THE DIGESTIVE AND RESPIRATORY SYSTEM 283 



nammals in which the milk set is represented by useless rucli- 

 nents that are either absorbed before birth or expelled soon 

 after. In view of all facts thus far presented it seems that 

 the monophyodont condition has been secondarily acquired as 

 a special adaptation in certain forms by the suppression of 

 one or the other of the two typical generations (Fig. 78, B). 

 Although fishes possess no functional tongue the material 

 out of which a tongue is to be constructed is present in the 

 form of the anterior part of the hyo-branchial apparatus. 

 The anterior border of this complex lies in the floor of the 

 mouth cavity, following the outlines of the jaw, and by certain 

 actions of the visceral muscles may be projected upwards so 

 as to form a noticeable elevation. When in amphibians these 

 parts become relieved from the gill-bearing function, they 

 form the basis of the tongue, often fusing into a complex, 

 moved by muscles and supporting a fleshy organ of some sort. 

 In that type of tongue which leads to the higher vertebrates 

 the skeletal basis consists of two to four of the visceral arches, 

 beginning with the hyoid, and, although admitting of many 

 varieties, possesses as essential a median basi-branchial piece, 

 called here the os entoglossum, and two posteriorly projecting 

 cornua. The tongue of the Sauropsida is a direct continuation 

 of this, and in both cases the principal motion is a protrusion 

 and withdrawal of the organ as a whole, which is effected by 

 means of the two posterior cornua, which lie in sheaths from 

 which they may be everted. When the tongue is unusually 

 long the sheaths and the enclosed cornua are of correspond- 

 ing length, and their disposition when retracted becomes a 

 problem variously solved in different cases. Thus in a cer- 

 tain salamander, Spelerpes fuscus, the sheaths of the cornua 

 run down the sides of the body and are attached to the ilia, 

 and in the woodpecker they come around the occipital region, 

 pass over the top of the head and terminate at the base of the 

 upper beak, near the anterior nares. In these cases the ends 

 of the cornua are attached to the bottom of the sheaths and as 

 they are withdrawn the sheath is turned inside out and adds 

 to the total length. 



