228 



THE CRANIAL NERVES. 



the cerebral peduncle, to which it is closely attached ; after this, becoming more 

 nearly cylindrical, it is continued between the tuber cinereum and the anterior 

 perforated space, adhering- to the basal grey matter of the cerebrum, and thus 

 reaches the optic commissure. 



th-i 



fithqerv. fe. sup. braclv. 



\nf, \irach. 



ttcl.l. 



.b. 



tania. sentif-. 



ank.perf sp 



Fig. 145. ORIGIN AND RELATIONS 



OF THE OPTIC TRACT. (Gr. D. T. ) 



The mid-brain has been divided 

 immediately above the pons, and the 

 part is viewed from below. 



ofc. mrtte 



The optic commissure 



or chiasma is of an oblong 

 form, the longer diameter 

 (10 12 mm.) being directed 

 transversely. It rests upon 

 the olivary eminence of the 

 sphenoid bone, and the in- 

 ternal carotid artery, ascend- 

 ing to the brain, is close to it 

 on each side. A large number 

 of the fibres of the two nerves decussate in the commissure, but some are continued 

 from the tract to the nerve of the same side, while those fibres of the tract which 

 come from the internal geniculate body do not pass into either optic nerve, but 

 cross in the hinder part of the chiasma to the opposite tract, constituting the 

 inferior commissure of Gudden (see p. 118). 



The optic nerve proper runs from the commissure forwards and outwards through 

 the optic foramen, having the ophthalmic artery to its lower and outer side. As it 

 enters the foramen it receives a strong sheath from the dura mater, and a slender 

 one from the arachnoid, both of which are prolonged as far as the eyeball. Appear- 

 ing in the orbit between the origins of the recti muscles, it continues its oblique 

 course, inclining at the same time somewhat downwards, to the eyeball, which it 

 enters on its posterior aspect about 3 mm. internal to its centre. Here the nerve 

 perforates the sclerotic and choroid coats, and terminates by expanding in the 

 retina (see the anatomy of the eye). The nerve has a diameter of 3 4 mm., and 

 is from 30 to 40 mm. long. Its intraorbital part measures from 20 to 30 mm., and 

 has a slightly flexuous course, its length exceeding the distance in a straight line 

 from the optic foramen to the eyeball by about 5 mm., so as to allow of the move- 

 ments of the latter. It is surrounded by the ciliary vessels and nerves, and is 

 pierced about 10 mm. behind its termination by the central artery of the retina, 

 which, with a companion vein, runs in the axis of the nerve to the eyeball. 



The number of fibres in the optic nerve is estimated at somewhat less than half a million 

 (Salzer) ; they are mostly of small size. 



Varieties. In a few rare instances absence of the optic commissure has been met with, 

 each optic tract being continued directly into the optic nerve, and passing- to the eyeball, of 

 the same side. In some cases it would appear that the decussation of the fibres in the 

 commissure is complete (see p. 118). 



III. OCULOMOTOK NERVE. 



The third nerve, the common motor nerve of the eyeball (motor oculi), gives 

 branches to all the muscles of the orbit, with the exception of the superior oblique 

 and external rectus. It also supplies, through its connection with the ciliary 

 ganglion, the sphincter muscle of the iris and the ciliary muscle of the eyeball. 



