CRANIAL NERVES. 135 



note about the first, second, third, fourth, and sixth cranial 

 nerves. 



The ophthalmicus profundus (Fig. 79, opv) is a well marked 

 and distinct nerve in most Elasmobranchs, but in some (e.g. 

 Scyllium} it is less marked. It arises either with or just in 

 front of (Laemargus) the main root of the trigeminal. After 

 emerging from the skull wall with the trigeminal it presents a 

 ganglion the profundus ganglion, and passes forwards in the 

 orbit, dorsal to the external and internal recti, but ventral to 

 the superior rectus and superior oblique. It penetrates the 

 anterior wall of the orbit, and is distributed to the skin of the 

 front of the snout. In the orbit it gives off long ciliary nerves 

 to the eyeball, and one or more nerves which anastomose with one 

 or more filaments from the ventral branch of the oculomotor 

 (om), a small ganglionic swelling being found at the point of 

 junction. This small ganglion is the ciliary or oculo-motor 

 (lenticular, ophthalmic) ganglion ; it gives off some ciliary 

 nerves (short ciliary) to the eyeball. The filaments connecting 

 this ciliary ganglion with the ophthalmicus profundus (some- 

 times with the profundus ganglion) constitute the radix longa, 

 while those passing to the oculomotor represent the radix 

 brevis. The filaments passing from the ciliary ganglion to the 

 eyeball represent the short ciliary nerves. 



The ciliary ganglion of Plagiostomes seems to be variable in its size and 

 position ; it is sometimes absent, sometimes close to the profundus nerve, 

 sometimes approximated to the ventral branch of the third nerve. In the 

 embryo the oculomotor nerve passes directly from its origin to the pro- 

 fundus ganglion ; later it becomes detached, the connecting cord being 

 the radix brevis. The ciliary ganglion is probably a detached portion of 

 the profundus ganglion, in which case its relation to this ganglion is exactly 

 that of a sympathetic ganglion to the ganglion of a posterior root of a 

 spinal nerve. The ophthalmicus profundus itself probably corresponds 

 to the nasal nerve of the M ammalia. The third nerve must be regarded 

 as the ventral root of a nerve of which the ophthalmicus profundus 

 is the dorsal (see p. 73). 



The trigeminal nerve arises usually by a single root from the 

 sides of the medulla. It swells either inside or outside the 

 cranium into the gasserian ganglion and there gives off the 



the cranial nerve? of Scyllium," Q, J. M. S., 21, 1881, p. 469. C. Gegen- 

 baur, " Die Kopfnerven von Hexanchus," Jena. Zeitsch., 6, 1871, p. 497. 

 F. J. Cole, " On the cranial nerves of Chimaera, " Trans. Roy. Soc. of Edin- 

 burgh, 38, 1896, p. 631. Also literature cited on p. 75. 



