628 STRUCTURE OF THE BULB, PONS, AND MID-BRAIN [CH. XLIV. 



space are made by the superior peduncles of the cerebellum, which 

 contain fibres going up through the mid-brain to the cerebrum. 

 The middle peduncles of the cerebellum are principally made up of 

 fibres running from one cerebellar hemisphere towards the other 

 through the pons. 



Eunning 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 strice acousticce), which join the 

 auditory nerve. 



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

 quadrigemina (a a, 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 second 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 to the cranial nerves. There are altogether 

 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. 



3. Motor oculi\ Thege three nerves Supp l 7 the muscles of the 



4. TroMear ^ 



6. Abducens J 



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 



