OF THE MESOCEPHALE.- The pyramidal and olivary colurnns may be readily traced, as already explained, from the medulla oblongata up to the cerebral hemispheres ; the former becoming united chiefly with the corpora striata, the latter with the optic thalami.
In that part of their course which is interme diate to the medulla oblongata these columns become mingled with certain transverse fibres, and with more or less of vesicular matter, and with them contribute to form a mass which is the connecting link between all the segments of the cerebellum, and may be compared to a rail road station, at which several lines meet and cross each other. This is the inesocephak or mesencephale. The name was suggested by Chaussier, inasmuch as it forms " to a certain extent the middle and centml part of the ence phalic organ, the bond which unites the several bundles of fibres which contribute to its for mation." The mesocephale may be isolated from the other segments by dividing the crura cerebri just beyond the anterior margin of the pons, and the crura cerebelli as they penetrate the he mispheres, and the medulla oblongata on a level with the posterior edge of the pons. The crura cerebri emerge from it in front : the medulla oblong,ata is connected with its posterior sur face : on either side it is prolonged into a crus cerebelli. Its inferior surface, which is very , convex and looks forwards, is composed of the I thick layer of arched fibres which form the pons ; and on its superior surface, which looks backwards, are the corpora quadrigemina, the processus cerebelli ad testes, and part of the floor of the fourth ventricle (fig. 386).
According to Chaussier, its weight is equal to about the sixtieth or sixty-tifth part of the entire brain.
We shall describe separately the inferior. and the superior surfaces of this segment of the en cephalon, and its intimate structure as unfolded by sections.
The inferior surface, (pons Varolii, annular protuberance,) convex from side to side, is inter rupted along. the median plane from behind for wards by a shallow groove in which the basilar artery usually lies, giving off in its course nu merous minute capillaries to the nervous struc ture of the mesocephale.
When the pia mater has been stripped off this surface, it is seen to be very evidently composed of a series of transverse fibres which take an arched course. The fibres are collected
into large fascicles separated from each other by very distinct intervals, so that there is no part where the fibrous structure is more appa rent than here. They form arcs of circles, not concentric, lying one behind the other in a se ries nearly parallel. Owing to this want of complete parallelism the width of this surface measured from before backwards is much less at each extremity than in the centre. The an terior margin is convex, and forrns a thick edge crossing the crura cerebri like a bridge ; hence the term pons was applied by Varolius to the whole series of fibres. The posterior border is concave, less curved than the anterior, and crosses the anterior pyramids and olivary co lumns, as the latter does the crura cerebri. The intervening fascicles of fibres become gradually less curved as they approach the posterior margin.
These transverse fibres form a stratum of considerable thickness at the inferior surface of the mesocephale. Some grey matter is depo sited between the less superficial layers which constitute it. The more deep-seated layers are penetrated and crossed at right angles by the ascending fibres of the anterior pyramids. A remarkabl interlacement takes place at this situation between the vertical and transverse fibres—the latter passing alternately in front of and behind adjacent bundles of the former. Some of the vertical fibres seem to sink into and connect themselves with the grey matter.
A transverse vertical section of the meso cephale gives a more complete view of the exact extent of the transverse fibres. They are found to occupy rather more than one-third of the depth of the exposed surfac.e. Their dispo sition in lamina. is very apparent. Those which are nearest the centre of the mesocephale have between them considerable intervals, which are filled up by grey matter, through which pass vertically the fibres of the pyramids. The intervals between the laminx gradually dimi nish towards the inferior surface of the pons, and the quantity of intervening grey matter becomes proportionally less, and disappears altogether from between those lamin the in tervals of which are not traversed by the fibres of the pyramids.