THE CRANIAL NERVES. 537 



The olfactory nerve. According to the observations of 

 Professor His, the mode of development of the olfactory nerve in 

 the human embryo is as follows. The olfactory lobe is formed 

 as an outgrowth of the cerebral hemisphere towards the end of 

 the fourth week, and very early becomes divided by a transverse 

 constriction into anterior or distal, and posterior or proximal 

 portions. 



At this stage, in embryos of from twenty-seven to twenty- 

 eight days, although the olfactory pit is well developed (Figs. 

 204 and 227), there is, according to His, no trace of either the 

 olfactory ganglion or olfactory nerve. A day or two later, the 

 olfactory epithelium begins to undergo changes similar to those 

 which occur in the wall of the brain, or spinal cord, preparatory 

 to the appearance of nerves. Neuroblasts are formed near its 

 inner or deeper surface : these soon become pyriform, and give off 

 processes which grow into the mesoblast, and towards the brain. 

 There is thus, early in the fifth week, a mass of neuroblasts 

 forming a ganglion in direct connection with the olfactory epi- 

 thelium : from the ganglion, nerve fibres grow out towards the 

 brain, but do not yet reach this. By the end of the fifth week 

 the nerve fibres reach the olfactory lobe, meeting it at the 

 constriction separating the proximal and distal portions, and 

 thus placing the olfactory epithelium in connection with the 

 brain. 



During the second month the distal portion, or bulb of the 

 olfactory lobe, which at first lies entirely in front of the nerve, 

 becomes bent down so as to lie in contact with this ; and by 

 the end of the second month the olfactory nerve arises by a 

 number of fibres from the olfactory bulb, instead of by a 

 single stem from the olfactory lobe behind the bulb, as in the 

 earlier stages. 



The roots of the adult olfactory nerve are formed by bundles 

 of ascending or centripetal nerve fibres, which grow from the 

 ganglion into the brain ; they are already present at, or shortly 

 after, the end of the second month. 



At first sight this account appears to differ widely from the 

 descriptions given above of the development of the other sensory 

 nerves ; but the differences are not really so great as they seem. 

 In the case of all sensory nerves the connection with the brain, 

 or spinal cord, is acquired by growth of nerve processes centri- 



