38 



INNERVATION. 



[CHAP. xvn. 



outside, and with the tuber cinereum on the inside. Whether any of 

 its fibres spring from the tuber cinereum is matter of great uncer- 

 tainty. Further back the optic tract adheres by its outer margin 

 to the crus cerebri, and is concealed by the middle lobe of the brain. 

 In this course it expands considerably, and at the posterior edge of 

 the crus cerebri it forms intimate connexions with certain gangliform 

 masses of the brain. First, we observe its connexion with the ex- 

 ternal geniculate body, a small prominent tubercle of darker colour 

 than the surrounding parts, and situate at the posterior margin of 

 the crus; it seems to involve the outermost fibres of the tract, as a 

 ganglion, and from it a band of fibres is continued back to the pos- 

 terior of the quadrigeminal bodies. Beneath the posterior extrem- 

 ity of the optic thalamus the innermost fibres of the tract form a 

 connexion with another similar body, the internal geniculate body, 

 from which fibres are continued backwards to the anterior of the 

 corpora quadrigemina. The tracts, thus, appear to divide, each into 

 two bands : of which the outer one, after passing through the ex- 

 ternal geniculate body, reaches the testes; and the inner one, simi- 

 larly related to the internal geniculate body, reaches the nates. 



The optic tracts are connected with the optic thalami chiefly 

 through the geniculate bodies. Each tract adheres to the outer side 

 of its corresponding thalamus for some distance, but whether any 

 fibres sink into it is not determined. In the horse, dog, sheep, 

 and monkey, this arrangement is very conspicuous, as the greater 

 portion of the fibres of the tract expands over the internal genicu- 

 late body, which is incorporated with the posterior extremity of 

 the thalamus. The diameter of the tubules of the optic tracts we 



have found to vary from 



Fig. 126. 



f an inch. 



The chiasma results from 

 the junction of the optic 

 tracts in front of, and inferior 

 to, the tuber cinereum. The 

 fibres which form the inner 

 margin of each tract, p, are 

 continued across from one 

 side of the brain to the other, 



Course of fibres in the chiasma, as exhibited by tearing off ^ J i w 1 



the superficial bundles from a specimen hardened in spirit. and I rm n Connexion With 

 a. Anterior fibres, commissural between the two retinae. o.v, r^tio -nov^roc. on A ^i' f 

 p. Posterior fibres, commissural between the thalami. a, p. tne OptlC nerves, and CXlSt 

 Diagram of the prec where ^^ ^^ ^ ^ 



exist, as in the mole. These fibres may be regarded as commissural 

 between the thalami of opposite sides. The remaining fibres of the 

 tracts go to form the optic nerves; the central ones pass into the 



