162 
DIGESTION AND ABSOEPTION. 
the blood, and is thrown out from the surface, mingled with 
animal matter. This hardens in a day or two, and the new 
covering is complete. The concretions in the stomach are 
then found to have disappeared; but they are gradually 
replaced, before the supply of lime they contain is again 
required. 
CHAPTEB IV 
DIGESTION AND ABSORPTION. 
171. Having now considered the nature of the food of 
Animals, and the sources from which it is obtained, we have 
next to consider the process by which the aliment is received 
into their bodies, and prepared to form a part of their own 
fabric. This process, termed Digestion , is naturally divided, 
among the higher animals at least, into various stages. In 
the first place, there is the prehension or laying hold of the 
aliment, and its introduction into the mouth or entrance to 
the digestive cavity. In the mouth it usually undergoes a 
preparation ; which consists partly in its being cut, ground, 
or crushed, by mechanical action, into minute pieces; and 
partly in the working-up of these pieces with a fluid that is 
poured into the mouth,—the saliva. These two processes are 
termed mastication and insalivation; similar processes are 
performed, in some animals, in a part of the digestive tube 
intermediate between the mouth and the stomach, and even 
in the latter itself. The stomach is usually situated at some 
distance from the mouth, and is connected with a tube called 
the oesophagus or gullet; and the passage of the food into 
this, constituting the act of swallowing, is termed deglutition . 
The food, having arrived in the stomach, is acted-upon by a 
peculiar fluid which it contains, and much of its alimentary 
portion is dissolved, so that a pulpy mass is formed which is 
termed chyme ; hence this process, which is the first stage of 
digestion properly so called, is termed chymification or the 
manufacture of chyme. The chyme, which passes into the 
intestines, is further acted-on by secretions that are poured 
into them ; and a certain nutritive combination of albuminous 
