SECTION XV 



SUMMARY OF THE CONNECTIONS AND FUNCTIONS 

 OF THE CRANIAL NERVES 



Cranial nerves. The cranial nerves are generally reckoned as 

 twelve in number : 1st, olfactory ; 2nd, optic ; 3rd, oculo-motor ; 

 4th, or trochlear ; 5th, or trigeminus ; 6th ; 7th, or facial ; 8th, 

 auditory ; 9th, glossopharyngeal ; 10th, vagus or pneumogastric ; 

 llth, spinal accessory ; 12th, hypoglossal. 



Of these the first two stand on a different footing from the rest, 

 which, like the spinal nerves, are outgrowths of nerve fibres from 

 the central tube of grey matter surrounding the neural canal or from 

 ganglia corresponding to the spinal posterior root ganglion. 



The olfactory bulb and the retinas, from which the majority of 

 the fibres forming the olfactory tract and the optic nerve respectively 

 take their origin, are analogous rather to lobes of the brain than to 

 peripheral sense-organs. Thus in the retina there are three relays 

 of neurons through which the visual impulse must pass before it 

 arrives at the optic nerve. The olfactory tract and optic nerve are 

 thus comparable with the association or commissural fibres connecting 

 different parts of the central nervous system. The connections of 

 these sensory fibres have already been fully dealt with, and the 

 structure of the peripheral sense-organ will be treated of under the 

 physiology of the special senses. Among the cranial nerves proper 

 we may therefore reckon the third to the twelfth. 



The third or oculo-motor arises from an extensive nucleus which 

 extends on either side along almost the whole length of the ventral 

 part of the aqueduct of Sylvius close to the middle line, the most 

 anterior part lying in the back part of the third ventricle (Fig. 204). 

 The anterior part is composed of small cells which give origin to the 

 fibres innervating the intrinsic muscles of the eye, namely, the ciliary 

 muscle and the sphincter pupilla3. The rest of the nucleus is made 

 up of large multipolar cells, arranged in groups, and gives origin 

 to the fibres passing to most of the extrinsic muscles of the eye. The 

 fibres of the third nerve pass through the tegmcntum to emerge 

 at the inner margin of the crusta of the same side. The fibres from 



the posterior large- celled nucleus supply the following muscles : 



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