The vesicular layer of the posterior portion of the pons Varolii underlies the floor of the fourth ventricle, and being elevated upon its posterior surface is called the fasciculi teretes. This layer is continuous below with the gray col umns of the spinal cord, which, in the floor of the fourth ventricle, are separated behind, so that the posterior cornua are external upon the sides, and the anterior cornua lie together along the median fissure of the ventricle. On ac count of this disposition of the anterior and posterior coruna, the centres of the motor nerves of the medulla oblongata arc placed along the median line of the ventricle, and those of the sensory nerves upon its sides.
Tim FOURTH VENTRICLE:—The fourth ventricle is situated behind the pons Varolii and the upper half of the medulla oblongata. It is bounded below, on each side, by the rest iforna bodies, and above by the inferior margins of the pro cesses. Its roof is formed by the processes on each side, and by the valve of Vieussens in the middle. It is about one inch and a half in length from above downwards, and about three quarters of an inch in width at its widest part, which is be tween the upper portion of the restiform bodies, its depth from above downwards, froul the valve of Vieussens to the median fissure, is about five-eighths of an inch. Its floor is lozenge shaped and pointed at either extremity, its upper extremity is continued by the iter to communicate with the third ventricle. Its lower extremity is contimmus with the central canal and the posterior median fissure of the spinal cord. The median fissure of the fourth ventricle divides its floor into lateral halves, and is the posterior boundary of the raphe. The resti form bodies are elevated on each side above the level of the floor of the ventricle, and their gradual approach and junction at the lower extremity of the ventricle resembles a pen in appearance, hence the lower extremity of the ventricle is called the calamus scriptorius.
Below are two slight elevations or grey tubercles, one upon each side of the apex of the ventricle, the clavate nucleii, which are the terminations of the posterior pyra mids of the medulla oblongata. Directed upwards and outwards from these tubercles, along the floor of the fourth ventricle internal to the restiform body on either side, is a tract of white fibres which passes through the restiform trian- • gle, to join the. fillet. This tract forms the posterior layer of the fillet, and a portion of it turns outward with the restiform body to the cerebellum. It also receives the anterior root of the eighth nerve as this root turns downward from the resti form triangle into the fourth ventricle to reach the auditory nucleus, which is situated beneath the floor of the ventricle at the side of calamus scriptorius.
At the lower part of the restiform body, external to the ventricle, and above the elevate nucleii, is a broad elevation covered by white fibres which marks the situation of the restiform nucleus. The restiform nucleus is the superior ter mination of the posterior column of the medulla oblongata and spinal cord, and it is the origin of a large portion of the fibres which constitute the restiform body.
The floor of the fourth ventricle is gray in color, some parts of it presenting bluish elevations which are the centres of nerves, and are called loci cerulii. If the sides of its me dian fissure are separated, the torn surfaces present a striated appearance of autero-posterior oblique fibres, which inter mingle across the median line. The fibres this separated compose the raphe before described.
Crossing the surface of the floor of the fourth ventricle, about its middle, are several strim, which are the stria? trans transversm. The inferior striae are derived externally from the posterior root of the eighth nerve (auditory), which pass inward beneath and across the flexion of the restiform body to the floor of the fourth ventricle, to enter the raphe, in the me dian line, among the fibres of which they are lost. The superior striae transverse commence above the arch of the restiform bodies upon the outer margin of the floor of the ventricle, and are directed downward and inward, obliquely from the restiform triangle to the raphe.
They may be traced as far forward, and below, as the inferior margin of the pons Varolii, at the anterior me dian fissure of the medulla. These fibres are continuous out wardly on the same side with the seventh nerve (facial), and, internally, at the anterior median fissure, with the anterior pyramid of the opposite side.
The roof of the fourth ventricle is arched from side to side, and diminishing in size gradually towards its anterior extrem ity encloses a conical shaped cavitiy which is contracted above to be Continuous with the iter. This cavity lodges the inferior vermiform process of the middle lobe of the cerebellum, the superior vermiform process resting upon the upper surface of the parts forming the roof of the fourth ventricle.
The parts which form the roof the fourth ventricle, viz. : the processes and the valve of Vieussens, which enter the upper part of the hilus of the cerebellum are connected behind with the dentate body. In front they assist in the formation of the peduncles of the cerebrum.
To recapitulate: The pons Varolii is divided into two por tions, an anterior or cerebellar, and a posterior or spinal. These portions include from before backwards: first, a trans verse layer which is the commissure of the cerebellum; second, a reticulated and vesicular layer, which is the motor tract ; third, a longitudinal layer, the lemniscus or fillet ; fourth, a vescicular layer or the fasciculi teretes ; and fifth, the resti form bodies or inferior peduncles of the cerebellum, which will be described with the medulla oblongata in connection with the tracts from which they are derived. The relation of these layers to each other should be recollected,in order to trace their distribution above in the formation of the erns cerebri and internal capsule, as well as their connections below with the medulla oblongata and spinal cord.