26 



DR. A. iMILNfiS MARSHALL. 



continues the course of the main trunk, crosses the ophthalmic 

 nerve (fig. 26, V) nearly at right angles, lying to its inner side 

 (fig. 17, III), and then curves slightly forwards, passing in front 

 of the rectus externus (fig. 26, re), behind the rectus internus 

 {ri), and behind and below the optic nerve (II). This is the 

 precise course of the third nerve and its branches in the adult. 



According to the description given above, the third nerve 

 must be a true cranial segmental nerve. It resembles the other 

 cranial nerves in arising from the neural ridge; in shifting down- 

 wards towards the mid ventral line at an early period ; in being 

 ganglionic at its origin; in having a distal ganglionic swelling, 

 from which two branches are given off, of M'hich the posterior is 

 the larger ; and, lastly, in its primitive straightness and general 

 direction. It will be seen by comparing figs. 21, 22, 2-'3, and 26 

 that the posterior cranial nerves run parallel to one another, but 

 that in front of the ear, owing to the distortion produced by 

 cranial flexure, their distal extremities converge. The direction 

 of the trunk of the third nerve will be seen to be precisely that 

 which a true segmental nerve springing from the mid brain would 

 necessarily have. 



With the exception of a very brief notice in my former paper^ 

 I am not acquainted with any previous account of the develop- 

 ment of the third nerve. 



Foster and Balfour state •} — " It is worthy of note that of the 

 third, fourth, and sixth nerves no such early rudiments appear; 

 and there are reasons for thinking that these are in reality inter- 

 cranial branches, the third and fourth of the fifth, and the sixth 

 of the seventh nerves.'^ 



Huxley^ is " greatly disposed to think" that the motor nerves 

 of the eye '' are really the motor portions of the nerves of the 

 orbito-nasal cleft; the third and fourth appertaining to the inner 

 divisions of the ophthalmic, the sixth to its outer division.''^ 



Allen Thomson holds that'^ " the third, fourth, and sixth pairs 

 of nerves are of subordinate importance, and may be considered 

 as related, the two first to the fifth pair, and the last to the facial 

 nerve. Their peripheral ])arts are developed in connection with 

 the muscles of the eyeball, but the mode of the formation of 

 their roots in connection with the nervous centres has not been 

 ascertained." 



None of these accounts are based on direct embryological 

 evidence. It is obvious from my descriptions and figures that 

 the third nerve has nothing whatever to do with the fifth, and 



' Loc. cit., p. 510. 



- Op. cit., p. 13S. 



^ ' Anatomy of Vertebrated Animals,' p. 73, note. 



* ' Quain's Anatomy,' 8th edition, vol. ii, p. 761. 



