THE NEKVOUS SYSTEM. 435 



as the "gray commissure," in the centre of which is a narrow longi- 

 tudinal canal, the " central canal," but little over 0.2 millimetre in di- 

 ameter, and lined internally with epithelium. 



The anterior and posterior portions of gray substance, in each lateral 

 half of the cord, are called the anterior and posterior horns. Imme- 

 diately in front of the gray commissure is a transverse band of white 

 substance, the " white commissure" of the cord. 



The spinal nerves originate from the cord on each side by two dis- 

 tinct sets of fibres, forming the anterior and posterior roots. The 

 anterior root (Fig. 148, d) passes out from the surface of the cord near 

 the extremity of the anterior horn of gray matter. The posterior root 

 (e) originates at a point corresponding with the posterior horn of gray 

 matter. Both roots are composed of a considerable number of fibres, 

 united with each other in parallel bundles. The posterior root is dis- 

 tinguished from the anterior by the presence of a small rounded mass 

 of gray matter, or ganglion, with which it is incorporated and through 

 which its fibres pass. The two roots unite with each other soon after 

 leaving the cavity of the spinal canal, and mingle their fibres in a common 

 trunk. 



The white substance of each lateral half of the spinal cord is thus 

 divided into three portions or " columns ;" so called because the nerve 

 fibres composing them run, for the most part, parallel with each other, 

 in a longitudinal or vertical direction. The portion which is included 

 between the anterior median fissure and the origin of the anterior nerve 

 roots is the anterior column ; that between the anterior and posterior 

 nerve roots is the lateral column; while that between the posterior 

 nerve roots and the posterior median fissure is the posterior column. 

 As the posterior median fissure penetrates deeply into the substance of 

 the cord, quite down to the gray substance, the posterior columns ap- 

 pear entirely separated from each other in a transverse section ; but the 

 anterior median fissure is more shallow and stops short of the gray 

 matter, so that the anterior columns are connected with each other by 

 the white commissure above mentioned. 



The brain, or "encephalon," is that portion of the cerebro-spinal 

 system contained in the cranial cavity. It forms a more or less 

 rounded mass of nervous matter, consisting, as in the spinal cord, of 

 right and left lateral halves which remain connected with each other by 

 their central parts. In man and the higher vertebrate animals, it pre- 

 sents, above and behind, two principal divisions, namely, the cerebrum 

 and cerebellum, which are composed externally of a convoluted layer of 

 gray substance, these two divisions together forming at least nineteen- 

 twentieths of the whole encephalon ; while beneath them is a smaller 

 portion composed externally of white substance, like the spinal cord, and 

 forming the communication between the cord below and the brain above. 

 This inferior portion is called the "isthmus," and comprehends the 

 medulla oblongata, the tuber annulare, and the peduncles of the cere- 

 brum. Beside, however, the portion visible externally, there are, in 



