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, glossopharyn- 

 geal ; 10th, vagus or pneumogastric ; llth, spinal accessory ; 12th, hypo- 

 glossal. 



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 retinae, 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 physi- 

 ology 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 com- 

 posed of small cells which give origin to the fibres innervating the intrinsic 

 muscles of the eye, namely, the ciliary muscle and the sphincter pupilbe. 

 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 tegmentum 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 : levator 

 palpebrarum, superior rectus, inferior rectus, internal rectus, and inferior 

 oblique. 



Stimulation of the trunk of the third nerve causes the eyeball to look 



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