128 ELEMENTARY ANATOMY. [LESS. 



the upper beak, being thin and elastic, are more or less mov- 

 able on the rest of the skull, and in the Parrots even by a 

 joint. In Lizards the anterior part of the skull is also more or 

 less movable on the posterior part. In Fishes we meet with 

 an extreme mobility of the jaws (the mandible being sus- 

 pended by a chain of bones), as also in Serpents, in which 

 not only is each side of the lower jaw separately movable 

 and united at the symphysis by a very elastic ligament, but 

 each part of the upper jaw can also be advanced singly ; and 

 thus, by successively advancing one tooth-bearing part of 

 the jaw after another, these animals rather drag themselves 

 over their prey than swallow it. 



34. The general conformation of the skull shows that 

 almost always the proportion borne by the jaws to the cranium 

 is greater than in man ; and often, as in the Stork, the Whale, 

 the Gavial, or the Sword-fish, it is the facial part which is 

 enormously predominant. Thus the skull may be extraordi- 

 narily elongated as compared with that of man, and tapering 

 anteriorly, or it may be much broader in front than behind, 

 as in the Hammer-headed Shark. It may also be very elon- 

 gated but cylindrical, as in Centetes. It may be strangely 

 flattened, as in the Matamata Tortoise (Chelys), or it may be 

 singularly compressed from side to side, and high, as in the 

 fish Argeriosus. 



Considering now the EXTERNAL SURFACE of the skull, the 

 superior region of man's is very exceptional in its extent, its 

 smoothness, and its rotundity. Generally in man's own class 

 a prominent sagittal ridge runs from in front backwards in the 

 line of the sagittal suture. This may be enormous, as in some 

 Seals and Carnivora, and even in the very Anthropoid Gorilla. 

 One of the most exceptional conditions which this region 

 may present is that exhibited by the Sperm Whale, or Cacha- 

 lot, where the cranium forms above, a great semicircular basin 

 for holding the spermaceti. In this and certain other animals 

 of the same order there is a want of symmetry in the skull 

 when looked at from above ; the two nostrils and pre-maxillas 

 being more or less unequally developed. This asymmetry is 

 carried much further in some of the true Fishes (namely, the 

 Pluronectidce, e.g. the Sole, Turbot, Flounder, &c.), where the 

 anterior part of the skull is extraordinarily twisted so as to 

 allow both the eyes to come to be on one side of the head. 



Generally when the upper surface of the skull is looked at, 

 the anterior nares are more or less inconspicuous, and placed 

 in front. In certain animals, however, as the Elephant, the 



