THE NERVOUS SYSTEM. 205 



THE SPINAL CORD. 



The spinal cord extends from the foramen magnum 

 through the vertebral canal. It is more or less cylindrical 

 throughout and has a diameter of about one centimeter 

 except in the lumbosacral region, where it grows gradu- 

 ally smaller until it is only one or two millimeters in 

 diameter. In order to study the cord, one should have 

 a mounted transverse microscopic section, an entire cord 

 in situ with the dorsal wall of the vertebral canal re- 

 moved, so that the exit of the spinal nerves may be seen. 



The membranes which envelop the spinal cord are the 

 same as those that envelop the brain. The dura mater 

 is the external tough coat; the pia mater, the internal 

 delicate vascular coat sending a process deep into the 

 anterior fissure on the ventral side; and the arachnoid, 

 the very thin membrane between the two preceding. 

 The arachnoid lies close against the dura mater, but is 

 separated from the pia mater by the subarachnoid space, 

 which is filled with a serous fluid called the cerebrospinal 

 fluid. It is apparently the same as that in the ventricles 

 of the brain, and seems to be of a lymphoid nature. 

 In fact, the subarachnoidean space is merely a large 

 lymph space similar to the cavities of the pleura and 

 peritoneum. This lymph, or cerebrospinal fluid, proba- 

 bly escapes from the thin walls of the capillaries covering 

 the pia mater and is taken up by lymphatic vessels which 

 begin in open mouths on the walls of the space. 



The cord presents two enlargements, the cervical, 

 whence issue the nerves of the forelimbs, and the lumbar, 

 giving origin to the nerves of the posterior limbs. Tw r o 

 deep fissures, the anterior median and the posterior 

 median, penetrate about one-third through the cord, 

 incompletely dividing it into halves longitudinally. The 



