COMPARATIVE ANATOMY OF THE NERVOUS SYSTEM 311 



turns ventral to it, and then runs forward in the floor of the orbit to the inferior 

 oblique. Note also that the branch which supplies the inferior rectus gives off 

 a branch, one of the ciliary nerves, which passes into the eyeball in company 

 with an artery. Along this nerve small brown masses can be noticed; they are 

 the ciliary ganglia belonging to the sympathetic system. The deep ophthal- 

 mic nerve also supplies ciliary branches into the eyeball. The function of the 

 ciliary nerves is to control the smooth muscles of the iris, regulating the size of 

 the pupil; they are visceral motor nerves, making a relay in sympathetic 

 ganglia. 



It will be seen from the foregoing account that the oculomotor nerve supplies four eye 

 muscles, namely, those which develop from the so-called first head myotome. It is a somatic 

 motor nerve originating from the somatic motor column in the floor of the midbrain. It 

 also, however, carries with it sympathetic fibers of visceral motor function. 



e) The sixth or abducens nerve: The abducens originates from the somatic 

 motor column on the ventral surface of the anterior end of the medulla. Its 

 origin will be seen later. It penetrates the orbit at the point of origin of the 

 external rectus muscle and passes along the ventral surface of this muscle to 

 which its fibers are distributed. It will be seen as a white ridge on the ventral 

 surface of the muscle. 



It will now be seen that the third, fourth, and sixth cranial nerves are somatic motor 

 nerves to the muscles of the eyeball. The reason for the allotment of three cranial nerves 

 to six small muscles has already been indicated. To every segment of the vertebrate body 

 there should be theoretically a pair of motor and sensory nerves. Since as already ex- 

 plained the six eye muscles come from three head myotomes, it is readily understood that 

 a somatic motor nerve should be present to supply the derivatives of each of these myo- 

 tomes. The sensory nerves of these segments will be indicated later. 



/) The fifth or trigeminus nerve: The trigeminus is a very large nerve with 

 four main branches in elasmobranchs (three in land vertebrates) . The trigemi- 

 nus is attached to the medulla near the anterior end of the somatic sensory 

 column, just behind the auricles of the medulla. Its roots here are inextricably 

 mingled with the roots of the seventh and eighth nerves, the three together 

 forming a conspicuous mass at the place stated. The trigeminus passes through 

 the adjacent wall of the orbit and should be followed into the orbit by carefully 

 picking away the cartilage around it. As soon as it penetrates the orbit the 

 trigeminus divides into four branches. The first of these, the superficial oph- 

 thalmic branch, is part of the superficial ophthalmic trunk which has already 

 been mentioned several times. This large trunk passes forward in the dorsal part 

 of the cartilage of the medial wall of the orbit. Trace it forward. It passes out 

 of the orbit through the ophthalmic foramen in the chondrocranium, and above 

 the olfactory bulb. Only a small part of this trunk is trigeminal; this is sensory 

 to the skin dorsal to the orbit. The second branch of the trigeminus is the deep 



