VI.] THE SPINAL NERVES. 12.9 



The history of the third nerve is still imperfectly known. 

 There is developed early on the second day from the nem-al 

 crest, on the roof of the mid-brain, an outgrowth on each side, 

 very similar to the rudiment of the posterior nerves. This out- 

 growth is believed by Marshall to be the third nerve, but it must 

 be borne in mind that there is no direct evidence on the point, 

 the fate of the outgrowth in question not having been satisfac- 

 torily followed. 



At a very considerably later period a nerve may be found 

 sp-inging from the floor of the mid-brain, which is undoubtedly 

 the third nerve. If identical with the outgrowth just spoken of, 

 it must have shifted its attachment from the roof to the floor of 

 the brain. 



The nerve when it springs from the floor of the brain runs 

 directly backwards till it terminates in the ciliary ganglion, 

 from which two branches to the eye-muscles are given ofi". 



[A. Marshall. " The development of the cranial nerves in the 

 Chick." Quart. Journal of Microscop. Science, Vol. xviii.] 



In the case of the spinal nerves the posterior roots 

 originate as outgrowths of a series of median processes 

 of cells, which make their appearance on the dorsal side 

 of the spinal cord. The outgrowths, symmetrically 

 placed on each side, soon take a pyriform aspect, and 

 apply themselves to the walls of the spinal cord. They 

 are represented as they appear in birds in Fig. 43, sp. g., 

 and as they appear in a lower vertebrate form in Fig. 44, 



The original attachment of the nerve -rudiment to 

 the medullary wall is not permanent. It becomes, in 

 fact, very soon either extremely delicate or absolutely 

 interrupted. 



The nerve-rudiment now becomes divided into three 

 parts, (1) a proximal rounded portion; (2) an enlarged 

 middle portion, forming the rudiment of a ganglion ; (3) 

 a distal portion, forming the commencement of the nerve. 

 The proximal portion may very soon be observed to be 

 F. & B. 9 



