CEKKBRO-SPESTAL AXIS. 259 



membrane, in two layers, composed chiefly of inelastic tis- 

 sue, which, lines the cranial cavity and is adherent to the 

 bones. In certain situations, its two layers become sepa- 

 rated and form what are known as the venous sinuses. The 

 dura mater also sends off folds or processes of its internal 

 layer ; one of these passes into the longitudinal fissure, and 

 is called the falx cerebri ; another lies between the cerebrum 

 and the cerebellum, and is called the tentorium; another 

 is situated between the lateral halves of the cerebellum, and 

 is called the falx cerebelli. The dura mater is closely at- 

 tached to the bone at the border of the foramen magnum, 

 From this point, it passes into the spinal canal and forms a 

 loose covering for the cord. In the spinal canal, this mem- 

 brane is not adherent to the bones, which have, like most 

 other bones in the body, a special periosteum. At the fora- 

 mina of exit of the cranial and the spinal nerves, the dura 

 mater sends out processes which envelop the nerves, with 

 the fibrous sheaths of which they soon become continuous. 



The arachnoid is an excessively delicate serous membrane, 

 in two layers, the surfaces of which are nearly in contact. The 

 external layer lines the internal surface of the dura mater. 1 

 Like the other serous membranes, the arachnoid is covered 

 with a layer of tessellated epithelium. There is a small 

 amount of liquid between the two layers of the arachnoid ; 

 but by far the greatest quantity of liquid surrounding the cere- 

 bro-spinal axis lies beneath both layers, in what is called the 

 subarachnoid space. This is called the cerebro-spinal, or 

 cephalo-rachidian fluid. The fact that it exists in greatest 

 quantity beneath both layers of the arachnoid was first 

 pointed out by Magendie. 3 The arachnoid does not follow 

 the convolutions and fissures of encephalon or the sulci of 



1 According to Kolliker, the arachnoid consists of a single layer, the layer 

 attached to the dura mater being not properly a membrane, but simply an 

 epithelial covering (Handbuch der Gewebelehre, Leipzig, 1867, S. 308). 



2 HAGEXDIE, JRecherches physiologiques et cliniques sur le liquide cephalo- 

 rachidien, Paris, 1842. 



