The Structure of the Mammalian (Ksophagus. 37 
but if we examine the matter more closely we find that as a matter 
of fact the csophagus of these animals has been specialized on 
account of the coarse character of the food but in other ways than by 
the production of mucous glands. On the other hand the theory that 
the glands are developed in proportion to the needs of the animal 
for lubrication of the food bolus does not adequately explain the fact 
that in many Carnivora glands are few in number or wholly wanting 
whle in others they are very abundant. If we examine the cases 
of those animals which have a very large number of cesophageal 
glands we find that there is no common quality as regards efficiency 
of mastication relative to development of the salivary buccal and 
cesophageal glands, or consistence of the food which would serve to 
explain their presence. They are, however, without exception mixed 
feeders, and this suggests that it is the composition of the food 
rather than its consistence which has called forth the development 
of mucous glands in the esophagus, or to express it differently, that 
the secretion of these glands has a chemical value rather than a 
mechanical one. This view is supported by the twofold cellular char- 
acter of the glands, for it is difficult to understand why a serous 
gland should have been developed where the need was simply the 
mechanical need of lubrication. 
In contrast to the glands the epithelium shows a very definite and 
constant relation to the character of the food, and it is in the epithe- 
lium that we find the esophagus undergoing specialization in those 
animals which live on coarse vegetable food. The structure of the 
epithelium is thus an accurate index of the character of the food 
swallowed, inasmuch as we find a thickened and highly cornified 
epithelium in those animals which live on coarse food, particularly 
vegetable feeders, and a thin slightly cornified epithelium in animals 
living on soft food, for example, the carnivors. 
Correlated with this thickening of the epithelium we find, as 
might be expected, an increase in the development of the muscularis 
mucose, the probable function of this layer being to retract the 
mucous membrane above the descending bolus of food. Conversely, 
the muscularis mucose and the epithelium exhibit these specializa- 
tions in a less degree in those animals in which large numbers of 
