194 ELEMENTS OF MAMMALIAN ANATOMY. 
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 ante- 
rior 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, probably 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. Two deep 
fissures, the anterior median and the posterior median, 
penetrate about one-third through the cord, incompletely 
dividing it into halves longitudinally. The pia mater is 
prolonged into the anterior fissure, but not into the pos- 
terior fissure. Slightly laterad of the anterior median 
fissure issue the anterior roots of the spinal nerves, and at 
about the same distance from the posterior median fissure 
are the posterior roots of the spinal nerves. These two 
roots unite about one centimeter from their origin (Fig. 
93). On the posterior or sensory root is a small ganglion 
located very near the junction of the two roots. The 
common spinal nerve, formed by the union of the two roots, 
almost immediately divides into four branches, one of 
which, the dorsal, supplies the muscles and skin along the 
vertebral column; a second, the ventral branch, supplies the 
limbs or intercostal spaces; while the other two branches, 
