296 STRUCTURE OF THE VERTEBRATES 



form a tract connecting this ganglionic mass with the higher 

 centers. In the lower forms the optic axones pass through the 

 diencephalon to the midbrain, but in the higher groups some 

 of the fibers make connection in the thalamic nuclei of the 

 diencephalon. 



There is a complete decussation of the optic fibers in lower 

 vertebrates, forming a complete optic chiasma, but in the mam- 

 mals half the fibers cross to the opposite side while half do not 

 cross. These relationships will be considered more fully in the 

 chapter on organs of special sense. 



Nerves III, IV, VI; Oculomotor, Trochlear, Abducens. 

 These are the motor nerves controlling the muscles of the eye. 

 It is now known that each carries a small bundle of sensory 

 fibers, transmitting proprioceptive muscle sensations to the brain. 



The eye muscles arise from three small metameric muscle 

 bundles, the most anterior (innervated by Nerve III) splitting 

 to form four of the muscles. The second and third myotomes 

 form one muscle each, with the result that each has a cranial 

 nerve, while the oculomotor supplies the remaining four. The 

 six eye muscles are arranged in two groups; (1) the rectus 

 muscles, four in number, having their insertions equally spaced 

 around the equator of the eye, and arising together on the inner 

 surface of the eye socket; and (2) the two oblique muscles, one 

 dorsal and one ventral, which pass from the eye to the anterior 

 (median in the human) surface of the socket. The rectus muscles 

 are the dorsal and ventral, and the antenor and posterior. In 

 the human these are called the superior and inferior, and in- 

 ternal and external recti. 



Nerve III, oculomotor, innervates the muscles arising from 

 the first myotome: the dorsal and ventral recti; the anterior 

 (internal) rectus; and the ventral oblique. 



Nerve IV, trochlear, goes to the dorsal (superior) oblique. As 

 these muscles, when they contract synchronously, pull the eyes 

 medially and upward, the alternative name patheticus is often 

 given them. The trochlear leaves the skull through the most 

 dorsal of the nerve foramina. 



Nerve VI, abducens, innervates the posterior (external) rectus. 

 The fourth and sixth nerves are the smallest of the cranial 

 nerves and are frequently lost in dissection. 



