26 ON THE NERVODS SYSTEM IV. 



eyes are developed,* and yet the optic lobes do not remain undeveloped to a cor- 

 responding degree, but, on the contrary, acquire a proportional size nearly equal 

 to that of other Fishes with normal vision. While, on the one hand, there is suffi- 

 cient evidence to show that the optic lobes are connected with vision, there is, on the 

 other, evidence to prove that they exist without vision, or entirely out of proportion 

 to the visual organ, as in Proteus. 



The instances of Proteus and Amblyopsis naturally suggest the questions, whether 

 one and the same part may not combine functions wholly different in different 

 animals, and whether the same may not hold true with regard to the cerebral or- 

 gans which is known to obtain with regard to the skeleton, the teeth, the tongue, 

 and the nose, that identical or homologous parts in different animals may per- 

 form functions wholly distinct. If the doctrine here suggested can be admitted 

 (and if this were the place facts could be cited in support of it), may we not 

 find in it an explanation of many inconsistencies which now exist between the re- 

 sults of comparative anatomy and physiology "? 



III. Motor Communis (oculo-motor). (Plate I. Fig. 1, in.) In the whole Ver- 

 tebrate series of animals, there is generally but little variety in this nerve ; but the 

 exceptional cases have given rise to some difference of opinion as to its true nature, 

 and as to its relationship to the trigeminus. The ordinary distribution of this 

 nerve is as in the human body. On entering the orbit, it divides into two princi- 

 pal branches ; a superior, which is distributed to the upper rectus and levator of the 

 upper eyelid, and an inferior, distributed to the rectus internus, and the rectus 

 and obliquus inferior. In Batrachians the distribution is exceptional, and is differ- 

 ently described. Cuvier leaves us to infer that it is the same as in Man. Stannius 

 describes it as dividing into two branches, " and which are distributed to the rectus 

 superior and inferior, and obliquus inferior ; at the same time, in Salamanders and 

 Tritons, the rectus superior receives its filaments from the ophthalmic nerve " (oph- 

 thalmic branch of the trigeminus). 



As seen in my dissections the motor communis arises as a minute filament from 

 the under surface of the medulla oblongata near the median line at its anterior 

 part and just behind the pituitary body ; it is then directed outwards and forwards, 

 perforates a little obliquely the cartilaginous sides of the cranium, just in front of 

 the trigeminus ; on reaching the orbit, it crosses the ophthalmic nerve in close 

 contact, and has the appearance of becoming incorporated with it, which has led 

 to an erroneous description on the part of some anatomists. The nerves, however, 

 simply cross, there being no intermixture of filaments whatever. The branches 

 given off by the motor communis are, 1st. a branch passing along the border 

 of the rectus internus to the inferior oblique ; 2d. a branch to the rectus internus ; 

 3d. to the rectus inferior ; and, 4th. to the rectus superior. (Plate II. Figs. 4, 5, 6.) 



* Johannes Muller thinks that an eye actually exists ; but according to his description, it is " an 

 extremely small black point, without a cornea, of which the pigment forms the external layer, and 

 under which lies a colorless membrane ; nothing was determined on with certainty with regard to the 

 contents " ; no optic nerves were traced. Memoir on the Blind Fishes and some other Animals living in 

 the Mammoth Cave, Ky. By Theodore A. Telkampf, M. D. New York Journ. Med., July, 1845. 



