SENSE 0? SIGHT. 187 



are supplied by the 3rd nerve (Fig. 152). The muscle plate of 

 the 2nd head segment produces the superior oblique. In the 

 course of evolution the superior oblique of the right side has 

 shifted to the left and the left to the right (Gaskell), hence the 

 decussation of the 4th nerves (the motor nerves of this segment) 

 on the anterior part of the roof of the hind-brain — the valve of 

 Vieussens. The muscle plate of the third cephalic segment gives 

 rise to the external rectus ; the 6th nerve is the motor nerve of 

 the segment. 



The sensory nerves of these three segments are fused together 

 to form the ophthalmic division of the 5th nerve. The ciliary 

 ganglion is the splanchnic (sympathetic) ganglion of the first 

 segment (see Tig. 180, p. 220). The nerves for the choanoid 

 (Miiller's) muscle, the non-striated muscle of the upper eyelid, 

 and the dilator fibres of the iris, issue from the upper three dorsal 

 segments of the spinal cord, and reach the eye by the cervical 

 sympathetic chain and cavernous plexus. The nerve fibres for 

 the orbicularis palpebrarum pass out with the facial, but they 

 arise from cells in the first segment of the neural canal (oculo- 

 motor nucleus). The ophthalmic division of the fifth nerve 

 represents the sensory part of the 1st segment ; hence the reflec- 

 tion of pain along this nerve (frontal headache) in disorders of 

 accommodation, the muscle of accommodation being the ciliary, 

 and its nerve, the oculo-motor, both also derivatives of the first 

 segment. 



Development of the Nerve Centres concerned with Sight. 

 — Five parts of the brain are concerned with vision. They 

 are : 



(1) The optic tracts. 



(2) The basal centres surrounding the termination of the aque- 

 duct of Sylvius in the 3rd ventricle. 



(3) The optic radiations. 



(4) The occipital lobes — in part at least. 



(5) The angular gyrus. 



(1) The optic tracts are made up of fibres developed from 

 the ganglionic cells of the retina and also in part of efferent 

 fibres developed from cells of the basal ganglia in which the 



