THE CENTRAL NERVOUS SYSTEM. 



1029 



Fig. 886. 



I c- 



Each crescent of gray matter is divisible into three parts — the ventral and the 

 dorsal extremity, that. project beyond the transverse gray commissure and constitute 

 the anterior ^xiA posterior horns or cornua of the gray matter (columnae griseae), and 

 the intermediate portion (pars intermedia) that connects the cornua and receives the 

 commissure. The two horns differ markedly from each other and, although varying 

 in details in different levels, retain their distinctive features throughout the cord. 



The anterior cornu (columna grisea anterior) is short, thick and rounded, and 

 separated by a considerable layer of white matter from the surface of the cord, through 

 which the ventral root-fibres proceed to their points of emergence in the root-areas. 

 The blunt tip of the anterior horn is known as the caput cornu, and the dorsal por- 

 tion by which it joins the commissure and the pars intermedia as the basis cornu. 



The posterior cornu (columna grisea posterior) presents a marked contrast in 

 being usually relatively long, narrow and pointed, and 

 in extending peripherally almost to the postero-lateral 

 sulcus. The tip or apex of the dorsal horn is formed 

 of a A-shaped stratum of peculiar character, the sub- 

 stantia gelatinosa Rolandi, that appears lighter in 

 tint (Fig. 885) and somewhat less opaque than the 

 subjacent and broader portion of the horn, caput cornu, 

 which it covers as a cap. More ventrally the posterior 

 horn is usually somewhat contracted, to which portion 

 the term, cervix cornu (cervix columnae posterioris) is 

 applied. In the lower thoracic cord, however, this 

 constriction is replaced by a slight bulging located on 

 the mesial side of the junction of the posterior cornu 

 with the gray commissure. This enlargement corres- 

 ponds to the location of a longitudinal group of nerve- 

 cells constituting the column of Clarke. 



The fairly sharp demarcation between the gray 

 and white matter is interrupted along the lateral border 

 of the crescent by delicate prolongations of gray matter 

 into the surrounding lateral column (Fig. 888). The 

 subdivisions of these processes unite to form a reticulum 

 of gray matter, the meshes of which are occupied by 

 longitudinally coursing nerve-fibres, the whole giving 

 rise to an interlacement known as \k\Q processus ox for- 

 matio reticularis. Although to some extent present 

 in the greater part of the cord, this structure is most 

 marked in the upper cervical region, where it exists as 

 a conspicuous net-work filling the recess that indents 

 the lateral border of the pars intermedia and the neck 

 of the posterior horn of the gray crescent. In the 

 thoracic and upper parts of the cervical cord, therefore 

 in regions in which the enlargements are wanting, the 

 formatio reticularis is condensed into a compact process 

 of gray matter that is directed outward (Fig. 885) and 

 known as the lateral cornu (columna lateralis). 



iT- 



I L — 



I S— 



'1 



Taken as a whole, the gray matter, which in cross-sections 

 appears as the H-shaped area formed by the two crescents 

 and the commissure, constitutes a continuous column, whose 

 irregular contour depends not only upon the peculiar disposi- 

 tion of the gray matter, but also upon the variations in its 

 amount at different levels of the cord. Thus, at the level of 

 the third cervical nerve the gray matter constitutes somewhat 

 more than one-fourth of the entire area of the cord ; at that of 

 the seventh nerve about one-third, while in the thoracic region, 



between the second and eleventh nerves, it is reduced to about one-sixth. At the last thoracic 

 nerve it again forms one-fourth, and at the third and fifth lumbar two-fifths and three-fifths 

 respectively. In the sacral cord the relative amount of gray matter increases until, at the level 



Diagram showing amount of gray 

 and white matter in relation to entire 

 area of cord, and relative lengths of 

 cord-segments; the latter are indicated 

 by divisions on left margin of figure — 

 I C, I T, I L, I S, first segment of cervi- 

 cal, thoracic, lumbar and sacral regions 

 respectively ; dark zone next left bor- 

 der represents the gray matter, light 

 zone the white matter, outer dark zone 

 the entire area of cord. {Donaldson.) 



