The laminm which form the superior surface of the median lobe, (or the superior vermiform process,) are considerably fewer than those ol the hemispheres. This explains the less depth of the median lobe, when measured from before backwards, than of the hemispheres. Two or more of the laminm of the latter are united tc a single lamina of the former, and thus the superior vermiform process serves as a trans verse commissure to the superior larninm of the hemispheres.
The inferior surface of the median lobe, or inferior vermiform process, is likewise com posed of laminm, which take a transverse di rection and present a free convex border, with some resemblance to the rings of a worm in action. (Fig.391.) These laminre are not all of equal transverse extent. The middle and pos terior are the broadest; the anterior gra dually diminish in size. Ilence the body which results from the conjunction of all the laminm has a triangular form, its apex being anterior and its base posterior, corresponding to the notch between the hemispheres. The laminm which occupy its middle have a greater depth than the rest, and consequently the body is more prominent at this situation.
Certain deep fissures divide the inferior ver miform process into segments which evidently correspond with and connect the lobes into which the hemispheres are subdivided on their inferior surface.
These segments may be very readily distin guished from -rch other, and the names which the accurate Reil has given them are suffi ciently appropriate. By separating each SC7, ment from the adjacent ones and tracing its lateral relations the anatomist may form a better idea than 'by any other means of the way, in which this portion of the cerebellum is con nected with the hemispheres.
The anterior extremity of the inferior venni.; form process projects into the cavity of the fourth ventricle, and serves to close it at its inferior ex-7 treinity. It is a pointed process, furrowed trans versely, continuous by its base with the rest of the vermiform process. Reil has named it the Nodule. From either side of it a valve-like membrane of exquisite delicacy extends forwards and outwards towards a lobule which is at tached to each crus cerebelli near to the origin of the auditory nerve. These membranes ro,
semble very much in shape the semilunal valves of the aort.a. By their attached margin they adhere to the crus cerebelli, and their fi et margin projects into the cavity of the fourth ventricle. Their inner extremities adhere to nodule, and are connected to each other by e thin membrane of precisely similar texturo which is a commissure to them. Reil gives tr. the two membranes and their intermediate cow necting one the name of posterior mcdullolj vcluni.* The lateral membranes were first do • The valve of Vieussens is the anterior met1.0 lary velum.
scribed by Tarin and Malacarne. When the fourth ventricle has been carefully opened in a recent cerebellum, it is very easy to demon strate them by passing the handle of a knife under them.
The structure of these lateral wings of the inferior medullary velum is readily ascertained. Their delicacy is such that they admit of being -examined by the microscope without pressure or other manipulation. They consist of tubular fibres of various sizes, talcing a transverse di rection, that, namely, of the long diameter of each wing, covered by a layer of nucleus-like particles as an epithelium. They seem to con nect the nodule to the small lobules of the pneumo-gastric nerve above mentioned (the flocks of Iteil), or to connect those lobules them selves as a commissure.* The nodule pushes befoiv it, into the fourth ventricle, a fold of the pia mater, connected with which on either side are several small granulations, or Pacchionian bodies. It is called the choroid plexus of the fourth ventricle. We can easily trace it to be continuous with the pia mater which covers the lobules of the seventh pair of nerves.
Next to the nodule, below and behind it, is a small lobe, called by Reil the spigot ( Zapfen), with a pointed extremity directed downwards and forwards. It consists of several small la minm separated by their fissures. Behind it is a larger lobule, which forms the most prominent portion of the inferior vermiform process, called by Iteil, from its furm, the pyramid. Its apex is directed downwards and backwards, and it likewise consists of numerous small lamin.