320 DK. A. MILNES MARSHALL. 



their length., and only present nerve cells in any considerable 

 number at certain points, the olfactory nerve still presents 

 nerve-cells along its whole length. This second difference 

 appears, however, to depend on the first, since as the nerve 

 elongates with age, we find it gradually taking on the histo- 

 logical characters of the other nerves, i.e. the greater part of 

 its length becomes fibrillar, and the nerve-cells confined to 

 the two extremities, where they form ganglionic swellings. 

 The practical importance of these differences is, however, 

 considerable, since, ovving to the olfactory nerve consisting 

 for some time after its first appearance almost entirely of 

 rounded cells, it is very difficult to distinguish from the 

 surrounding mesoblast, and may, therefore, very readily be 

 overlooked. 



The olfactory nerves are by most authors considered as of 

 totally differentmorphological value to the otliercranial nerves. 



According to Gegenbaur, "the cerebral nerves 



are seen to break up into two very distinctly marked 

 divisions, when examined after the comparative method. 

 One division, the larger, contains nerves which more or less 

 agree with, or might even be derived from, spinal nerves, 

 while the other contains those which have not the fai7itest 

 resemblance to spi?ial nerves. This latter divison contains 

 two specific sensory nerves, the olfactory and the optic. "^ 



Again, Prof. Huxley says, ''The greatest number of pairs 

 of nerves ever given off from the vertebrate brain is twelve, 

 including the so-called olfactory nerves and the optic nerves, 

 which, as has been seen, are more properly diverticula of the 

 hraiyi than nerves in the proper sense of the icord. The 

 olfactory ' nerves ' {olfactorii) constitute the first pair of 

 cerebral nerves. They always retain their primarxj connec- 

 tion with the cerebral hemispheres, and frequently contain, 

 throughout life, a cavity, the olfactory ventricle, which com- 

 municates with the lateral ventricle."^ 



Finally, Balfour considers that the "very late appearance 

 and peculiar relations" of the olfactory nerve " are, at least 

 for the present, to my mind sufficient grounds for excluding 

 it from the category of segmental cranial nerves. "^ 



1 have already attempted to show that the existence of 

 an olfactory lobe or vesicle can in no way be s^^ud to mili- 

 tate against the establishment of a complete homology 



1 'Elements of Comparative Anatomy,' English translation, p. 515. 

 The italics are mine. 



2 'Anatomy of Vertebrated Animals,' p. 7i. The italics again are 

 mine. 



3 Op. cit., p. 215. 



