THE DIGESTIVE AND RESPIRATORY SYSTEM 285 



veloped upon the dorsal side of the former, and gradually 

 usurping its function. An opposing opinion rejects the claim 

 of the sub-lingua as homolog of the sauropsidan tongue, and 

 finds the rudiment of the os entoglossum in the septum lingua, 

 a band of connective tissue running lengthwise through the 

 tongue and serving as an attachment for the muscles, or more 

 definitely in the lyssa, a vermiform structure composed of 

 connective tissue, fat, cartilage, and muscle fibers, which oc- 

 curs in the tongue of certain mammals, and is often asso- 

 ciated with the septum. 



Distinctive glands in the mouth cavity other than simple 

 mucous cells are not found in fishes or wholly aquatic am-i 

 phibians and appear only as an adaptation to the terrestrial/ 

 life for the primary purpose of keeping the surface moist. \ 

 Such glands are thus constantly present in terrestrial verte- 

 brates and become secondarily reduced in those that become 

 readapted to the water, as in the case of the Cetacea. These 

 glands are often voluminous, occur in all available positions, 

 and are named accordingly, buccal, labial, lingual, sub-lingual, 

 sub-maxillary (more properly sub-mandibular) , etc. Al- 

 though the primary function of all these was undoubtedly that 

 given above, they have in numerous instances assumed more 

 special functions and have altered the chemical nature of their 

 secretion accordingly. Thus the intermaxillary glands of frogs 

 and toads, which open into the roof of the mouth, secrete a 

 viscid fluid by means of which the tongue is rendered adhesive 

 for the apprehension of insects, the lingual glands of many 

 salamanders and lizards have a similar function, and certain 

 buccal glands in poisonous serpents become the elaborators of 

 the venom, which is inoculated into the victim by means of 

 the poison fangs. These teeth are provided either with a 

 groove along the external surface or possess a minute lumen 

 through the center as in the case of a hypodermic needle. 

 Another secondary function extensively employed is that of 

 lubricating dry food to render it more easily swallowed, and 

 in association with this the secretion often develops ferments, 

 of use in the digestion of starch, and forms the saliva. The 



