NEUROLOGY. 111 
or horn of the grey matter. The anterior roots of the nerves are connected 
with them, and the posterior roots adhere to them when the cord is split up 
along the plane of the posterior horn. The posterior columns are small, 
triangular in section, and placed in opposition with each other by their inner 
surfaces. 
The spinal nerves. There is a pair of spinal nerves for each interverte- 
bral foramen, and for that between the axis and occiput. There are thus 
thirty-one pairs of nerves haying their origin from the spinal cord, this 
number being exclusive of the spinal accessory nerve, which is connected 
with the upper part of the cervical region. Hach spinal nerve has its origin 
from the cord, by two roots, of which the anterior is distinctly inferior in 
size to the posterior. The ligamentum articulatum (a narrow longitudinal 
band on each side of the cord, and intervening between the dura mater and 
the pia mater, attached to the latter by tooth-like triangular processes) is 
placed between these roots. Each root passes out through a distinct open- 
ing in the dura mater. Immediately after its emergence a ganglion is 
formed on each posterior root, and the anterior root lies imbedded in the 
anterior surface of the ganglion, and involved in the same sheath, but with- 
out mingling its fibres with those of the ganglion. Beyond it, the nervous 
fibres of both roots intermingle, and a compound spinal nerve results. The 
trunk thus formed passes immediately through the intervertebral tube, and 
divides into an anterior (and usually larger) and a posterior branch, which 
are distributed to the muscles and integument of the trunk and the extre- 
mities. 
The anterior roots derive their fibres wholly from the antero-lateral 
columns. Of these fibres, it is probable that some are continuous with the 
longitudinal fibres of the cord, and that others pass into the grey matter. 
The posterior roots adhere to the posterior part of the antero-lateral columns, 
and derive their fibres chiefly from that source. It is probable, though by 
no means certain, that the fibres of the posterior roots have a disposition 
similar to that described for the anterior, and that while some pass into the 
posterior horn of the grey matter, others are continuous with the longitudi- 
nal fibres. 
In conclusion, the spinal cord may be said to be of a cylindroidal shape, 
slightly flattened on its anterior and posterior surfaces. At its inferior 
extremity it gradually tapers to a pomt. In the cervical region there is a 
swelling or enlargement, which begins a short distance beneath the medulla 
oblongata, and gradually passes into the dorsal portion, which is the smallest 
as well as the most cylindrical portion of the cord. This cervical enlarge- 
-ment begins opposite the third cervical vertebra, and ends about the third 
dorsal. About the ninth or tenth dorsal vertebra, the lumbar swelling occurs, 
occupying a space corresponding to about two vertebre. The cervical 
swelling corresponds to the point at which the nerves of the upper extremi- 
ties are given off, and the lumbar has the same relation to the lower limbs. 
Pl. 187, fig. 3, spinal marrow and pons from the anterior surface: ', dura 
mater; *, ligamentum dentatum; ***, single dentations of do.; *, a spinal 
nerve. Fg. 4, a portion of the spinal marrow represented on a larger 
ICONOGRAPHIC ENCYCLOPADIA.— VOL. II. 52 817 
