DIGESTION. 161 



Birds which feed on fish have no separate dilata- 

 tion for this purpose, probably because the great 

 width of the oesophagus, and its having the power 

 of retaining a large mass of food, render the further 

 dilatation of any particular part of the tube unne- 

 cessary. The lower portion of the oesophagus 

 appears often, indeed, in this class of animals, to 

 answer the purpose of a crop, and to effect changes 

 in the food which may properly be considered as a 

 preliminary stage of the digestive process. 



Chapter VII. 



Difrestion. 



All the substances received as food into the 

 stomach, whatever be their nature, must neces- 

 sarily undergo many changes of chemical composi- 

 tion before they can gain admission into the general 

 mass of circulating fluids ; but the extent of the 

 change required for that purpose will, of course, be 

 in proportion to the difference between the qualities 

 of the nutritive materials in their original, and in 

 their assimilated state. The conversion of vege- 

 table into animal matter necessarily implies a 

 considerable modification of properties , but even 

 animal substances, however similar may be their 

 composition to the body which they are to nourish, 

 must still pass through certain processes of decom- 

 position, and subsequent recombination, before they 

 can be brought into the exact chemical state in 

 which they are adapted to the purposes of the 

 living system. 



VOL. n. ]\i 



