CAfiPENTEE: DEVELOPMENT OF THE OCUIiOMOTOR NERVE. 153 



ventro-lateral position. The situation of the nidulus in the cerebro- 

 spinal axis corresponds to that of the somatic motor column of ganglion 

 cells (Gaskell) of the spinal cord. The neuraxons of the cells pass 

 ventrad to emerge from the ventral face of the mesencephalon as the 

 third uerve, which runs ventrad and cephalad through the oculomotor 

 foramen, and then horizontally forward (Plate 1, Fig. 1, n. oc'mot.). 

 Passing ventrad of the posterior rectus muscle, it sends a small branch 

 (rm. mu. rt. d.) dorsad and cephalad to the posterior edge of the base 

 of the dorsal rectus muscle. Just distal to this branch, a large ventral 

 ramus (rm. v.} is given o£^ on the opposite or ventral side of the nerve 

 trunk. This ventral ramus passes beneath the ventral rectus muscle, 

 and runs cephalad along the floor of the orbit to terminate, as a brush of 

 fine fibres, on the ocular face of the ventral oblique muscle, about mid- 

 way of its length. A slender bundle of neuraxons (rm. m,u. rt. v.) arises 

 from the main trunk of the nerve in close connection with the ventral 

 ramus, and innervates the lower face of the ventral rectus muscle, near 

 the proximal end of the latter. From the ventral ramus, soon after it 

 passes the anterior border of the ventral rectus muscle, a small branch 

 (rm. mu. rt. a.) is given off to the adjacent edge of the anterior rectus 

 muscle. 



Immediately distal to the origin of the ventral ramus, the remainder 

 of the neuraxons of tlie third nerve enter the spindle-shaped ciliary 

 ganglion (gn. cil.), which measures approximately two mm. in length, 

 and has a gi'eatest diameter of a little less than one mm. No radix 

 brevis can be said to exist in the hen, since it is possible, in serial 

 sections, to trace the cells of the ciliiiry ganglion back as far as the 

 level of the ventral ramus (Plate 2, Fig. 3, 0). From the distal end 

 of the ciliary ganglion, a comparatively large ciliary nerve (Plate 1, Fig. 1. 

 n. cil. oc'niot.) is given off. This runs parallel with the optic nerve, and 

 penetrates the sclerotic coat of the eyeball. Ou its way, the ciliary 

 nerve gives rise to a variable number of branches of microscopical size 

 (Plate 1, Fig 2, rm. n. cil. oc'mot.), which accompany it to the eye. 

 The ciliary nerve receives, about one mm. distal to the region of 

 the cells of the ciliary ganglion, a slender communicating ramus 

 (Figs. 1 and 2, rm. comn.) from the ophthalmic branch of the fifth 

 nerve. 



Trochlear Nerve. The nidulus of the trochlear nerve is found in the 

 somatic motor column in the ventral part of the mesencephalon, pos- 

 terior to the nidulus of the oculomotor. The nerve (Fig. 1, n. trch.) 

 takes its superficial origin from the dorsal surface of the brain, between 



