SENSE ORGANS OF AMIURUS. 365 



tapering may be observed till the upturned portion of the notochord 

 is reached where the cord suddenly loses its cylindrical form and 

 dilates into a pyramidal swelling. This is, no doubt, owing to the 

 greater size of the ventral as compared with the dorsal columns in 

 this region where two pairs of powerful ventral roots are given off 

 behind the last dorsal roots. 



II. PERIPHERAL NERVOUS SYSTEM. 



The intracranial course of the" cranial nerves has been described 

 at page 355. It remains to follow them to their terminations out- 

 side the skull. Nothing further need be said with regard to the 

 olfactorius and opticus. 



Owing to the small size of the eyes, the dissection of the motor 

 nerves of the eyeball is a matter of some difficulty, which may 

 account for the fact that I have not been able to find any trace of 

 an oculomotor or ciliary ganglion, although I have examined the 

 whole of the third and ciliary nerves within the orbit for that pur- 

 pose. 



In the course of passing through the skull the third nerve leaves 

 the infero-medial strand of the trigeminus, to enter a special canal in 

 its course to the orbit which it reaches between the R. ophthalmicus 

 profundus and the R. ciliaris. It divides immediately into the 

 superior and inferior divisions, the former of which runs at once to 

 the rectus superior while the latter crosses obliquely over the rectus 

 inferior and medius, supplying them, to end by the long branch in the 

 obliquus inferior. 



In dissecting from the floor of the mouth, (Figs. 1, 2, 3, PI. IV.) the 

 rectus externus has to be l-efiected to expose the inferior division of the 

 third taking this course. 



The trochlearis accompanies the R. ophthalmicus profundus into the 

 orbit and leaves it there about the middle of its course to pass 

 obliquely forwards and outwards to end in the obliquus superior. Tn 

 its course there, certain fibres from the ophthalmicus may be asso- 

 ciated with it (Figs. 4, PI. IV.) which end in the fat near the superior 

 oblique muscle. 



The abducens also leaves the ventral edge of the infero-medial 

 strand, and crosses to accompany the third into the orbit ; this it 

 does apparently in the same sheath, although it may be readily 



