NATURE AND PURPOSES OF ANIMAL SECRETIONS. 297 



especially those of the nervous system. This sometimes 

 happens in consequence of disease, and it may be imitated by 

 experiment ; for when the trunk of the blood-vessel convey- 

 ing the blood to the liver or kidney is tied, the excretion is 

 necessarily checked, and the same results take place as when 

 the stoppage has depended on want of secreting power. The 

 biliary and urinary matters have the effect of narcotic poisons 

 upon the brain ; when they have accumulated in the blood, 

 their symptoms begin to manifest themselves ; and these 

 symptoms increase in intensity, as the amount of the sub- 

 stances becomes augmented, until death takes place. 



352. Besides the Excretions, we find various Secretions 

 elaborated in different parts of the bodies of animals, with a 

 view not so much to the purification of their blood, as to the 

 fulfilment of special purposes in their economy. These vary 

 considerably in the different classes of animals ; though some 

 of them, being concerned in functions almost universally per- 

 formed, are equally general in their range. Thus we find the 

 Salivary and Gastric fluids poured into the mouth and stomach, 

 for the reduction and solution of the food ( 190 and 204); 

 and the Lachrymal secretion poured out upon the surface of 

 the eye, for the purpose of washing it from impurities ( 541) : 

 while the secretion of Milk for the nourishment of the 

 young is limited to Mammals ; and poisonous secretions are 

 formed in Serpents ami Insects, for the destruction of their 

 prey or for means of defence. Any one of these may be 

 checked, without rendering the blood impure by the accumu- 

 lation of any substances that should be drawn-off from it ; 

 but its cessation may produce effects fully as injurious, by 

 disordering the function to which it is subservient. Thus, if 

 the salivary and gastric secretions were to cease, the reduction 

 of the food could not be effected, and the animal must starve, 

 though its stomach were filled with wholesome aliment. It 

 is to be observed, in regard to nearly all these secreted fluids, 

 that they contain but a small quantity of solid matter, and 

 that this matter seems to be formed from the albumen of the 

 blood by a process of incipient decomposition, which gives it 

 the character of a " ferment." 



353. The various acts of Secretion and Excretion which are 

 continually taking place in the living body, are, like those of 

 Nutrition, completely removed from the influence of the will; 



