DEVELOPMENT OF PERIPHERAL NERVES. 



1377 



is no longer within the primary epithelium, but deeper and within a cell aggregation known 

 as the olfactory ganglion. The neurones, however, retain connection with the olfactory epithe- 

 lium by means of their peripherally directed processes, which correspond to dendrites, and with 

 the brain by means of their axones. With the thickening of the olfactory epithelium which sub- 

 sequently occurs, the peripheral fibres and their nuclei comes to lie entirely within the epithelial 

 stratum and persist as the olfactory cells, whose centrally directed processes form the olfactory 

 filaments that end as arborizations within the characteristic olfactory glomeruli. The first 

 cranial nerve is peculiar in the superficial position of its cell-bodies and in the extreme shortness 

 of its dendrites, which are represented by the rod-like fibres of microscopic length extending 

 from the cell-bodies toward the free surface of the olfactory mucous membrane. This superficial 

 position of the olfactory neurones is regarded as an unusual persistence of the primary condition 

 of all sensory elements and as evidence of the archaic nature of the olfactory nerves. 



Fig. 1141. 



Superior coUiculus 

 Mid-brain \ 



pineal body 



piencephalon 



Median geniculate body 

 Pallium 



Inferior coUiculus 

 Oculomotor nerv 

 Trochlear nerve 



Cerebellum 

 Trigeminal nerve 

 Auditory nerve 



Glosso-pharyngeal nerve 

 Vagus nerve 



Spinal accessory nerve 



Rhinencephalon 



, \ Optic stalk 



;V Inferior part of III. ventricle 



''t- Facial nerve 



ifr- Abducent ner\'e 



Hypoglossal nerve 



Reconstruction of brain of human embryo of four and one half weeks (10.2 mm.); outer surface, showing 



developing nerves. X 12. Drawn from His model. 



The optic nerve is so inherently a derivative of the cerebral and optic vesicle, that its develop- 

 ment is appropriately considered with that of the eye (page 1482) ; moreover, its morphological 

 significance being so at variance with that of the other nerves, it may be omitted from further 

 discussion in the series now being described. 



The oculomotor nerve being strictly a motor nerve has much in common' in its mode of 

 formation with the ventral root of a spinal nerve, with which it is homologous. The nerve 

 originates as an outgrowth from a group of neuroblasts, which occupies the ventral zone about 

 the middle of the mesencephalon. From these neurones, visible in the fourth week in the 

 human embryo, the axones proceed as a converging group of fibres which, piercing the wall of 

 the brain-tube close to the mid-line, appear on the ventral surface of the brain-stem as the fibres 

 of the third nerve. Although by some regarded as possessing a transient rudimentary dorsal 

 root that early entirely disappears, thus bringing the nerve of a cranial myomere into close 

 correspondence with those of the spinal series, it is doubtful whether such structure is usually 

 present, the suppression of the dorsal portion of the nerve being complete. Soon after its for- 

 mation, the main trunk undergoes division into a smaller upper and a larger posterior limb, 

 which foreshadow the superior and inferior divisions of the mature nerve. 



The trochlear nerve, although springing from a central group of neuroblasts in close 

 proximity with those giving rise to the third, is peculiar in the course of its axones. Instead 

 of maintaining a ventral course, these proceed dorsally and become superficial on the upper 

 (dorsal) aspect of the hind-brain, piercing the plate which later becomes the superior medul- 



87 



