OPTIC TRACTS. 



117 



around the walls of the tubules. The lymphatics of the organ originate in cleft- 

 like spaces between the tubules and pass to a network in the capsule. In its 

 microscopic structure the anterior lobe of the pituitary body bears a resemblance 

 to the thyroid body, the vesicles of which are also originally a network of anasto- 

 mosing tubules, and in some animals remain throughout life in this condition. 

 Moreover, a colloid substance like that in the thyroid vesicles, is found sometimes 

 in the alveoli of the anterior lobe of the hypophysis. 



In the middle line of the base of the brain, in front of the optic commissure, is 

 the anterior portion of the great longitudinal fissure, which separates the two 

 hemispheres. At .a short distance in front of the chiasma, this fissure is crossed 

 transversely by the anterior recurved extremity of the corpus callosum. On gently 



int.qen-. b. sup. fcrach/. 



oulvinar 



\.rtf. larach. 



rfc.gen-.b. 



opt,, traiet 4- 



anb.perf so 



opt. nerve 



Fig. 86. ORIGIN AND RELATIONS OF 

 THE OPTIC TKACT (G. D. Thane). 



The parts are viewed from below, 

 the mid-brain having been divided 

 transversely immediately above the 

 pons, and the pons, cerebellum and 

 medulla oblongata removed. The 

 lower part of the figure is the more 

 anterior. 



turning back the optic com- 

 missure, a thin connecting 

 layer of grey substance, the 

 lamina cinerea. is seen oc- 

 cupying the space between 

 the corpus callosum and the 

 chiasma, and continuous above 

 the chiasma with the tuber 



cinereum. It is connected at the sides with the grey substance of the anterior 

 perforated space, and forms part of the anterior boundary of the third ventricle 

 (fig. 57, p. 70) : it is somewhat liable to be torn in removing the brain from the 

 skull ; and, in that case, an aperture is made into the fore part of the third ventricle. 



The optic tracts and optic commissure or chiasma. The optic tracts curve 

 round the crusta on each side to unite with one another immediately in front of the 

 tuber cinereum, where they form the X-shaped commissure which is known as the 

 chiasma. The optic tracts form the posterior limbs of the X ; the optic nerves, 

 passing into the optic foramina, the anterior limbs (fig. 86). 



Each tract arises posteriorly by a broad root, which is divided by a longitudinal 

 groove into two distinct parts, a lateral and a mesial. The lateral root is the larger. 

 It is connected with and emerges from the posterior and ventral part of the thalamus 

 (lateral geniculate body and pulvinar), and is partly continuous with the brachium 

 of the superior quadrigeminal body. The mesial root, when traced backwards, is 

 seen to curve round the crusta and then to lose itself beneath the mesial geniculate 

 body, in which it appears to end, although it may perhaps ultimately pass into the 

 inferior quadrigeminal body as its brachium. Although this root appears connected 

 with the internal geniculate body, it is doubtful if there is any functional connexion 

 between this body and the optic nerve, the part of the tract which enters the mesial 

 geniculate body being Gudden's commissure, which joins the mesial geniculate bodies 

 of the two sides (see below). Before reaching the chiasma the optic tract lies 

 postero-mesially to the anterior perforated space. 



Darkschewitsch describes the passage of some of the fibres of the optic tracb to the 

 ganglion habenulae and pineal peduncle, and ultimately by the posterior commissure to the 

 oculomotor nucleus (compare pp. 109 and 114). He regards these as fibres subserving the reflex 

 changes of the pupil. 



As for the course of the optic fibres in the tract and chiasma it may be stated 



