35 



m ' -" 



later, weighs strongly in favor of considering it an independent cranial 

 nerve. 



Description of the new Nerve in adult Selachians. 

 A. Forms in whicli the nerve has a dorsal or semi-dorsal 

 attachment to the brain-wall. 

 There is considerable variation in adult selachians as to the point 

 at which these nerves are attached to the brain -wall. The actual 

 termination of their fibres is another question, but as regards surface 

 connections with the brain, we may distinguish two groups, viz. those 

 forms in which the nerves have a dorsal i) or semi-dorsal attachment 

 and those having a ventral attachment. 



In some cases, as in Trygon, Raja, JViyliobatis etc. the superficial 

 connection is truly dorsal, being on the anterior summit of the pros- 

 encephalon, but, in others like Squalus, Heptanchus etc., the attach- 

 ment is within the median furrow of the prosencephalon, neither dorsal 

 nor ventral, but often nearer the dorsal than the ventral surface. There 

 is greater uniformity of position in the second group where the nerve 

 is always connected with the ventral surface. 



Remarks on finding and exposing the Nerve. — This nerve 

 will usually be removed in the process of clearing the brain of membranes, 

 connective tissue and blood vessels. The safest way of preserving it 

 intact is to stain the entire brain after it is well exposed, but betbre 

 completely removing the cartilaginous envelop. The dissection of the 

 stained specimen should be carried on under fluid with the aid of a 

 lens. The staining diflerentiates the nerve and renders the ganglion 

 more distinct. Even after these precautions the dissection of the nerve 

 requires great care. It is slender, and more or less overrun by strands 

 of connective tissue and blood vessels, which vary in amount in the 

 different forms. The use of artist's brushes for the finer manipulations 

 will be found very helpful. The nerve is to be looked for along the 

 line of the tractus and running over the anterior brain surface and 

 the olfactory bulb. Among the commoner sharks, its exposure is re- 

 latively easy in the sandshark (Carcharias littoralis), Scylliura stellare 

 and Galeus canis, but it is very difficult to dissect in Myliobatis and 

 others. In general, it is easier to dissect in those forms in which it 

 has a ventral connection with the brain-wall. 



I n S q u a 1 u s a c a n t h i a s. — The anatomical relations of this new 



1) Sewertzoff proposes the name "Nervus praeopticus" for this 

 nerve, but, its dorsal position in several selachians makes the name 

 inappr^t)riate. Later, I shall suggest the designation Nervus terminalis 



3* 



