318 FISHES. 



pectoral and of the ventral fins, may be compared to the short flexors 

 and short extensors of the toes of the lizards, and especially the 

 crocodile ; animals in which these parts are usually in the same way 

 disposed in two layers, on each surface of the hand and foot, but 

 more defined and assisted by long muscles, which are entirely wanting 

 in the fishes. 



Muscles of the Jaws. 



These compose one single mass (No. 20), which is common to the 

 two jaws, and which closes the mouth by bringing the latter 

 together. 



The mass is connected with the whole external surface of the 

 posterior portion of the palato- temporal arch, and with all the bones 

 of which it is composed, including the anterior edge of the preoper- 

 culum. It is most usually divided into three tubercles or bellies, and 

 sometimes four ; its form is very nearly quadrangular, and from its 

 anterior border it gives off two tendons united by a single apophysis. 

 That which is sent off by the superior angle, and which is the 

 longest, reaches the upper part of the superior jaw ; that coming 

 from the opposite angle, which is much shorter, is inserted into the 

 lower jaw behind its coronoid apophysis. The aponeurosis dilates 

 itself on the membrane which joins the two jaws. 



This organization, it will be seen, is very different from our 

 crotaphyte and our masseter ; but we shall find it constant in all the 

 osseous fishes, and in some of them we have never been able to 

 detect any thing which could be said to resemble the pterygoidean 

 muscles. As to the cartilaginous fishes, the muscles of their jaws 

 present some important distinctions, which will be described in the 

 proper place. 



A difference not less remarkable, is the absence of a digastric, or of 

 any representative of that muscle, for depressing the lower jaw ; the 

 jaw does not move in that direction, and, consequently, the mouth can 

 only open by the simultaneous action of the muscles between the 

 shoulder and os hyoides, and between the latter and the lower jaw. 



This last muscle (No. 27) answers to the genio-hyoideus and we 

 shall return to it. 



But the lower jaw in fishes being able, in numerous species, to 

 approximate more or less, the two branches is furnished with a 

 proper muscle destined for this office (No. 21). It is placed across 

 the angle formed by the two branches and behind their symphysis, 

 above the anterior termination of the genio-hyoideus. 



Muscles of the Palato- Tympanal Arch. 



There is one muscle always occupying a considerable part of the 

 palatine arch ; it is a very thin layer of tranverse fibres which pass 

 from a part, more or less extended under the sphenoid and the great 

 wing, tranversely to the superior border of this arch and its internal 

 surface, being inserted chiefly in the temporal, and the adjacent part 

 of the box and internal pterygoid. This muscle depresses the arch, 

 and brings it to its fellow, a circumstance that contracts the lateral 

 space for the branchial apparatus. 



