THE NERVOUS SYSTEM 



445 



mucous membrane and to the retina. The morphological 

 position is doubtful, for, while they are considered by some 

 to be the first true cranial nerves and to belong to a much 

 earlier period than any of the rest, others deny them the right 

 to be called nerves at all, and treat them as parts of the brain, 

 the olfactory lobes (rhinencephalon) and the optic stalks re- 

 spectively. 



Attempts have been made to bring this condition- into ac- 

 cord with that found in Amphioxus, for here the archen- 

 cephalon, a rudimentary brain formed by the enlargement of 

 the anterior end of the spinal cord and possibly the equiva- 

 lent of the telencephalon of vertebrates, bears two rudimentary 

 sense-organs, the first an olfactory pit and the second a pigment 

 speck. Of these the first is connected with the brain by a short 

 diverticulum, while the second is embedded within the brain 

 wall. Although similarity of function of the two sets of organs 

 in the two cases tempts one to believe in an homology between 

 them, the decision really hinges upon the identity of these 

 sense-organ rudiments and the perfected organs of the higher 

 vertebrates ; for if the olfactory groove and the pigment speck 

 are historically the anlagen of the nose and eye, a point not 

 definitely established, then the identity of the nerves with 

 the corresponding parts of the archencephalon naturally fol- 

 lows. In favor of this latter assumption is the fact of the 

 origin of these two nerves from the primary fore-brain, while 

 none of the others arise anterior to the metencephalon. An 

 entirely problematical element belonging to this region is that 

 of a definite pair of nerves, Nervus terniinalis, which occur in 

 all selachians, and extend from the anterior part of the telence- 

 phalic lobes, where they originate, along the anterior aspect 

 of the olfactory stalks. As they have been but recently dis- 

 covered they have escaped enumeration with the classical 

 twelve pairs; their origin from telencephalon is also anoma- 

 lous. Nothing can as yet be predicted of their morphological 

 significance. 



II. THE MOTOR NERVES OF THE EYEBALL. (MotOY OCllli, 



Trochlcaris and Abducens.) 



