THE NERVOUS SYSTEM 



447 



about the developing eyeball and furnish it with muscles. 

 From the first arise three of the straight muscles and one 

 oblique, from the second the other oblique, and from the 

 third the remaining straight muscle. These three myotomes 

 are innerved by the three nerves under consideration, and in 

 their natural order of succession, as follows : 



These relationships are constant throughout all vertebrates, 

 corroborating the idea that we have here the enumeration of 

 some very primitive morphology. In certain Orders, in re- 

 sponse to special needs, other special muscles appear in con- 

 nection with the eyeball, but these are seen to be differentia- 

 tions of certain of the above, and retain the same innervation ; 

 thus the retractor bulbi * arises from the external rectus, and, 

 like it, is innerved by the sixth nerve. 



The relation of these three nerves to adjacent sensory ele- 

 ments and their right to be considered ventral roots are matters 

 concerning which, although much has been done, few definite 

 conclusions may be drawn as yet. The Motor oculi, although 

 its fibers are purely motor, yet becomes connected with the 

 small ciliary ganglion, through which its fibers innerve the 

 ciliary muscles and the iris. This ganglion may have the 

 morphological value of the one belonging to a sensory root 

 now lost, a conclusion which would make this nerve an entire 

 spinal element with a reduction of the sensory root. Other 

 views associate with it as its sensory element a portion of the 

 Trigeminus. The Trochlearis, although essentially a motor 

 nerve, possesses in fishes and amphibians a few sensory fibers, 



* This muscle is rudimentary or wanting in the Anthropoidea. 



