CROCODILIA. Ml 



quarter of an inch broad, and near half an inch long ; the formation of 

 the amis is a slit about an inch long, and has a little of the external 

 skin carried a little way into the rectum, but there it is very fine. The 

 sphincter ani is very strong ; the whole of the clitoris is hid within the 

 anus 1 . 



The centre of motion of the lower jaw is nearly in the same line with 

 the centre of motion in the head on the neck ; but it is a little further 

 back, nearly as far back as the union of the first and second vertebrae 

 of the neck ; so that there is one centre of motion to the jaws, and to 

 the first vertebra upon the second. Now, as all the openers of the 

 mouth are behind this centre of motion, and their origin from the head 

 is at a greater distance from the centre of motion than the insertion is, 

 it may be owing to this that the head moves upon the lower jaw. The 

 raisers of the jaw or depressors of the head arise from the head ; their 

 origins are very extensive ; they arise as far forward as half-way 

 between the anterior and posterior nares, making part of the roof of 

 the mouth posteriorly and laterally, but not in the middle, where there 

 is an interval between the two muscles ; they make part of the bottom 

 of the orbit. This origin is continued backward along the base of the 

 skull under the ear as far back as the [posterior margin of the entoptery- 

 goid bone]. Prom this extensive origin the muscles pass backward and 

 downward in different directions, and the anterior fibres almost directly 

 back, the others almost directly down, and are inserted into the upper 

 edge and inner surface of the lower jaw principally before the centre of 

 motion, but some are inserted behind it. The use of these is to raise 

 the lower jaw or depress the upper one, and those that arise so far 

 forwards will rather depress the upper jaw. As they are so far before 

 the centre of motion at the origin, and so near it at the insertion, one 

 would imagine that those fibres that are inserted behind the centre of 

 motion, would depress instead of raising it ; but tbe lower jaw is carried 

 further back, and these fibres in some measure pass over a ridge of 

 bone, as over a pulley, when the mouth is open, so that by these means 

 these fibres are upon the stretch at that time. 



There are two bony processes, one on each side 2 , passing down on 

 the middle of the lower jaw, just before the insertion of these muscles. 

 These processes are covered on their exterior surface by a smooth articu- 



1 [Hunt. Prep. Phys. Series, No. 2725. The peritoneal canals terminate on two 

 papilla, placed on either side the root of the clitoris or penis, and communicate, at 

 about a line distant, with the cavernous structure of those parts ; they oppose the 

 ingress of fluids from without.] 



2 [The ' entopterygoids.' See ' On the Homologies of the Vertebrate Skeleton,' 

 8vo. 1848, figure 22, 24-] 



