CH. XLIV.] THE CRANIAL NERVES. 625 



Origin of the Cranial Nerves. 



Each cranial nerve arises from a collection or collections of 

 nerve-ceils situated in the grey matter ; these collections are 

 called nuclei of origin. The nerve-fibres run a certain distance 

 through the brain substance and then pass away to their destina- 

 tions. The region of the brain's surface where the nerve leaves 

 it, is often spoken of as the superficial origin of the nerve. 



There are twelve pairs of cranial nerves altogether, and of 

 these ten originate from the floor of the fourth ventricle or the 

 neighbouring grey matter. Fig. 464 is a diagram to show the 

 nuclei of origin of these ten nerves (3rd to the i2th). The first 

 two pairs of nerves will be described more fully later. In the 

 following brief account of the ci-anial nerves, it will be noticed 

 that many of them are either wholly motor, or wholly sensory in 

 function. 



1. Olfactory nerve. This is the nerve of smell. 



2. Optic nerve. This is the nerve of sight. 



3. Motor oculi. This is wholly motor. It supplies the follow- 

 ing extrinsic eye-muscles : superior rectus, inferior rectus, internal 

 rectus, inferior oblique and levator palpebra) ; and the following 

 intrinsic eye-muscles (i.e., within the eyeball itself) : the ciliary 

 muscle, and the sphincter fibres of the iris. Its deep origin is in 

 the grey matter on the side of the Sylvian aqueduct underneath 

 the corpora quadrigemina close to the middle line. The anterior 

 part of the nucleus is composed of small cells from which nerve- 

 fibres originate for the ciliary muscle and sphincter of the 

 iris ; the other muscles mentioned are supplied by fibres from 

 the larger cells of the posterior part of the nucleus. 



4. Trochlear. This is wholly motor too. It supplies the 

 superior oblique muscle of the eyeball. It takes origin from the 

 grey matter immediately below the centre of the third, but 

 slightly more lateral in position ; its centre is connected with 

 that of the third and sixth nerves. 



6. Abducent. It is convenient to take this next. It also is 

 wholly motor, supplying the external rectus of the eyeball. Its 

 centre is beneath the eminentia teres in the upper part of the 

 floor of the fourth ventricle, near the middle line. By the 

 posterior longitudinal bundle it is connected to the nucleus of 

 the third nerve of the opposite side, and thus it partially supplies 

 the internal rectus of the opposite side. 



5. Trigemincd. This is a mixed nerve. Its smaller motor 

 division supplies the muscles of mastication, the tensors of the 



K.I'. 8 8 



