56 BIOLOGY. [BOOK i. 



3. Principles non-crystallisable, but coagulable, met with in all 

 the humours, except the bile, the urine, and the sudor. These last 

 immediate principles are the albuminoidal substances properly so 

 called, and the saccharine substances, both of them having the 

 property of dissolving certain mineral or mineralised compounds 

 little or not at all soluble in water. It is thus that albumine 

 fixes silica, phosphate of lime, urates, and so on. 



The humours produced or humours of secretion, are formed in the 

 economy at the expense of the constituent humours, and generally 

 by special organs called glands. By and by we shall have occa- 

 sion to study their process of formation. They comprehend the 

 extremely aqueous liquids, produced on the surface of the mem- 

 branes, called serous, which cover certain viscera (brain, heart, 

 lungs, intestines, and so on) ; the liquids bathing the articular 

 surfaces or synovial liquids ; lastly the sperm, the milk, the muci, 

 the salivae, the bile, the intestinal juice, and so on. 



In connection with the produced humours we may view the 

 liquids simply excreted, that is to say, separated from the con- 

 stituent humours without chemical modifications (sudor, urine, 

 amniotic liquid, allantoi'dian liquid, pulmonary exhalation). 



All these liquids, save three, are alkaline. The three liquids 

 habitually acid are the gastric juice, the sudor, and the urine. 

 However, this last liquid is sometimes alkaline, sometimes acid, 

 sometimes neutral, in man, at the different stages of digestion. 

 In the herbivorous mammifers it is normally alkaline. But here 

 also the alkalinity depends on the digestion. In effect the urine 

 of the herbivora becomes acid if we feed them on animal ali- 

 ments, or, which comes to the same thing, if dieting them, subject- 

 ing them to inanition, we force them to live at the expense of their 

 own substance. The alkalinity of the animal humours is usually 

 due to salts of bibasic or tribasic soda ; but free soda we never 

 meet with in them. 



In connection with secretion and excretion we shall speak in 

 detail of the produced humours, contenting ourselves here with 

 indicating their principal distinctive characteristics. 



