124 



ELEMENTAL Y ANA TOMY. 



[LESS. 



The body of the hyoid may be in quite a rudimentary con- 

 dition, as in the Sheep, or swollen and inflated to an enor- 

 mous relative size, as in the Howling Monkey. It may 

 develop in front a long, median, projecting process, termed 

 a glosso-hyal, as in the Horse, which in man is only repre- 

 sented by the vertical ridge on the anterior convex surface ot 

 the body of the os hyoides. 



When we descend below man's class we may find (e.g. in 

 Birds) that the cornicula more or less abort, while the cornua 

 are very long and slender. A glosso-hyal may not only 

 extend forwards from the basi-hyal, but another azygos 

 median part (the uro-hyal) may extend backwards from the 

 basi-hyal. 



A most unexpected condition may exist, as we see by the 

 Woodpeckers, in which both the elongated cornua curve over 

 the back part of the cranium, and are together inserted just 

 above and behind the right nostril ! 



In Reptiles we may (as in the Crocodiles) find a car- 

 tilaginous body together with one pair 

 of cartilaginous cornua, and these not 

 joining the skull. But generally both 

 cornua and cornicula are developed, and 

 may be large and complex, as in the true 

 Lizards, and the glosso-hyal may be 

 enormous, as in the Chameleon. When, 

 however, we descend to the class Batra- 

 chia we begin to perceive the full signi- 

 ficance of the hyoidean cornua, and this 

 by means of the transformation under- 

 gone by the Frog in its passage from 

 its larval (and fish-like) state as a tad- 

 pole to its adult condition. 



In the fully-developed state the os 

 hyoides of the Frog consists of a body 

 with a pair of cornua and a pair of corni- 

 cula. But the process of development 

 shows that the pair of cornua are the last 

 rudiments and relics of those cartilagi- 

 nous arches which exist on each side of 

 the neck in the larva and support the gills. 



These cartilaginous gill-arches of the tadpole evidently* 

 answer to the great cartilaginous branchial (or gill) arches r 

 of the Sharks, and to the bony branchial arches of the osseous 



1 For further details concerning these arches see Lesson XII. 



FIG. 1^2 HYOID OF A 



LIZARD -Lacerta. 



(After Cztvier.) 



M, body of the hyoid : 

 c l , corniculum ; c 2 , 

 cornua ; gh, glosso- 

 hyal ; ?/, uro-hyal. 



