316 DR. A. MILNES MARSHALL. 



olfactorius when wishing to speak of the third or distal ele- 

 ment, the olfactory nerve proper ; in this case, however, it 

 must be clearly understood that olfactory nerve and nervus 

 olfactorius are by no means equivalent or mutually con- 

 vertible terms. ^ 



Though the hulbus olfactorius and tractus olfactorius are 

 considered as together equivalent to the olfactory lobe of the 

 embryo, it must be noticed that the proximal ganglion of the 

 nervus olfactorius may fuse so completely with the bulbus, 

 that it is, even in comparatively early stages, " rather diffi- 

 cult to fix on the exact line of demarcation between the bulb 

 and the nerve."" 



The three elements of the olfactory nerve, but especially 

 the first and third, vary much in the relative proportions 

 they attain in the adult. Thus, in the dogfish there is a 

 large biilbus olfactorius, connected proximally with the 

 hemispheres by a short, thick, tractus olfactorius, and giving 

 origin distally to the numerous filaments of the nervus 

 olfactorius. In the skate, while the bulbus and nervus 

 retain much the same proportions as in the dogfish, the tractus 

 olfactorius is of very great length. Among osseous fishes the 

 variations are still greater; in the pike, salmon, perch, 

 gurnard, &c., on the one hand, there is a very long nervus 

 olfactorius, springing from a bulbus olfactorius which is in 

 close contact with the hemispheres; on the other hand, in 

 the cod, carp, &c., as in the skate, the bulbus olfactorius is 

 situated near the olfactory organ, and is far removed from 

 the rest of the brain, with which it is connected by a long 

 tractus olfactorius. 



A question of far more morphological interest is the rcdation 

 of the olfactory nerve to the other cranial nerves. My ob- 

 servations, if confirmed, prove that in the chick up to the 

 end of the sixth day, in the dogfish up to stage o, and in the 

 salmon and trout, at any rate up to the time of hatching, the 

 olfactory nerve agrees very closely in histological characters 

 and in general anatomical relations with the other cranial 

 nerves. I propose now to consider these resemblances more 

 in detail, and specially in reference to the question of the 

 segmental value of the olfactory nerve. 



Certain of the cranial nerves — e.g. the facial and glosso- 

 pharyngeal — have long been acknowledged to possess seg- 

 mental value. If we consider the mode of development of 

 these segmental cranial nerves, we find that they agree 

 aniorg themselves, and differ sharply from other nerves 



^ Fide 'Qiiain's Anatomy,' 8tli edition, vol. i, p. 526. 

 2 Balfour, op. eit., p. 178. 



