STRUCTURE OF THE SPINAL CORD. 599 



of the nerves on either side; but this can scarcely be said to have a real ex- 

 istence; and the separation of the "antero-lateral columns" into the "ante- 

 rior" and the "lateral" columns (A, A and L, L) is only marked externally 

 by the attachment of the nerve-roots. It is made more obvious internally, 

 however, by the peculiar distribution of the fjnnj matter, which, though by 

 no means uniform throughout the Cord, usually presents (in a transverse 

 section) the form of two somewhat crescent-shaped masses, whose convexities 

 are turned towards each other, and are connected by the gray commissure, 

 whilst their cornua are directed toward the surface of the cord. The poste- 

 rior peak on each side nearly reaches the posterior lateral furrow, whilst 

 the anterior, though the larger cornu, does not approach quite so near the 

 surface. The gray matter is enveloped by the white substance of the columns, 

 which are entirely composed of nerve-tubes, whose general direction is longi- 

 tudinal. The Spinal Corel of Man is by no means of uniform dimensions in 

 every part of its length ; and the proportions which the gray and white sub- 

 stances bear to one another in different parts, are extremely diverse. Two 

 principal enlargements are seen in the cervical and lumbar regions, at the 

 origins of the large nerves forming the brachial and crural plexuses; and 

 these enlargements are chiefly due to an increase of the gray substance, 

 which is comparatively deficient in the intervals. On the other hand, there 

 is a regularly progressive increase in the white substance, as we proceed from 

 the lower to the higher portion of the cord ; and this fact of itself serves to 

 indicate the probability, that the longitudinal columns serve (as formerly 

 supposed) to establish a direct connection between the Encephalic ceutres 

 and the roots of the Spinal nerves. 



481. The gray matter or vesicular substance of the Spinal Cord, which is 

 best seen in transverse section (Figs. 207 and 208), is by no means uniform 

 in its texture throughout. Its anterior cornua, which are thicker and shorter 

 than the posterior, are of a uniform gray color; and they consist of large 

 well-developed nerve-cells (d, Fig. 207 and /, Fig. 208), svhich usually pre- 

 sent many radiating processes, one of which always passes into the anterior 

 root of the nearest spinal nerve ; whilst an.oth-er ascends on its own side, and 

 others form communications either with adjoining cells or with those in the 

 horn, or on the opposite side of the Cord. The cells of the anterior horn 

 connected together in this way form groups which may reasonably be sup- 

 posed to constitute centres for the numerous fibres distributed to each muscle, 

 so that an impulse of the will or other stimulus acting upon one of the cells 

 may excite the whole to equal and consentaneous action, and thus abolish 

 the necessity that would otherwise exist for every cell to possess its own motor 

 fibre. The central portion, which contains the canal, and which also forms 

 the commissure, has a similar composition ; but the cells are smaller, though 

 still having long branching radiations; and the fibres are extremely fine, 

 their tubularity being often indistinguishable. The anterior portion of the 

 commissure, however (n, Fig. 208, i, Fig. 209), is purely fibrous, and is dis- 

 tinguished by some as the "white commissure;" it does not, however, form 

 an immediate connection between the two anterior columns, but brings each 

 of them, as will be presently seen, into relation with the vesicular matter of 

 the anterior coruu of the opposite side. The posterior cornu (Fig. 208, g h 

 /./), longer and narrower than the anterior, is divided by Mr. Lockhart 

 Clarke into two parts, the Caput and the Cervix Cornu. The Caput Cornu 

 (gh), or bulbous extremity of the horn, consists of an outer and compara- 

 tively transparent gelatinous portion (g') } the substantia gelatiuosa of Rolando, 

 and of an inner and more opaque portion (h). The former contains nerve- 

 cells of various shape and size, often multicaudate, the caudate prolonga- 

 tions being continuous with nerve-tubules of small average size (varying 



