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 and 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 accumn- 
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 eftected, 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; 
