762 A MANUAL OF PHYSIOLOGY 



logical series, it will be well to begin with the spinal nerves, 

 since their motor and sensory fibres are gathered into different 

 and definite roots, whose course within the cord is, in general, 

 more easily traced than the course of the cerebral root-bundles 

 within the brain. 



Arrangement of the Grey and White Matter in the Spinal 

 Cord. The grey matter of the spinal cord is arranged on 

 each side in a great unbroken column of roughly crescentic 

 section, joined with its fellow across the middle line by a grey 

 bar or bridge, which springs from the convexity of the crescent, 

 and is pierced from end to end by the central canal. The 

 anterior horn of the crescent, although it varies in shape at 

 different levels of the cord, is, in general, broad .and massive, 

 in comparison with the slender and tapering posterior horn. In 

 the lower cervical and upper dorsal region a moulding or pro- 

 jection, forming a lateral horn, springs from the fluted outer 

 side of the grey substance. Within the grey matter nerve- 

 cells are found, sometimes so regularly arranged that they form 

 veritable cellular or vesicular strands. Of these the best marked 

 are : (i) The tract or tracts made up by the cells of the anterior 

 horn (Fig. 313), which practically run from end to end of the cord, 

 swell out in the cervical and lumbar enlargements, where the 

 cells are very numerous and of great size (70 p to 140 // in 

 diameter), and contract to a thin thread in the thoracic region, 

 where they are relatively few, scattered, and small. In the 

 enlargements there are several groups of these cells corresponding 

 with the segments of the limbs, the movements of the hand, 

 forearm, and upper arm being each represented by a group in 

 the cervical, and those of the foot, leg, and thigh by groups in 

 the lumbar swelling. In the rest of the cord only two well- 

 marked groups of cells are present in the anterior horn, a mesial 

 and a lateral. (2) Clarke's column, whose cells, mostly of good 

 size and somewhat rounded in outline, are situated at the inner 

 side of the root of the posterior horn just where it joins on to the 

 grey cross-bar. It gradually increases in size from above down- 

 wards, usually appearing first at the level of the seventh or 

 eighth cervical nerve, attaining its maximum development at 

 the eleventh or twelfth dorsal and disappearing altogether, as a 

 continuous strand, at the level of the second or third lumbar 

 nerves. Scattered nerve-cells, however, constituting the so- 

 called cervical and sacral nuclei of Stilling, are frequently found 

 occupying the same position towards the upper and lower ends 

 of the cord, and may be looked upon as isolated portions of 

 Clarke's column. (3) A tract of small cells called the intermedia- 

 lateral tract, lateral cell column, or lateral horn, situated at the outer 

 edge of the grey matter, about midway between the anterior and 



