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. p 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, 



