CH. XLVI.] THE CRANIAL NERVES 663 



Running down the centre of the floor of the fourth ventricle is 

 a shallow groove; on each side of this is a rounded longitudinal 

 eminence called the eminentia teres ; running across the middle of 

 the floor are a number of fibres (the striae acousticce), which join the 

 auditory nerve. 



In the upper part of the diagram, the mid-brain, with the corpora 

 quadrigemina (a a, I I), is shown. Here there is once more a canal 

 which penetrates the substance of the mid-brain, and is called the 

 aqueduct of Sylvius, or the iter a tertio ad quartum ventriculum ; it 

 leads, as its name indicates, from the third to the fourth ventricle. 



The Internal Structure of the Bulb, Pons, and Mid-Brain. 



The structure of the interior of these parts is complex, and the 

 complexity arises from the circumstance that here we have to deal 

 not only with parts running upwards from cord to brain, or down 

 from brain to cord, but also with a considerable amount of grey 

 matter in which some of the white tracts terminate, or from which 

 new tracts issue. The most important stretch of grey matter is that 

 which appears on the floor of the fourth ventricle, and which is 

 continued upwards around the Sylvian aqueduct, and downwards 

 into the spinal cord ; here are situated groups of nerve-cells, which 

 are spoken of as centres, or nuclei. The most important of these are 

 those which are connected with the cranial nerves. There are 

 twelve pairs of cranial nerves, and of these the last ten pairs originate 

 from the floor of the fourth ventricle or the neighbouring grey 

 matter. 



The following is a list of the cranial nerves : 



1. Olfactory. This is the nerve of smell. 



2. Optic. This is the nerve of sight. 



A rrij OCU \ These three nerves supply the muscles of the 



4. Trocklear JT !, 



6. AUucens J e ^ ebaU ' 



5. Trigeminal. This is the great sensory nerve of the face and 

 head. Its smaller motor division supplies the muscles of mastication 

 and a few other muscles also. 



7. Facial. This is mainly the motor nerve of the face muscles. 



8. Auditory. This is divided into two parts, one of which, called 

 the cochlear nerve, is the true nerve of hearing, and is distributed to 

 the cochlea of the internal ear ; the other division, called the vestibular 

 nerve, is distributed to the vestibule and semicircular canals of the 

 internal ear. 



9. Glosso-pharyngeal. This is a mixed nerve ;. its motor fibres pass 

 to certain of the pharyngeal muscles ; its sensory fibres are mainly 

 concerned in the sense of taste. 



