150 MR HAROLD AXEL HAIG ON THE 



trapezium, the pyramid bundles form two very distinct masses, and between them lies 

 grey matter representing the nuclei pontis ; two small masses of grey matter dorso- 

 external to the pyramid bundles may possibly represent superior olivary nuclei. 

 Pontine nuclei are also to be seen lying ventral and external to the pyramids. 



The nucleus of the 7th cranial nerve lies at the outer and upper angle of the 

 formatio reticularis, and the issuing fibres of the 7th nerve pass down close to and 

 parallel to a wide bundle of fibres which arises (partly) from a nucleus close to the inner 

 side of the restiform body ; this bundle is the issuing root of the auditory or 8th 

 cranial nerve. Outside the restiform body and the above-mentioned nerves lies part 

 of the white matter of the cerebellar hemisphere (c). 



Pontine fibres pass ventrally to the pyramid bundles, and form characteristic 

 parallel strands. 



The mid-pontine region shows a very thick layer of grey matter beneath the 

 ependyma of the floor of the 4th ventricle, on either side of the middle line : beneath 

 this, the posterior longitudinal bundle forms a conspicuous band, the two lying close 

 together in the middle line. The tract of the fillet is also well defined, lying just above 

 and to the outer side of the pyramid bundle ; whilst the intercrossing fibres of the pons 

 alternate with parallel streaks of grey matter, breaking up the majority of the central 

 part of the section (PI. I. fig. 7) into a reticular formation of wider extent than that in 

 the lower pontine region. 



The nuclei of the 5th cranial nerve (motor and sensory nuclei close together) 

 appear as a large group of nerve-cells just internal to the restiform body, which latter 

 is now known as the superior cerebellar peduncle, and the issuing fibres of the 5th 

 nerve are seen at the lateral part of the section just ventral to the white matter of the 

 cerebellar hemisphere. 



The trapezium is represented by the dorsal portion of the reticular formation, and 

 can hardly be distinguished histologically from that, except by appropriate methods, 

 inapplicable in the case of the present material. The central bundle of the 5th nerve 

 forms a rather narrow band of fibres lying just beneath the grey matter of the floor 

 of the ventricle. 



The pontine fibres which lie at the lower (ventral) aspect form very marked parallel 

 strands curving outwards towards the 5th nerve : and the grey matter just dorsal to 

 these fibres (nuclei pontis) are distinguished by their relatively wide extension laterally. 



The upper pontine region appears in some respects very similar to the mid-region ; 

 the tract of the fillet is, however, becoming divided into two main portions, viz. the 

 intermediate fillet and the lateral fillet (PI. I. fig. 8 /). 



The posterior longitudinal bundles are now slightly separated in the mid-line by 

 a small amount of grey matter, and the grey matter of the floor of the 4th ventricle is 

 not quite so thick as in the middle region of the pons. 



The crossing of the 4th cranial nerves is seen as a band lying just above the 

 ventricle, forming a kind of roof to the cavity at this point ; the superior cerebellar 



(ROY. soo. EDIN. TRANS., VOL. XLVIII., 858.) 



