IT. OF RANA Pn>IEXS. 25 



by him as the representative of the united frontals of serpents, and by others as 

 the ethmoid. The nerve trunks, after passing tliese openings, divide into two 

 branches, each of which breaks up into a brush of fihiments, and are distributed 

 to the upper and under surfaces of the olftictory pouches, but principally to the 

 former. 



The minute structure of this nerve, as in other animals, is quite different from 

 that of the other sense nerves. Its fibres have not the appearance of being tubu- 

 lar, are not varicose, and are flattened and closely matted together, resembling the 

 fibres of the nerves of organic life. 



II. Optic Nerves. (Plate I. Figs. 1 and 3, ii.) — Tbese, when stripped of their 

 thick sheath exteriorly to the cranium, or when examined within the cranial cavity, 

 are a little less voluminous than the preceding pair, though the organ of sense with 

 which they are connected is larger and more complex. Behind the decussation, 

 each nerve, as it advances towai'ds its ganglionic centre, divides into two distinct 

 fasciculi of fibres ; the anterior fasciculus is directed upwards, its fibres being dis- 

 tributed to the anterior portion of the optic lobe, and in part to the adjoining 

 portion of the optic thalamus, while the posterior, passing beneath the lobe, enters 

 it upon its posterior face. If the brain have been previously macerated in strong 

 alcohol, this demonstration is rendered still more clear by cutting through the 

 optic nerves at the decussation, and on tracing up each nerve towards its origin, 

 it will be found that the only direction in which the fibres may be separated 

 without rupture is towards the ganglionic masses above mentioned. The connec- 

 tion of these nerves both with the optic lobes and optic thalami seems unequivo- 

 cal, and is a fact of interest bearing on the connection of both these organs 

 with the sense of vision, and this view is supported by the evidence derived from 

 pathology and experiment. Many observers have shown, by dissection, that blind- 

 ness of long standing is followed by atrophy of the optic lobes (agreeably to the 

 well-known law of atrophy following disuse), and that extensive lesions of the 

 lobes are attended by either impairment or loss of vision. In recent dissections 

 of Frogs, similar results have been observed. One in which the ri(jht eye had 

 been destroyed had atrophy of the left lobe, which was reduced by nearly one 

 third of the dimensions of that of the opposite side. The condition of the nerve 

 w^as not examined. In a second instance, there was blindness from destruction 

 of the left eye, in which there was atrophy of the left nerve before the decussa- 

 tion, of the right nerve behind the decussation, and of the right optic lobe.* In 

 neither of these cases was there any marked alteration in the proportions of the 

 optic thalami. 



The facts furnished by comparative anatomy, however, tend to sho\v that vision 

 is not the sole function of the o|)tic lobes. There are Avell-known instances of 

 animals, whose lives are passed either in caverns or localities from -which the light 

 is excluded ; one of the most remarkable of these animals is the Blind Fish {Am- 

 hlyopsis spelicns, Dekay) from the Mammoth Cave of Kentucky. In these no 



* American Journal of Medical Sciences, edited by Isaac Hayes, M. D., for October, 1852. 



