220 DR J. 0. WAKELIN BARE ATT. 



These become invested by the arachnoid membrane when the 

 nerve has reached a length of ten millimetres, and by the dura 

 mater about five millimetres further on. Until it approaches 

 its termination in the external rectus muscle of the eyeball 

 (fig. 5, sections 55 to 65) no large branches are given off from 

 the fibres of the main trunk proper, though a few exceedingly 

 small twigs composed of small medullated fibres, with a few 

 medium-sized medullated fibres among them, join the main 

 trunk at twenty-five millimetres from its origin, leaving it again 

 about ten millimetres further on. In text-books on anatomy 

 the 6th nerve is described as communicating with the carotid 

 and cavernous plexuses, and with the ophthalmic branch of the 

 5th. Probably these small twigs are the branches referred to. 



Towards its termination the 6th nerve divides into several 

 sets of branches (fig. 5, sect. 60). 



At the superficial origin of the 6th nerve a small amount of 

 amorphous granular material may be recognised; occasionally 

 careful examination fails to reveal its presence. This material, 

 which, as has already been stated, is regarded as vestigial in its 

 origin, can only be demonstrated when extreme care is taken in 

 the manipulation to which the nerve is subjected. 



The medullated fibres of this nerve range from IIjul to IT^a 

 for the largest fibres, and ojul to 6^ for the smallest. 



DESCRIPTION OF PLATES XXVII. -XXXI. 



Fig. 1. A series of transverse sections of the right 3rd cranial 

 nerve, taken at intervals of five inillimetres, from its superficial origin 

 to the termination of its muscular branches. On the left is seen a 

 magnified sketch of the nerve, accompanied by a millimetre scale, 

 also magnified. The niunbers given to the transverse sections corre- 

 spond to the various levels on the millimetre scale. 



The second transverse section (5) exhibits a collection of amor 

 ])hous material, vest, str., the limits of wliich are also shown in the 

 diagram of the nerve, and which is present in less amount than in 

 the succeeding figure. The jDosition of the fibres of the inferior 

 division, inf. div., passing to the ciliary ganglion is indicated by the 

 shaded area in sections (5) to (45). Tlie pia-araclnioid is seen covering 

 the main trunk at about twenty millimetres from its origin, and the 

 dura mater, d.m., becomes closely applied to it five millimetres 



