STRUCTUPE OF THE PONS VAEOLH :—The several layers of which the pone Varolii is composed, will be more easily under stood by dividing it in the median line from before backward, into lateral halves. After the cerebellum has been removed, if the pons Varolii is torn in a longitudinal direction along the floor of the fourth ventricle in its posterior median fissure, it will separate into striated surfaces to the depth of about a quarter of an inch in the middle of the ventricle, and above the pons, between the cruri cerebri, it will divide as far for ward as the posterior perforated space, and below to the anterior median fissure of the medulla oblongata. The adjacent surfaces, thus separated, will be seen to form each a plaino-coucave surface, concave in front and straight upon its posterior margin along the floor of the fourth ventricle. If the remaining portion of the pons is now divided in the median line by the knife, the pons will be separated into lateral halves. The surface, presented by a section of the anterior portion of the pens, is lenticular in shape and covers the anterior two-thirds of the whole surface thus exposed.
It will be observed that in a division of the pone Varolii, transverse to the fibres on its anterior surface, and longi tudinal to the cerebro spinal axis, the divided surface presents two distinct portions for examination; an oval portion in front which is connected with the anterior pyramid of the medulla oblongata, and with the crnsta of the crus cerebri; and behind, a plane-concave surface of radiating fibres, the raphe, which separates the lateral halves of the crura cerebri, the posterior part of the pons, and the medulla oblongata. The raphe is composed of anterio-posterior fibres, and is an important structure, because, within it are the decussations of the nerves of the medulla oblongata and the commissures of the nerve centres situated in the floor of the fourth ventricle.
Upon each side of the posterior margin of the raphe, upon the floor of the fourth ventricle, and that of the, iter, are two rounded cords, the fasciculi teretes. These are broad in the
ventricle below, narrow above in the floor of the iter, and are continuous in front with the gray matter upon the lateral walls of the third ventricle. These columns attend the pos terior margin of the raphe as far as the floor of the third ventricle. The superior margin of the raphe curves abruptly in the floor of the third ventricle forward to the base of the brain in front of the corpora albicantia.
In that part of the raphe, which separates the crura cere bri an oval opening about the size a split pea, around which the fibres of the raphe separate to form its margins. This opening in the raphe is filled by the extremity of a tract of broken fibres which are those of the decussation of the pro cesses, or superior peduncles of the cerebellum. The fibres of the processes pass beneath the fasciculi teretes, and decussate within this opening in the raphe. The prO cessus of the opposite side crosses the raphe at this point to reach the red nucleus which is situated external to the raphe above the point of decussation. (The letters RN Figure 26, are placed over the situation of the red nucleus which is buried within the tegmentum of the cr:us cerebri.) The opening in the raphe, just described, is above the superior margin of the pons, and in the middle of that portion of the raphe which separates the crurte cerebri from each other.
-By the removal of the raphe there will be seen, behind the pons, the margin of a longitudinal tract of white fibres extending from the medulla oblongata to the erus cerebri above. 'fhis tract. is the internal lateral margin of the lemniseus or fillet, which is separated from_its fellow by the raphe. This tract is crescent in shape, with its concavity applied to the posterior convexity- of the lenticular section of the pons before described. Behind the fillet is a layer of Efray matter which underlies the floor of the fourth ventri cle, and constitutes the fascicula teretes.