MEMBRANES OF THE BRAIN. 173 



tains more or less fluid, which has been named cerebro-spinal. The space 

 is largest at the under part of the great brain, about its middle, and in the 

 fissure between the hemispheres of it and the cerebellum. Under the 

 arachnoid, in the fissure between the halves of the cerebellum, is the aper- 

 ture of the fourth ventricle, by which that cavity communicates with the 

 subarachnoid space. 



The PIA MATER closely invests the different parts of the brain, and dips 

 into the fissures, as well as into the sulci between the convolutions and 

 laminse. Besides covering the exterior of the brain, it sends processes 

 into the interior to supply vessels to the walls of the inclosed space : thus, 

 one penetrates into the cerebrum below the corpus callosum, and is named 

 velum interpositum ; and two vascular fringes, which project into the fourth 

 ventricle, are known as the choroid plexuses of that cavity. 



This membrane is a network of vessels, and is constructed of the minute 

 ramifications of the arteries and veins entering into or issuing from the 

 cerebral substance ; whilst the intervals between the vessels are closed by 

 fine areolar tissue, so as to form a continuous thin layer. From the under 

 surface of the membrane proceed numerous fine vessels for the nutrition of 

 the brain. 



Vessels and nerves. The arachnoid membrane has but few vessels, 

 whilst the pia mater is composed almost entirely of vessels. The pia 

 mater is largely supplied by offsets of some cranial nerves, and by branches 

 of the sympathetic which accompany the vessels at the base of the brain 

 (p. 33). Bochdalek has described branches to the arachnoid from some 

 cranial nerves. 



SUBDIVISIONS OF THE ENCEPHALON. Before the description of the 

 arteries is given, the chief subdivisions of the encephalon will be shortly 

 noticed. 



The cranial or encephalic mass of the nervous system consists of cere- 

 brum or great brain, cerebellum or small brain, pons, and medulla oblon- 

 gata. Each of these parts has the following situation and subdivisions : 



The medulla oblongata, or the upper end of the spinal cord, lies in the 

 groove between the halves of the small brain, and is divided into two 

 symmetrical parts by a median fissure. To it several of the cranial nerves 

 are united. 



The pons Varolii is situate in front of the medulla oblongata, and is 

 marked along the middle by a groove, which indicates its separation into 

 halves. Anterior to it are two large processes (crura cerebri) connecting 

 it to the great brain ; on each side it is united to the small brain by a sim- 

 ilar white mass (crus cerebelli) ; and behind it is the enlarged upper part 

 of the cord. 



The cerebellum, or the small brain, is separated into two by a median 

 fissure, and each half will be subsequently seen to consist of lobes. 



The cerebrum, or the large brain, is divided into hemispheres by a 

 longitudinal fissure in the middle line ; and each half is further subdivided 

 into two by a transverse sulcus the fissure of Sylvius. In the centre of 

 the cerebrum, between the hemispheres and in front of the pons, are seve- 

 ral small bodies that will be afterwards enumerated. 



Dissection. To follow out the arteries, let the brain remain upside 

 down, and let the remains of any arachnoid membrane be removed. Hav- 

 ing displayed the trunks of the vertebral arteries on the medulla oblon- 

 gata, and those of the carotid in front near the median fissure of the large 

 brain, the student should lay bare on one side the branches to the large 



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