CEREBELLUM.
It will not assist us to the understanding of the architec ture of the central nervous system to enter into a minute description of the external surface and shape of the cere bellum. Its external appearance and the divisions of its sur face will be readily understood by a study of the plates in this work. It is sufficient to say that it is composed of a middle and two lateral lobes. The upper portion of the mid dle lobe is called the superior, and the lower part the inferior vermiform proces§ of the cerebellum. The superior vermi form process rests upon the upper surface of the valve of Vieussens, and the inferior vermiforin process is beneath the valve, and within the fourth ventricle. The lateral lobes project backwards and outwards, and are large masses sep arated in front by a notch, the incisura anterior, aud behind by a deep and narrow fissure, the incisura posterior. In front of the lower surfaces of the lateral lobes, beneath the lower margin of the pons, are two small lobes, one on each side of the medulla oblongata, called the flocculi.
The cerebellum is connected with the cerebrum by the processes which are its superior peduncles; with the pons in front by its middle peduncles; and with the spinal cord below by two rounded cordlike bodies, the restiform bodies, or its inferior peduncles.
A section of the middle lobe of the cerebellum in the median 'line from before backward, exposes the fourth ventri cle. The surface of the section presents a beautiful arbores cent distribution of white matter radiating from the end of a longitudinal section of the valve of Vieussens the rami from which divide into small branches as they approach the surface. These subdivide into minute stems of white fibres terminating in small leaflets which line the sides of the deep fissures that project inward from the surface of the lobe. The appearance presented by this section is called the arbor vitfe.
A. section across a lateral lobe, outward and backward, shows within, a large mass of white matter, an arbores cent border, and imbedded in the anterior part of the white substance a convoluted layer of gray matter surrounding a portion of white substance. of irregular shape. This body is
called the corpus dentatum of the cerebellum and is its sensory or internal basal ganglion.
I11 a properly hardened cerebellum, if the sides of its transverse fissure are separated, and the upper half gently torn from the lower aud broken off ia front, it will be ob served that the surface of either lobe will separate behind and in front over the dentate body, that the fractured fibres are external to this body, aud are those of the middle peduncle of the cerebellum.
The upper surface of the dentate body is elevated above the remaining portion of the surface thus expcs.ed upon the inferior segment, or lower portion of the cerebellum. This surface presents numerous Hues radiating outward from the corpus dentatum, and others directed inward and backward, from the broken middle peduncle of the cerebellum. The fibres from these separate sources commingle to form the corona radiata of the cerebellum.
It will be observed that the processus, or superior pedun cle of the cerebellum, of either side passes directly down.
ward and backward into the centre of the corpus dentatnm, entering its hilus upon the under surface. The lower extrem ity of the processus, on its outer side, is overlapped by a tract of radiating fibres which spring upward from the inter val between the middle and superior peduncles of the cere bellum as they approach each other. This tract emerging from between the superior and middle peduncles, is the upper extremity of the restiform body, or inferior peduncle of the cerebellum, and is distributed. to the upper surface of the corpus dentaturn.
The attention is directed to the fact, that the corpus dentatum is connected with the superior and the inferior peduncles of the cerebellum, directly associating this body with the cerebrum above, the spinal cord below, and the cerebellum behind. The middle peduncle is composed of two distinct layers of fibres, differing in function, con nected directly with the gray matter of the surface of the cerebellum and having no connection with the corpus den tatum.