On the Cranial Nerves of Elasmobranch Fishes. 527 



several small branches. On escaping from, the cranial wall the 

 ophthalmicus prof undus separates from the trigeminal and presents a 

 slight swelling, the meso-cephalic ganglion (Beard), or ciliary gan- 

 glion (Gegenbaur and others). This ganglion lies dorsal to, but only 

 very slightly in front of, the large Gasseriaii ganglion of the trige- 

 minal (3, fig. 1). From the ganglion the trunk extends forwards over 

 the external rectus muscle to pass under the rectus superior towards 

 the eyeball, from which it bends inwards between the superior oblique 

 and internal rectus muscles, to reach the snout by penetrating the 

 pre-orbital process of the cranium. 



The more important branches of the ophthalmicus profundus are 

 (1), a small branch which passes outwards above the superior rectus 

 muscle; (2), two or three ciliary branches (ci., fig. 1), which run 

 forwards under cover of the rectus superior to enter the eyeball to 

 these ciliary branches delicate filaments pass from the deep branch 

 of the oculo-motor ; (3), small branches which pass outwards in 

 front of the eyeball ; (4), branches to the skin, and subcutaneous 

 tissue of the snout and to the rostral cartilage. I have been unable to 

 trace any branches from the ophthalmicus profundus to either the 

 mucous canals or the ampullae of the sensory tubes : long and slender 

 branches, however, seem to be distributed to the tubes which extend 

 from the ampullae to open through the skin. 



2. The Oculo-motor Nerve. Although this nerve does not necessarily 

 stand in the same relation to the ophthalmicus profundus as does the 

 ventral root to the dorsal root of a spinal nerve, it will be con- 

 venient to deal with it before considering the trigeminal. The oculo- 

 motor has been ranked very differently by different observers. 

 Marshall and Spencer considered it of segmental value, and Gaskell 

 has recently stated that it retains in its root vestiges of a ganglion. 

 Van Wijhe looks upon the oculo-motor as forming the ventral (motor) 

 root of the ophthalmicus profundus, whilst Gegenbaur neither admits 

 that it has the rank of a segmental nerve nor feels satisfied that it 

 represents the ventral root of the ophthalmicus profundus. 



The oculo-motor (2, fig. 1) in Lcemargus arises by a number of 

 delicate rootlets (5 7) from the under-surface of the mid-brain, on 

 a level with the posterior end of the optic lobes and in line with the 

 abducens and spinal nerves. Passing outwards it escapes from the 

 cranial cavity by a special foramen, and bends round the orbital pro- 

 cess of the palato-pterygoid arch to reach the rectus superior, where 

 it divides into a superficial and a deep branch. The superficial 

 supplies the superior and internal recti muscles, the deep branch 

 passing under the rectus superior sends filaments to the inferior 

 rectus andanferior oblique muscles, and, as it runs over the pedicle, 

 it sends one or two exceedingly delicate twigs to the ciliary branches 

 of the ophthalmicus profundus. I have been unable to find any 



