756 THE CENTRAL AXIS OF THE NERVOUS SYSTEM. 



commencing at the occipital foramen, where it continues the medulla oblongata, 

 and terminating in a point at the upper third of the sacral canal, or a little 

 beyond that, giving off at each side, during its course, 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, 5i 

 ounces ; Cow, 7i ounces ; Sheep and Goat, If ounces ; Pig, 2^ ounces ; Dog, li 

 ounces ; Cat, 4^ drams ; and Rabbit, 3i drams. 



Shape a?id volume. — The spinal cord is slightly flattened above and below, 

 throughout its whole length ; in whatever part we examine a transverse section 

 of it, we shall always find the lateral diameter greater than the vertical, and that 

 this section appears regularly elliptical. 



The flattening is more marked in the lumbar than in any other region. 



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 cervical enlargement (or bulb). 

 Beyond this, it assumes its original volume, and even becomes gradually smaller 

 than in the cervical region. Towards the middle of the loins, it again enlarges 

 to constitute the lumbar enlargement (or bulb), which extends to the entrance of 

 the sacral canal. After this dilatation comes a conical prolongation (con us 

 medullaris), the point of which represents the terminal extremity of the cord. 



If we compare the diameter of this cord with that of the spinal canal, we shall 

 observe, as has been already said, that the capacity of the containing cavity 

 is generally related to the volume of its contents, and that the former is, as a 

 rule, most capacious at the cervical and lumbar enlargements. It will even be 

 noticed that the dilatation the spinal canal offers at these two points, is relatively 

 more considerable than the increase in volume of the cord. This is because the 

 mobility of the spine, which is very great in these two regions, requires this 

 difference to secure the spinal axis from injury during the movements executed 

 by the vertebral column. This protective combination is also found elsewhere — 

 at the atlas, for example, where we know motion is considerable ; and throughout 

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

 dorsal region. 



Means of fixture. — The cord is relatively fixed in its envelopes by: 1. Its 

 continuity with the brain. 2. The coccygeal ligament. 3. The dentated 

 ligaments. 



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 middle plane, and are collected in fasciculi opposite the intervertebral 

 foramina. 



In the middle line, and throughout the entire length of the cord, 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, by the names of superior and inferior collateral fissures (or sulci) ; but the 

 two superior alone exist, and even these are often scarcely noticeable, being 

 rather indicated by the origin of the nerve-roots. 



