THE VISUAL APPARATUS 211 



lobe of the fish brain (Figs. 43, 44). Here visual impressions 

 are brought into physiological relations with those of the tactual 

 and auditory systems received by the lemnisci. The chief effer- 

 ent pathway from this center is by way of the underlying cere- 

 bral peduncle (Fig. 75). Here reflex connections are effected 

 directly with the nuclei of the III and IV cranial nerves for the 

 eye muscles, and through the fasciculus longitudinalis medialis 

 with the centers for all other cranial and spinal muscles. This 

 fasciculus is a strong bundle composed of both descending and 

 ascending fibers whose function is the general coordination of 

 reflex motor responses, and in particular those of the conjugate 

 movements of the two eyes (see p. 186). 



The accommodation of the eye for distance is effected by 

 changes in the curvature of the lens, and the adaptation for 

 differences in illumination is effected in part by changes in the 

 diameter of the pupil (this is in addition to the changes in the 

 retinal pigment referred to on p. 207 and to changes in the rods 

 and cones and other neurons of the retina which may be excited 

 by the centrifugal fibers from the brain to the retina referred to 

 on p. 209). The nerves controlling the movements of the lens 

 and the pupillary reactions belong to the visceral motor system. 

 They leave the central nervous system in part through the ocu- 

 lomotor nerve and in part (for dilation of the iris) from the lower 

 cervical region of the spinal cord. The latter fibers pass by way 

 of roots of spinal nerves into the superior cervical sympathetic 

 ganglion (p. 234 and Fig. 41, p. 107) and then forward to the 

 eyeball. We cannot here enter into further details of the mech- 

 anism of accommodation or of the diopteric apparatus and the 

 accessory parts of the eye; see the larger text-books of anatomy 

 and physiology. 



The thalamic connections of the optic tracts in the lowest 

 vertebrates are very insignificant, collaterals of these fibers 



Fig. 101. A diagram of the visual tract, illustrating the significance of 

 the partial decussation of nerve-fibers in the optic chiasma so as to ensure 

 the representation in the cerebral cortex of nervous impulses excited by ob- 

 jects on the opposite half of the body only. ///, Oculomotor nerve; L, 

 medial lemniscus; M, mammillary bodies; RN, red nucleus (nucleus ruber); 

 SN, black substance (substantia nigra) ; TG, optic tract to corpora quadri- 

 gemina (cf. Fig. 75). (From Starr's Nervous Diseases.) 



