450 THE NERVOUS SYSTEM. 
gracilis of the cord. It is true that a surface inspection of the medulla might very 
naturally lead the observer to this supposition, because there is no sharp line of 
demarcation marking it off from the tubercles of these strands. Such a conclusion, 
however, would be altogether erroneous, because it is questionable if any of the fibres 
of the posterior columns of the cord are carried beyond the gracile and cuneate 
nuclei of the medulla. A study of the surface of the medulla y ields some important 
information regarding the constitution of the restiform body. Thus the direct 
cerebellar tract from the lateral column of the cord can be traced into it, and large 
numbers of fibres which take a curved course on the surface of the medulla may 
likewise be followed into it. These are the superficial arcuate fibres. Numerous 
other fibres enter the restiform body on its deep aspect, but these will be studied 
at a later stage. 
Superficial Arcuate Fibres (fibre arcuate externe).—These fibres enter 
into the constitution of the restiform body, and they may be regarded as con- 
sisting of two sets, viz. the anterior superficial arcuate fibres and the posterior 
superficial arcuate fibres, both of which present this feature in common that they 
run on the surface of the medulla. 
The anterior superficial arcuate fibres are more particularly seen in the neighbour- 
hood of the olive, round the lower border of which, and also over the surface of 
which, they may be observed coursing in the form of a number of coarse curved 
bundles. They vary greatly in number and in distinctness, and they are sometimes 
so numerous as to cover over almost entirely the olivary eminence. An attentive 
examination will show that they come to the surface in the antero-median fissure 
between the pyramids, and also not unfrequently in the groove between the 
pyramid and olive, or through the substance of the pyramid itself. The antero- 
median fissure in its upper part is often almost completely blocked up by these 
emerging fibres. The anterior superficial arcuate fibres reaching the surface of the 
medulla in this manner turn backwards, and the great majority enter the restiform 
body and form a considerable part of its outer portion. 
The posterior superficial arcuate fibres arise in the cuneate and gracile nuclei, and 
enter the restiform body of the same side. 
THE PONS VAROLII. 
The pons Varolii is a marked white prominence on the basal aspect of the brain 
which is interposed between the medulla and the crura cerebri, and which les in 
front of the cerebellum. It is convex from side to side, as well as from above 
downwards, and transverse streaks on its surface show that, superficially at least, it 
is composed of bundles of nerve-fibres which course transversely over it. On either 
side these transverse fibres are collected together in the form of a large compact 
strand, which sinks in a backward and outward direction into the white matter of 
the corresponding hemisphere of the cerebellum. This strand is termed the middle 
peduncle of the cerebellum, and the term “pons,” applied to the entire structure, 
expresses in an admirable way the arch-like manner in which this portion of the 
brain bridges across between the two cerebellar hemispheres. 
The ventral surface of the pons is in relation to the basilar process of the 
occipital bone and the dorsum selle of the sphenoid bone. It presents a mesial 
groove (sulcus basilaris), which gradually widens as it is traced upwards, and in 
which the basilar artery hes. This mesial depression is produced by the prominence 
which is caused on either side by the passage of the pyramidal tract of the medulla 
upwards through the pons. The trigeminal or fifth cranial nerve, with its large 
entering sensory root and its small emerging motor root, is attached to the side of 
the ventral aspect of the pons, nearer its upper than its lower border. It is usual 
to restrict the term “pons” to that portion of the structure which les between the 
two trigeminal nerves, and to apply the designation of middle cerebellar peduncle 
to the part which extends beyond the nerve into the hemisphere of the cerebellum. 
The sixth or abducent nerve, the seventh or facial nerve, and the eighth or auditory 
nerve are attached to the brain at the lower border of the pons. The sixth emerges 
at the outer border of the pyramid, the seventh immediately in front of the resti- 
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