MEN1NGES OR MEMBRANE OF THE BRAIN. 131 



their reunion the medullary fascicles. Opinions are not 

 fixed on the mode by which union is effected between the 

 nervous roots and the spinal cord, and we confine ourselves 

 to the simple statement that the cord appears to consist of 

 a greater number of nervous fibres than the nerves which 

 spring from it. 



The isthmus of the encephalon is formed by the expansion 

 of the superior portion of the spinal cord; it is the central 

 point of the greater and lesser brain, of which the hemispheres 

 of the cerebrum and cerebellum are but the terminal de- 

 velopments. 



Meninges or membranes. The three membranes, placed 

 one above the other, which line the cranium and the vertebral 

 canal, which envelop the encephalon and the spinal cord, 

 and extend to all their inequalities, are designated by this 

 term. They are divided into the dura mater, the arachnoid, 

 and the pia mater* The name mater, which is given to the 

 external and internal meninges, appears to have been derived 

 from the Arabs, who thus designated the covering of any body 

 whatever. 



The dura mater is a resistant, fibrous membrane, which 

 lines the cavity of the skull and of the spinal canal. It 

 adheres very slightly to the walls of this canal, but more 

 strongly to the arch of the skull, and closely to its base. 

 The arachnoid and the pia mater are interposed between the 

 dura mater and the nervous centre. It is separated from the 

 spinal cord by a space filled with the fluid peculiar to the 

 spinal column ; but it is, on the contrary, directly in contact 

 with the encephalon, some portions of which it holds in 

 place. 



On the median line of the cranial arch is a triangular 

 canal, circumscribed by the dura mater, called the superior 

 longitudinal sinus, which performs the functions of a large 

 vein. Below this sinus it forms a fold or vertical partition, 

 which is called \hzfalx cerebri (falx, a sickle), which descend- 

 ing into the great fissure, separates the cerebral hemispheres. 

 In its lower border is the inferior longitudinal sinus. Between 

 the posterior lobes of the cerebrum and the cerebellum the 

 dura mater covers the latter with the tentorium or tent of the 



