THE NERVOUS SYSTEM 569 



EXI'LAXATIUX OF FIG. 377. 



Fig. ,577. — Diagram of the Optic (II) Nerve and some ol its Principal Connections. 

 A, Level of nerves II and III; B, level of nerve IV; C, level of nerves VI and VII; D, 

 spinal cord. Neurone groups are represented by one or several individual neurones. 



The rods and cones (receptors) and the bipolar cells ( = Neurone No. i) of the 

 retina are not indicated. 



Neurone No. 2.- — 2a, Axones of ganglion cells in temporal part of retina pass to pulvi- 

 nar of thalamus of same side; 2 b, axones of ganglion cells in temporal retina pass to 

 superior coUiculus of same side; 2 c, axones of ganglion cells in temporal retina pass 

 to external geniculate body of same side; 2 e, axones of ganglion cells in nasal side of 

 retina cross in optic chiasma and pass to external geniculate body of opposite side; 2 /, 

 axones of ganglion cells in nasal side of retina cross in optic chiasma and pass to 

 superior coUiculus of opposite side; 2 g, axones of ganglion cells in nasal side of retina 

 cross in optic chiasma and pass to pulvinar of thalamus of opposite side. Macular 

 fibres are partly crossed and partly uncrossed. 



Neurone No. 3. — 3a, Axones of cells in pulvinar to cortex of occipital lobe of cerebrum 

 (this connection is disputed); 3 b, axones of cells in external geniculate body to cortex of 

 occipital lobe of cerebrum; 3 a and ^b constitute the primary optic radiation; 3 c, 3 </ and 

 3 e, axones of cells in deep (fourth) layer of superior coUiculus (see p. 522) decussate 

 ventral to medial longitudinal fasciculus (dorsal tegmental decussation or decussa- 

 tion of Meynert) and form the tractus colliculo-bulbaris et spinalis or predorsal 

 bundle. {Tr. tectobulb. et spin.) to bulb (medulla) and anterior column of cord, inner- 

 vating by collaterals and terminals, directly or indirectly, chiefly the nuclei of III, IV, 

 VI, and VII cranial nerves and motor nuclei of spinal nerves. 3 / and 3 g (possibly 

 another neurone intercalated between these and optic terminals), axones of cells in inter- 

 stitial nucleus of Cajal {Nu. fasc. long, post.) form part of medial longitudinal fasciculus 

 and descend on same side to anterior column of cord next to anterior median fissure, 

 innervating nuclei of III, IV, and VI cranial nerves and motor nuclei of spinal nerves. 



Neurone No. 4. — Axones of cells in above-mentioned motor nuclei. Axones from 

 cells in median nucleus of nerve III {Nu. med. N. Ill) and possibly in Edinger-Westphal 

 nucleus, probably innervate the intrinsic muscle of eyeball (ciliary and pupillary reflex 

 path). 



Pallio-tectal fibres, by means of which the coUicular reflex centre is brought under 

 the control of the cerebral cortex, are not indicated. 



It is evident from the diagram that the cerebral pathway of the optic nerve is via the 

 external geniculate body (and pulvinar of thalamus?), and the reflex pathway is via the 

 superior coUiculus. 



