DEVELOPMENT OF CRANTAL NERVES IN THE CHICK. 29 



is rather more prominent than it is either just in front of or just 

 behind this point. This prominence is the earliest rudiment of 

 the fifth nerve. 



In my previous account I have described and figured it at 

 forty-three hours as " an outgrowth of very slight vertical thick- 

 ness from the summit of the anterior dilatation of the hind 

 brain/''^ This slender outgrowth reaches about half way down 

 the side of the hind brain, with which it is still connected at the 

 extreme dorsal summit only. 



A little later on the attachment begins to shift, and the nerves 

 travel down the sides of the hind brain in a manner that will be 

 fully described under the seventh nerve. At the same time the 

 roof of the hind brain thins out very considerably (fig. 15). The 

 nerve increases rapidly in size, and about the fiftieth hour divides 

 distally into two branches, the ophthalmic and the inferior max- 

 illary. 



About the end of the fourth day the fifth nerve has the appear- 

 ance shown in fig. 21, which represents a longitudinal section 

 through the head of a ninety-six-hours^ chick, taken from the 

 same specimen as fig. 22, but a little further from the median 

 hne. The proximal part of the nerve is dilated into a large oval 

 ganglionic swelling. This springs from the floor of the hind-brain 

 {vide also fig. 15), but the connection between the two is not so 

 evident as in tlie earlier stages, owing to the white matter which 

 now invests the central ganglionic matter of the brain. The 

 nerve divides almost immediately into two branches, each of 

 which is swollen and ganglionic at its base. Of these the anterior 

 (fig. 21, V 1) passes at first forwards, then bends slightly down- 

 wards, crossing the third nerve almost at right angles, and lying 

 in a more superficial plane than this latter (fig. 17, V and III). 

 It then passes under the rectus superior (fig. 26, r s) but dorsad 

 of the other eye-muscles and of the optic nerve (fig. 26, II) . I 

 have traced it forward at this stage into close proximity with the 

 olfactory nerve and organ. This description leaves no room for 

 doubt that this branch is the rudiment of the ophthalmic branch 

 of the fifth in the adult. 



The posterior branch of the fifth passes downwards and back- 

 wards to the anterior border of the mandibular arch, along which 

 it runs (figs. 21, V 3, and 23 V b). Close to its distal extremity 

 it gives ott' a small branch forwards, which runs down the hinder 

 edge of the maxillary process (figs. 23, V 2 and 21). This latter 

 branch is the rudiment of the superior maxillary nerve of the 

 adult, while the main branch develops into the inferior maxil- 

 lary division of the fifth. 



The fifth nerve therefore arises as a single root, and is to be 

 I Loc. cit., p. 509, and plate xx, fig, 4. 



