342 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



end of the palatines. In correlation with the mode of 

 action of the tongue (Fig. 143 ; 't\ which is protruded 

 into the recesses from which the bird extracts the 

 insects on which it feeds, the hyoid bones that 

 support it (thJi) are greatly developed, the anterior 

 cornua passing backwards and then curving forwards 

 over the roof of the skull, in grooves of which they 

 lie. From the ends of these bones or their sheath, 

 there arises a muscle which is inserted into the lower 

 part of the same bones ; the contraction of this muscle 

 straightens the curve of the thyrohyals, but this can 

 only be effected by the protrusion of their free or 

 lower ends ; as the tongue is placed at this point, and 

 is supported by the bones, it is clear that it is 

 simultaneously protruded. (See page 156.) 



In the Mammalia the quadrate, of which we 

 have till now heard so much, is converted into one 

 of the auditory ossicles, the malleus ; the covering 

 bones of the mandible are reduced to the dentary. 

 Man, so far as is known, is the only animal, in addition 

 to the frog and the sturgeon, in which the meiito- 

 meckeliaii cartilage is found on either side of 

 the mandibular symphysis. The Marsupials are re- 

 markable for having the angle of their lower jaw 

 developed into a strong inflection, which is absent 

 only in Tarsipes, and, among the Eutheria, has only 

 been observed in the insectivorous Centetes (the 

 Tenrec). 



When we compare a carnivorous mammal, in 

 which the lower jaw moves up and down, with a 

 ruminant, in which it moves . from side to side, we 

 observe a difference in the characters of the glenoid 

 cavity by which the mandible is articulated with the 

 cranium ; in the dog, for example, this fossa is concave 

 from before backwards, and the projections in front 

 and behind limit the movements of the articular end 

 of the mandible to such as can be effected in a vertical 



