G68 TEE CENTBAL AXIS OF TEE NERVOUS SYSTEM. 



little beyond that, and giving rise at each side, during its course, to the 

 superior and inferior roots of the spinal nerves. 



Weight— In medium- sized animals the weight of the cord is represented 

 by the following approximate numbers : for the Horse, 10^ ounces ; the Ass, 

 51 ounces ; Cow, 7f ounces ; Sheep and Goat, If ounce ; Pig, 2^ ounces ; 

 Dog, II ounces ; Cat, 4^ drams. 



Figure and volume. — The medullary cord is slightly depressed above and 

 below, throughout its whole length; in whatever part we examine a 

 transverse section of it, we will always find the lateral diameter greater than 

 the vertical, and that this section appears regularly elliptical. 



Its volume is far from being uniform. In following it from before to 

 behind, we at first remark that it presents the same dimensions to the fifth 

 cervical vertebra, and that between this point and the second dorsal vertebra 

 it forms an oblong enlargement, designated the brachial (hracMo-racMdian) hulb 

 or enlargement. Beyond this, it assumes its primitive volume, and becomes 

 gradually smaller even than in the cervical region. Towards the middle of 

 the loins, it again augments to constitute the crural (lumbo-rachidian') hulb 

 or enlargement, which extends to the entrance of the sacral canal. After 

 this dilatation comes a conical prolongation, whose point represents the 

 terminal extremity of the cord. 



If we compare the diameter of this medullary axis with that of the 

 spinal canal, we will observe, as has been already said, that the capacity of the 

 containing favity is generally related to the volume of its contents, and that 

 the former is, as a rule, most capacious at the cervical and lumbar enlarge- 

 ments. It wUl even be noticed that the dilatation the spinal canal offers at 

 these two points, is relatively more considerable than the excess in volume of 

 the cord. This is because the mobility of the spine, which is justly very 

 great in these two regions, requires this difference to secure the spinal axis 

 from contusions during the movements executed by the vertebral column. 

 This protective combination is also found elsewhere : at the atlas, for 

 example, where we know the motion is considerable ; and in the entire 

 extent of the cervical region, which in this respect greatly exceeds the 

 dorsal region. 



External surface of the cord. — Covered by the pia mater, this surface 

 presents an extremely simple disposition. On its superior and inferior 

 planes, at each side, we remark the double series of sensitive and motor 

 roots of the spinal nerves, which are implanted in the same longitudinal 

 line to right and left of the median plane, and are collected in fasciculi 

 opposite the intervertebral foramina. 



In the middle line, and throughout the entire length of the organ, there 

 are two deep and narrow fissures : one superior (fissura longitudinalis 

 superior), the other inferior {fissura longitudinalis inferior), into which the 

 pia mater enters. Four other fissures have been described at the point of 

 emergence of the nerve roots, under the names of superior and inferior 

 collateral fissures (or sulci) ; but the two superior alone exist, and even 

 these are often scarcely noticeable. 



INTEENAL CONFOEMATION AND STEUOTUKE OP THE SPINAL COED. 



In making a transverse section of any portion of the cord, we may 

 convince ourselves that it has an internal cavity. This central canal is 

 elliptical, and lined by cylindrical ciliated epithelium, resting on a thin 

 connective membrane, the ependymis of Virchow. This section also shows 



