MODE OF EXIT FROM THE CRANIUM. 



223 



inwards to their so-called nuclei, in which they end without forming, so far as is 

 known, any direct connection with the nerve-cells. 



The superficial attachments of the cranial nerves are quite obvious : the 

 filaments of the first pair spring from the olfactory lobes of the cerebral hemi- 

 spheres ; the second pair arise from the optic thalami and the dorsal part of 

 the mesencephalon ; the third are attached to the crura cerebri or ventral part 

 of the mesencephalon ; the fourth to the valve of Vieussens ; the fifth to the 

 pons ; and the remainder to the medulla oblongata, with the exception of the 

 greater part of the eleventh pair, which arise from the cervical portion of the spinal 

 cord. The course of the fibres within the cerebro-spinal axis, and their connection 



Fig. 141. LATERAL VIEW OF THE 



BRAIN- STEM, SHOWING THE AT- 

 TACHMENTS OK THE CKANIAL 



NKRVE. (Allen Thomson.) 



The full description of this figure 

 will be found at p. 42. The follow- 

 ing references apply to the roots of 

 the nerves ; I, right olfactory tract. 

 cut near its middle ; II, the optic 

 nerves immediately in front of the 

 commissure ; the right optic tract is 

 seen passing back to the thalamus ( Tit ), 

 corpora geniculata (i, c), and corpora 

 quadrigemina (q) ; III, oculomotor 

 nerve ; IV. trochlear nerve arising at 

 v, from the valve of Vieussens ; V, 

 trifacial nerve ; VI, abducent ocular ; 

 VII, VIII, facial and auditory nerves ; 

 between them the pars intermedia ; 

 IX, glosso-pharyngeal ; X, pneumo- 

 ^astric ; XI, spinal accessory ; XII, 

 hypoglossal ; C I, the separate an- 

 terior and posterior roots of the sub- 

 occipital or first cervical nerve. 



rh. 



with the nerve-nuclei are 



more difficult to follow, and 



in many respects are as yet 



but imperfectly understood. 



They have been fully discussed 



in the accounts of the parts 



of the spinal cord and brain 



in which they occur, and to these reference may be made for farther details than are 



given in the following descriptions. 



Mode of exit from the cranium. Each of the cranial nerves, after traversing 

 the subarachnoid space and piercing the arachnoid, from which it receives a sheath 

 generally of very short extent (see p. 188), enters an aperture in the dura mater (in 

 the case of the third, fourth, fifth and sixth nerves at some distance from the osseous 

 foramina by which they emerge), and carries with it in its passage from the cranial 

 cavity a tubular prolongation of that membrane. Some of these nerves, or their 

 main divisions, are contained in distinct foramina of the cranium, others are grouped 

 together in one foramen. The numerous small olfactory nerves descend into the 

 nose through the cribriform plate of the ethmoid bone ; the optic nerve pierces the 

 base of the small wing of the sphenoid bone ; the third, fourth, and sixth nerves, 

 with the ophthalmic division of the fifth, pass through the sphenoidal fissure ; the 

 superior maxillary and inferior maxillary divisions of the fifth pass respectively 

 through the foramen rotundum and foramen ovale of the great wing of the 



p 2 



