DIGESTIVE SYSTEM 741 



is a little open place or pit in close relationship to the development of 

 the thyroid gland. 



It will thus be seen that the mammalian tongue is quite similar to 

 that of reptiles, and exceeds that of birds by having portions in it that 

 come from the mandibular arch. 



Two views are usually stated as to the relations of the mammalian 

 and amphibian tongue. One holds that the amphibian tongue is entirely 

 unrepresented in the mammals, unless it be by the sublingua. This is a 

 fleshy fold beneath the tongue of marsupials and lemurs, traces of which 

 occur in other mammals, even in man, as folds (plicae fimbriatae) be- 

 neath the tongue. In some cases (Stenops) the sublingua is supported 

 by cartilage, which may be the entoglossum. The other view is that at 

 least the anterior part of the tongue in amniotes is quite like that of am- 

 phibia. This view holds that the lyssa (a vermiform mass of cartilage, 

 muscle, and connective tissue, lying ventral to the median septum of the 

 tongue), is the equivalent of the entoglossum and its associated struc- 

 tures. 



The dorsal surface of the tongue is covered with a soft epithelium 

 with many mucous glands. There are also varying forms of papillae 

 (Fig. 433), some of which, the taste buds for example, are sensory, while 

 some become cornified to form epidermal teeth. A rasping type of tongue 

 in which many of the papillae have become cornified is that of the cat. 



GLANDS 



In animals that live under water it is quite natural that compara- 

 tively few glands should be found in the mouth cavity other than the 

 very simplest kind. These pour out a slight amount of mucus. If glands 

 were to exist there to any extent their secretionss would be washed away 

 with the incoming and outgoing water that passes through the mouth 

 cavity of such animals. Then, too, one can easily understand that, where 

 a secretion of an animal gland is soluble in water, if such animal lives 

 in water, the secretion could be of no value whatever. Contrasted to 

 this, it can also readily be understood that animals which breathe air 

 must have many glands moistening all surfaces constantly, or the ab- 

 sorption, which is always going on, would soon have all parts of our 

 bodies so dry that they could no longer function. For this reason ter- 

 restial animals have many more secreting glands than water animals. 



Mammals therefore have salivary glands. The saliva, which these 

 secrete, contains not only mucus, but a digestive ferment known as 

 ptyalin which changes starch into sugar. 



Glands are named largely after the position they occupy, such as 

 labial, lingual, sub-lingual, etc. 



In the air breathing amphibia, snakes, and lizards, there are labial 

 glands opening at the basis of the teeth as well as an intermaxillary or 

 internasal gland, in the septum between the nasal cavities, as well as 



