also of the white 

 cells of the blood. Uric acid is found in small quantities in herbivores; 

 generally speaking it varies directly with the amount of protein eaten. 



3. Kreatinin — containing the same elements as urea but closely allied 

 to a constituent of muscle. Some physiologists regard it as the decom. 

 position product of protein used for muscle repair. 



4. Pigments — these which give the characteristic colour to urine are 

 really altered bile pigments which have been absorbed from the gut. 



5. Sulphates. Proteins contain sulphur which, in the body, is oxidised 

 to sulphuric acid and leaves the body in the form of sulphates of sodium, 

 potassium, magnesium and calcium. 



6. Chlorides of Sodium, Potassium, &c. These are salts of the food 

 and of the blood and are continualh being excreted in the urine. 



7. Ammonia — fresh urine contains small quantities of this substance 

 probably formed bv the spontaneous decomposition of urea. 



Striking differences can be observed between the urine of herbivores on 

 the one hand and that of carnivores, of omnivores in whose food protein 

 preponderates, and of all voung mammals living exclusively on milk. The 

 urine of the latter class is acid in reaction, is transparent and contains 

 little or no sediment. It also contains phosphates of the four physiological 

 metals in considerable amount and its content of sodiiim is generallv 

 greater than that of potassium. Uric acid is also present in appreciable 

 quantity but aromatic bodies and oxalic acid are found only in traces. 

 Such urine mav occasionally show a sediment of uric acid and when, on 



* A litre =;= 1 1 pints. 

 V 2 



