716 THE CEREBELLUM 



CHAPTER LVIII 



THE PROTECTIVE MECHANISMS OF THE NERVOUS 



SYSTEM 



SLEEP AND NARCOSIS 



The Enveloping Membranes. — The encephalon is contained in a 

 rigid box formed by the cranial bones. The periosteal lining of the 

 latter is displaced by the dura mater, a strong fibrous membrane 

 which is lined internally with endothelial cells, and sends firm parti- 

 tions inward for the support and protection of the different parts of 

 the brain. A membranous process of this kind invades the great 

 longitudinal fissure separating the hemispheres of the cerebrum. 

 It is called the falx cerebri, because it possesses the shape of a sickle, 

 being narrow in front and broad behind. Another, the tentorium 

 cerebelli, extends transversely across between the cerebrum and cere- 

 bellum, while a third, the falx cerebelli, dips into the fissure between 

 the cerebellar hemispheres. In several places, the dura mater is 

 split into two layers for the reception of the sinuses which return the 

 blood from the brain. In the spinal canal, the dura is not attached 

 to the bone, but forms a long extended sac which closely invests the 

 spinal cord and is held in place by the prolongations which pass out- 

 ward to invest the individual spinal roots. Its outside surface is 

 covered with networks of veins. 



Directly underneath the dura and in intimate contact with it, 

 lies a delicate transparent membrane, known as the arachnoid. Its 

 outer surface is covered with endothelial cells and borders upon the 

 subdural space which, in reality, is of capillary size and does not seem 

 to have a special functional significance. Its under surface is placed 

 in relation with the pia mater, but in such a way that a distinct cleft 

 arises between them which is known as the subarachnoidal space. 

 The latter is intersected by fine fiber connections and septa of con- 

 nective tissue, the meshes of which are filled with a lymph-like fiuid. 

 It becomes especially conspicuous over the different sulci for the 

 reason that the pia mater follows the surface of the brain into these 

 furrows, while the arachnoid and dura pass directly across them in the 

 form of bridges. In certain localities, however, the subarachnoid 

 space is much increased in size, forming here the so-called cisternse which 

 in turn are connected with one another by delicate canals. Reflections 

 of the arachnoid frequently dip into the fissures. One of these is 

 found between the cerebral hemispheres and the third ventricle, 

 where it extends into the lateral ventricle, becoming covered on one 

 side by the ependyma of this cavity, and, on the other, by the epen- 

 dyma of the roof of the third ventricle. It envelops a rich network 



