THE EXITS OF THE CEREBRAL NERVES. 75 



The eleventh pair faccessoriij are cerebral only by courtesy, 

 as these nerves take their origin from the spinal cord, by 

 roots which issue between the proper anterior and posterior 

 roots of the spinal nerves, and, joining together, form, on 

 each side, a nei-re which passes out with the pneumogastric, 

 partly joining it, and partly going to muscles which arise 

 from the head and anterior vertebra, and are inserted into 

 the pectoral arch. 



The spinal accessory exists in no Ichthyopsid vertebrate, 

 but is found in all Scmropsida, with the exception of the 

 Ophidia, and in the Mammalia. 



The twelfth and last pair (hypocjlossi) are the motor neiwes 

 of the tongue, and of some retractor muscles of the hyoidean 

 apparatus. 



In the Ichthyopsida the first cervical nerve supplies the 

 distributional area of the hypoglossal; but in all the abran- 

 chiate Vertebrata there is a hypoglossal, which traverses a 

 foramen in the ex-occipital, though it often remains closely 

 connected with the first cervical, and may rather be re- 

 garded as a subdivision of that nerve, than as a proper 

 cerebral nerve. 



Thus the nerves arising from the hind-brain, in all the 

 higher Vertebrata, fall into three groups: 1st, a sensori- 

 motor, pre-auditory, set (3rd, 4th, 5th, 6th, 7th) ; 2nd, the 

 purely sensory auditory nerve (8th) ; 3rd, the sensori-motor, 

 post-auditory, set (9th, 10th, 12th). 



The apertures by which several of these nerv-es leave the 

 skuU. retain a vei-y constant relation to certain elements 

 of the cranium on each side. Thus : 



a. The filaments of the olfactory nerve always leave 

 the craniimi between the lamina perpendicularis, or body 

 of the ethmoid, and its lateral or prefrontal portion. 



b. The optic nerve constantly passes oiit behind the 

 centre of the orbitosphenoid and in front of that of the 

 alisphenoid. 



c. The third division of the trigeminal, or fifth nerve, 

 always leaves the skull behind the centre of the alisphenoid 

 and in front of the pro-otic. 



