IX.] THE NERVOUS SYSTEM. 387 



20. The two cords which have been spoken of as the 

 optic tracts arise, one on each side, from the optic thalami, 

 and run forwards obliquely across the under surface of the 

 brain, to join together immediately in front of the infundi- 

 bulum, whence they again diverge as the OPTIC NERVES. At 

 the point of junction the two tracts partially exchange 

 their fibres as follows : The outer fibres of each tract con- 

 tinue on to the optic nerve and eye of the same side. The 

 inner fibres cross over to the optic nerve and eye of the 

 opposite side. This decussation of fibres is called the optic 

 commissure or chiasma. Beyond the chiasma each optic 

 nerve passes through the corresponding optic foramen, in- 

 vested in a sheath of the dura mater, and surrounded by the 

 recti muscles. After piercing the outer coats of the eyeball, 

 it expands within that globe into what is called the retina. 



Posteriorly the optic nerves become connected with the 

 corpora quadrigemina, or optic lobes, but this connexion 

 does not exist at first. 



The structure of the EYE has been so amply described in 

 the Ninth Lesson of the " Elementary Physiology" that here 

 it is only necessary to notice its development and its more 

 important relations with the conditions presented by the eye 

 in Vertebrates generally. 



FIG. 340. DIAGRAM REPRESENTING THE DEVELOPMENT OF THE EYE IN 

 SUCCESSIVE STAGES, FROM FlG. I TO FlG. 3. 



;/, outgrowth from fore-brain becoming cup-shaped, applied to the back of the 

 vitreous humour and forming the reti)ia; i, the involution of the integument 

 becoming posteriorly the lens (/) and anteriorly the aqueous humour ; v, the 

 lateral ingrowth which intrudes between the l:ns and nervous outgrowth 

 and becomes the vitreous humour ; op, the optic nerve ~, cc, the incipient 

 skin-folds which become the eyelids ; they are connected by the conjunctiva. 



The eye is formed by the junction of an outgrowth from 

 the brain with an ingrowth from the skin. 



The outgrowth is from the fore-brainj and is in the form of 

 a hollow process containing a prolongation of the primitive 

 ventricular cavity. 



This process becomes cup-shaped, and embraces the back 

 of the part formed by the ingrowth of the skin, 

 c C 2 



