NERVOUS SYSTEM. 149 



into the carapace, beneath which it serves, together with the retra- 

 hentes capitis et colli, to retract the head of the animal. The 

 oblique and transverse muscles of the abdomen are of considerable 

 size, being important agents in the movements of respiration, and 

 there is found in this region of the body, as in Birds, a rudimen- 

 tary form of diaphragm, which arises as a broader thinner muscle 

 than in them, from the vertebral column and carapace, and is inter- 

 posed between the peritoneum and pleura, without, however, meet- 

 ing its fellow in the middle line from the opposite side. Beneath 

 the plastron lie the great pectoral muscles, which, like the large 

 glutaei, are strongly developed, this being the case also with the 

 muscles of the extremities, among which the flexor muscles of the 

 leg, the biceps femoris and semi-tendinosus, are remarkable for their 

 length. 



In the Ophidia the cranial and maxillary muscles, especially those 

 of the lower jaw, are distinguished by their great development ; for 

 instance, the temporal, from which a layer is given off over the 

 poison-sac of the Venomous species, and acts upon it as a com- 

 pressor. The muscles of the trunk, however, by which locomotion 

 is effected, are the most remarkable in the Serpents. It is here 

 chiefly the intercostales, as well as the spinales, semi-spinales, in- 

 terspinales and inter -transfer sales muscles that act upon the very 

 moveably united vertebrae and the free extremities of the ribs. Of 

 the intercostals, some pass directly from one to the next adjoining 

 rib, while others pass over several of these bones. Even the pelvis 

 and rudirnental feet of many Serpents (the osteological relations of 

 which have been described above) are provided with muscles that 

 bend the extremities as far as their ungual phalanx, and extend or 

 draw them in different directions. 



NERVOUS SYSTEM. 



THE Brain of the Amphibia ranks greatly inferior to that of 

 Birds in the relative proportional size which it bears to the bulk 

 of the body, though it fills up tolerably the cranial cavity, and is 

 surrounded by the usual membranes. What is remarkable, it ex- 

 hibits no very important differences in the several orders, though 

 in this respect we are confessedly still in want of more accurate 

 investigations. 



The Spinal Cord is prolonged into the caudal vertebrae, and is 

 generally furnished with two swellings corresponding with the 



