456 THE TISSUES. 



I. STEUCTUEE OF THE SPINAL COED. 



While the white substance of the cord is composed almost exclu- 

 sively of nerve-fibres, the gray portion is formed, in almost equal 

 proportions, of nerve-fibres and cells. A section of the cord shows 

 the gray matter forming a column in the central part of each half of 

 the cord (Fig. 302), whose transverse section is of a crescentic form, 

 its two extremities being termed the anterior and the posterior horns. 

 The white substance surrounds and incloses the gray on every side, 

 except towards the median line, where the gray matter projects 

 through the white, and thus comes into contact from the opposite 

 sides, constituting the gray commissure. This does not normally, in 

 man, contain any canal in its centre, but is constituted principally 

 of nerve-cells of a yellowish color, and is called the substantia grisea 

 centralis. The white matter in the two halves of the cord merely 

 comes nearly into contact behind the gray commissure ; but in front 

 of the latter it is continuous from one side to the other, constituting 



' O 



the anterior or white commissure. The portion of each half of the 

 cord between the anterior median fissure and the anterior roots of 

 the spinal nerves, is termed the anterior column; that between the 

 posterior median fissure and the posterior roots, the posterior co- 

 lumn; and the remaining portion between the anterior and posterior 

 roots, the lateral column. At the extremity of the posterior horns 

 of the gray matter is a more transparent layer, containing a prepon- 

 derance of smaller nerve-cells the substantia gelatinosa of Roland. 

 The gray columns vary considerably in their size and form in dif- 

 ferent parts of the cord, as shown by transverse sections; it being 

 most abundant in the lumbar, and next in the cervical region. 



1. On examining the intimate structure of the cord, we find in 

 the white matter two sets of fibres, the longitudinal and the hori- 

 zontal, or transverse. 



The longitudinal fibres are found in all situations except the ante- 

 rior commissure, are unmixed with horizontal fibres in every part, 

 as a rule, and everywhere run parallel to each other, without either 

 interlacement or being collected into smaller fasciculi. They in- 

 crease from below upwards, since they successively pass inwards 

 towards the 'gray matter in their descent. They average g ^ U75 to 

 ffq-Q of an in en, and the size of each fibre remains very nearly the 

 same in the white substance ; no divisions or other alterations in 

 diameter being found. These fibres are probably continuous with 



