32 ORGANIZATIONOFREPTILES. 



the nerve be one of motion; for the nerves of the body have very different offices 

 to perform, according to their connexion with the nervous centre;* — their fmictions 

 are all determinate and cannot be changed, for one nerve cannot perform the office 

 of another. The extremity of a nerve that touches the central mass is called its 

 origin; the peripheral end, or that spent on the various organs of the body, 

 its termination; and physiologists are still in doubt as to the ultimate disposition 

 of the peripheral extremity of the nerves. All nerves are protected at their 

 peripheral extremity, for in no instance does the body producing the impression 

 come in immediate contact with the extremity of the nerve; and we often find 

 peculiar structures frequently very complicated, to facilitate certain impressions; 

 thus, the optic nerve ends on the retina, which is beautifully arranged to receive 

 impressions from light; the auditory is spent on the internal ear, disposed to be 

 impressed by sonorous undulations; even the nerves of touch are covered by the 

 cuticle, through which the impression is made, and if this be removed, the sensation 

 of touch is weakened or entirely destroyed. 



Vision. — All Reptiles have organs of vision, and in many the eyes are prominent 

 and large; in some, however, as in the Siren and Amphiuma, they are exceedingly 

 minute, almost in a rudimental state; in all, they are movable and placed in 

 imperfect bony orbits. There is a manifest decrease in the developement and 

 degree of perfection of many parts of the organ when compared with the eye of 

 more elevated animals. The eye of the Reptile is intermediate in its structure 

 between that of Birds and Fishes. Blainvillet thinks that it has most relation 

 with that of Birds, while Carus;}: is of opinion that it approximates to that of Fishes, 



* Charles Bell, Phil. Trans, for 1822. The spinal marrow consists of four chords, two 

 anterior and two posterior; if a nerve be connected with both, or has a double root, it is then 

 a nerve of sensation and motion; if it be connected with only one of these chords, it is then a 

 nerve of sensation or motion, but cannot perform both functions; if it is connected with the 

 superior and lateral portions of the spinal marrow or medulla oblongata, then it becomes a 

 respiratory, vocal, or nerve of expression. 



t Princip. d'Anat. Comp., torn. i. p. 411. 



J Vergleich. Zoot. Erst. Theil., p. 394. 



