248 



SPECIAL PHYSIOLOGY. 



median fissure, being named the posterior median column. Opposite 

 to the lower part of the neck, and again towards its lower end, the 

 spinal cord presents an increase of substance, forming the cervical arid 

 lumbar enlargements of the cord. At its lower end, opposite the first 

 lumbar vertebra, it terminates in a point, which is fixed by a long fila- 

 ment, extending down the spinal canal, to the sacrum. The spinal 

 cord is composed of white substance externally ; but on making a 



Fig. 60. 



Fig. 60. Under surface or base of the cerebrum, and cerebellum, 

 and of the pons Varolii and medulla oblongata. also the anterior 

 surface of the spinal cord, to show the mode of origin of the cra- 

 nial nerves from the base of the brain, and that of the spinal 

 nerves from the spinal cord, a, a, cerebral hemispheres. 6, right 

 half of cerebellum, m, medulla oblongata; above this, is a trans- 

 verse white mass, the pons Varolii. c, </, the spinal cord, show- 

 ing its cervical and lumbar enlargements, and its pointed termi- 

 nation, e, the cauda equiiia, formed by the elongated roots of 

 the lumbar and sacral nerves. 1 to 9, the several cranial nerves, 

 arising from the base of the brain and sides of the medulla ob- 

 longata. Below these, on each side, are the roots or origins of 

 the spinal nerves, cervical, dorsal, lumbar, and sacral. In some 

 of these, the double root can be seen, and the swelling or gan- 

 glion on the posterior root, a x, the axillary or brachial plexus, 

 formed by the four lower cervical and first dorsal spinal nerves. 

 Z, the lumbar plexus, s, the sacral plexus, formed by the last 

 lumbar nerve, and first four sacral nerves, t, shows a piece of 

 the sheath of the cord cut open, and within it, a portion of the 

 ligamentum denticulatum which supports the cord. 



Fig. 61. A, a transverse section through the cord, to show the 

 form of the gray cornua, or horns, in the midst of the white sub- 

 stance. B, shows the same parts; and also the membranes of 

 the cord ; and the anterior and posterior roots of a pair of spinal 

 nerves springing from its sides. 



transverse section through any part of it, it is seen to contain, within 

 the external white matter, a quantity of gray matter. Quite in the 

 upper part of the cord, this is somewhat diffused ; but in the cord 

 generally, it is arranged in the form of two crescents, Fig. 61, A, one 

 in each half of the cord, and placed back to back, so that the horns of 



