C23 THE CHAXIAL NERVES. 



in the form of a flattened band, which is attached by its anterior surface 

 to the peduncle ; after this, becoming more nearly cylindrical, it adheres 

 to the tuber cinereum, from which, and from the lamina cinerea, it is 

 said to receive an accession of fibres, and thus reaches the optic com- 

 missure. 



Deep origin. — The fibres may be traced into the substance of the 

 optic thalamus and the corpora geniculata, and to the anterior of the 

 corpora quadrigemina. The fibres which come from the anterior corpus 

 quadrigeminum arise from the cells of that body. Those connected 

 with the internal and external corpus geniculatum are in part connected 

 with the cells of those bodies, in part pass through them to reach the 

 optic thalamus. The fibres which pass into the thalamus beneath the 

 inner corpus geniculatum, between it and the crus (and have been 

 described as a middle root of the tract), arise, according to Meynert, 

 from the cells of the lower stratum of the thalamus. The fibres which 

 arise in the base are derived from the lamina cinerea and also from a 

 " basal optic ganglion," which lies on the outer side of the tuber cine- 

 reum, and sends fibres to the optic nerve of the same side. 



Course and distribution. — In the rommissiire, or chiasma, the nerve 

 fibres of the two sides undergo a partial decussation. The outer fibres 

 of each tract continue onwards to the eye of the same side : the 

 inner fibres cross over to the opposite side ; and fibres have been de- 

 scribed as running fi'om one optic tract to the other along the posterior 

 part of the commissure, while others pass between the two optic 

 nerves in its anterior part (Mayo), The outer fibres of each tract 

 which do not decussate are much less numerous than those which cross 

 to the opposite side. The decussating fibres are arranged in alternate 

 layers. 



Tlie view before stated is that most commonly given as to the stiiictiu'e of the 

 commissure ; but it is right to state that its accuracy has been called in question 

 by several observers who hold, both on anatomical and physiological grounds, 

 that all the fibres from one tract cross into the optic nerve of the opposite side. 

 The point appears to be still undecided. In many fishes the optic nerves do not 

 unite in a commissure but merely cross, each to the side opposite to that of its 

 origin. (See "Waller, Proc. Eoy. Soc, vol, viii, Biesiadecki, Yv'ien. Sitzungsb. ISGl, 

 p, 86, and Michel, Archiv fur Ophthalm, 1873, p. 59.), 



In ft'ont of the commissure, the nerve diverges from its fellow and 

 acquires greater firmness. It enters the foramen opticum, by which it 

 reaches the orbit. 



Within the orbit it forms a cylindrical trunk, thick and strong, with 

 a uniform surface. On dissection it is seen to consist of a number of 

 separate bundles of nerve fibres, imbedded in tough fibrous tissue pro- 

 longed from the dura mater, and perforated in the centre by the small 

 arteria centralis retinae, which passes into it soon after it enters the 

 orbit. It is surrounded by the recti muscles, and, entering the eyeball 

 posteriorly a little to the 'inside of its middle, it pierces the sclerotic 

 and choroid coats, and expands in the retina. — (See the Anatomy of 

 the Eye.) 



THIRD PAIR OF NERVES, 



This nerve, the common motor nerve of the eyeball (motorius oculi), 

 gives branches to all the muscles of the orbit, wfth the exception of the 

 superior oblique and external rectus. 



