152 REPTILIA. 



pent establishes at the base of the cranium communications with 

 the n. trigeminus, vagus, glosso-pharyngeus, hypo-glossus, and 

 also with the facial nerve. External to the cranium we meet 

 with ganglia, which may be compared with the superior cervical 

 and spheno-palatine ganglion. Upon the heart and the greatest 

 portion of the trunk, and upon the internal processes of the ver- 

 tebral bodies, the sympathetic nerves may be traced as extreme- 

 ly delicate streaks. The ganglia are also very small even in the 

 largest Serpents. In the Crocodiles, the cervical portion of the 

 sympathetic is situated deep within the canal formed by the 

 transverse processes of the cervical vertebrae. The vagus is dis- 

 tributed in the Serpents as far as over a third of the cavity of the 

 trunk. 



The lateral branch, that arising chiefly from the nervus vagus, 

 runs along the sides of the trunk as far as the tail, and is of gen- 

 eral occurrence in the class of Fishes, is met with also in the 

 Perennibranchiate Amphibia, the Proteidea, and the larvae of Ba- 

 trachia ; but in the metamorphosis which the latter undergo, it 

 disappears by degrees, and becomes finally reduced to the auricular 

 branch of the nervus vagus, or some tegumentary offset correspond- 

 ing to the former. In the Serpents and Lizards, no lateral nerve 

 exists. 



ORGANS OF THE SENSES. 



Organs of Vision. 



THE Eyeball with its several component parts, approximates in 

 the naked Amphibia most in structure to that of Fishes, since 

 they live in the same element as the latter, and the globe of the 

 eye having to receive the rays of light through the water, is ac- 

 cordingly flatter in front, and the lens more spherical. In the 

 Squamigerous Reptiles, the structure of the globe of the eye agrees 

 more with that of Birds, yet even in them presents certain charac- 

 teristic differences, as may be readily perceived, upon examining 

 a longitudinal section of the eyeball. In Proteus the eye is very 

 small, but provided with a lens, and the usual tunics ; in the 

 genus Typhlops, the eye is still more rudimentary. The sclerotic 

 frequently contains a bony plate, or several bony scales united to 

 form a ring, as in Birds. This is the case with the Tortoises, 

 where ten bony pieces may be most usually counted, and in many 



