536 THE HUMAN EMBRYO. 



the human embryo, nerve fibres have been traced growing out 

 from the nerve cells of the ganglion into the brain, the attach- 

 ment taking place immediately dorsal to the point of emergence 

 of the facial nerve. Beyond its root of attachment, the gang- 

 lion of the auditory nerve divides into two main portions, the 

 cochlear and vestibular ganglia : these diverge from each other, 

 and between them the root of the facial nerve is wedged. The 

 auditory ganglia very early acquire connection with the wall 

 of the auditory vesicle, and the several ganglia of the adult 

 ear are formed by further division of the two ganglia of the 

 embryo. 



It is stated that the geniculate ganglion of the facial nerve 

 is derived from the same ganglionic mass from which the auditory 

 ganglion is formed. 



The sensory root of the trigeminal, or fifth cranial nerve. 

 The ganglion of the trigeminal nerve is, from the first, of great 

 size (Fig. 227). The three principal branches of the nerve- 

 ophthalmic, maxillary, and mandibular (Fig. 227, V, a b c) are 

 already present, and of large size, before the end of the fourth 

 week ; as these nerves lengthen, the originally single ganglion 

 gradually breaks up, small portions becoming detached, and 

 moving out along the growing nerve stems. In this way, early 

 in the fifth week, the ciliary, sphenopalatine, and otic ganglia 

 are established ; the submaxillary ganglion is not separated 

 until a rather later stage. The main ganglion persists as the 

 Gasserian ganglion of the adult, and the motor root of the 

 trigeminal nerve lies along its inner side, and in close contact 

 with it (Fig. 227, v). 



The optic nerve. The optic vesicle and optic stalk are parts 

 of the brain, and cannot be compared with nerves, either sen- 

 sory or motor. There is, however, strong reason to think that 

 the actual optic nerve fibres do not arise in the optic stalk, but 

 are formed independently, as outgrowths from the retinal cells 

 which grow inwards to the brain, following the line of the optic 

 stalk, but being fundamentally independent of this. Even then, 

 however, inasmuch as the retina is developmentally part of the 

 brain, the optic nerve would rather resemble the intra-cerebral 

 fibres of the brain than the ordinary sensory nerves ; and for 

 the present the relations of the optic nerves to the other nerves 

 must be left undecided. 



