Chap, x.] DIGESTIVE APPARATUS IN THE ANIMAL SERIES. 167 



membrane sometimes furnished with tufts or villosities clothed 

 besides with cells comparable with the cells of the epidermis, 

 and called epithelial cells : a muscular layer of which the 

 . contractile elements are in part longitudinal, in part arranged as 

 a ring round the digestive tube : a layer of cellular tissue, which 

 gives a surface of insertion to the contractile elements, and to the 

 canal a sufficient degree of solidity and of resistance. It deserves 

 remark that the villose tufts which garnish the internal face of 

 the stomachal and intestinal mucous membrane, to the number of 

 many thousands the square inch in man, are lacking in the 

 invertebrates. Lastly to the digestive apparatus are annexed 

 glands, some of them voluminous, having their body outside of 

 the digestive wall, which only their secretory conduits traverse, 

 others in the very substance of that wall. 



To conclude the very general morphological description of the 

 digestive tube, it remains for us to say a few words on those 

 glands. 



0/ the Glands of the Digestive Apparatus. 



It is especially by the glandular apparatus that the digestive 

 system of the vertebrates in general and of the mammifers in 

 particular is distinguished from that of the inferior animals. 

 The digestive glands, in efiect, are more numerous, more dif- 

 ferentiated as to function, and of a more complex structure. 



The salivary glands are numerous, in clusters granulous and 

 closely pressed. Already in the tortoise we find under the 

 tongue a pair of glands which are probably salivary glands. 

 Analogous glands, but larger still, exist in birds. In the mam- 

 mifers they are distinguished into three couples, the maxillary, 

 the sublingual, and the parotid glands. It is in the herbivorous 

 mammifers that the three pairs of salivary glands attain their 

 greatest volume, and we shall see that this fact is in relation to 

 the kind of alimentation. 



As Claude Bernard has shown, these salivary glands, though 



