CHAP, ix.] NERrOUS SYSTEM AND ORGANS OF SENSE. 257 



applied to the surface of the cerebro-spinal axis and conveys its 



minute arteries to it (Fig. 123, c). It is thicker and less vascular on 



the spinal cord than on the hrain. 



At the roots of the nerves it 



becomes continuous with the 



neurilemma. 



The third membrane is called 

 the arachnoid, and is a serous 

 membrane forming a closed sac 

 and containing a fluid. 



The outer, or parietal, wall 

 of this sac consists of a layer 

 of flattened and nucleated, poly- 

 gonal, epithelial cells, intimate- 

 ly connected with the dura 

 mater (both cranial and spinal) 

 external to it. 



The inner, or visceral, wall 

 of the sac is a distinct mem- 

 brane investing, but not inti- 

 mately, the pia mater, the 

 foldings of which it does not 

 follow a space, the sub-arach- 

 noid space, existing between 

 them and containing a watery 

 secretion. A series of attach- 

 ments, called the ligament-urn 

 denticulatum, connects the spinal 

 part of the arachnoid to the 

 spinal dura mater one existing, 

 on each side, between each pair 

 of nerves issuing from the spinal nervous axis. 



6. The SPINAL CORD, or MYELON, is the more or less cylindrical 

 mass of nervous matter, of varying dimensions, enclosed within the 

 neural canal of the spinal column, extending backwards from the 

 margin of the foramen magnum of the skull. 



It remains of considerable size thence backwards till it reaches 

 the hinder part of the lumbar region and sacrum, where it contracts 

 to a slender filament, the filum terminate, which extends on into 

 the tail. 



The general form of the spinal cord is cylindrical, but it is a little 

 flattened from above downwards. It becomes somewhat broadened 

 out in two places. The first of these, called the cervical enlarge- 

 ment, extends from the third cervical to the first dorsal vertebra. 

 The second, termed the lumbar enlargement, is situated at the last: 

 dorsal vertebra, whence the myelon tapers till it ends in the " filum 

 terminale." 



The cord is traversed by a deep median fissure both below and 



Fig. 123. BRAIN IN SITU, THE UPPER PART OF 

 THE SKULL BEING REMOVED. 



On the right side all the membranes are re- 

 moved. The pia mater (with its vessels) 

 remain on the left side. 



c. Cerebrum invested by pia mater. 



ce. Cerebellum. 



cr. Crucial sulcus. 



/. Frontal bone. 



fs. Frontal sinus. 



i. Superior lateral gyrus. 



m. Middle lateral gyrus. 



s. Inferior lateral gyrus. 



z. Zygoma. 



