ON THE SPIRAL MARROW AND SPINAL NERVES. 183 
anti you have the most perfect illustration of benevolent contriv¬ 
ance of which you can form any conception. Compare the in¬ 
crease of this cellular interposed substance in the spinal canal 
of the quadruped in whom the rapidity and extent of the changes 
in the direction and calibre of this canal, whether in the natural 
motion of the animal or the forced exertion which we sometimes 
brutally extort from him, require additional provision against in¬ 
jury —compare, I say, the relative proportion of this interposed 
substance in the horse and in the ox, both within and without the 
theca, and our subject is complete. 
The Ligament urn Denticulatum. —This interposed cellular sub¬ 
stance assumes a more condensed form at the side of the spinal 
chord, and is there closely connected with the pia mater within, 
and, by a line of tooth-shaped prolongations, with the dura mater 
without; hence it is termed the ligamentum denticulatum. It is 
not, however, any separate ligament, but a condensation of the 
arachnoid membrane or interposed substance, and which is thus 
deprived of its cellated structure and fluid contents. I need not 
point out to you the additional security to the spinal chord which 
this affords,"by limiting the motion of the chord in a way which 
the extensible strings above and the elastic cushion below could 
not effect; and by giving sufficient firmness of attachment to 
prevent laceration of the roots of the nerves in sudden and forci¬ 
ble flexion of the spine. It passes, in its condensed form, be¬ 
tween the superior and inferior root of the spinal nerves, being 
attached to the pia mater during its whole extent, and to the 
dura mater in the intervals between the nerves. 
The sub-occipital nerve, or Jirst, cervical nerve, or first nerve of 
the spine, arises, as I have described, at the very commencement 
of the medulla spinalis, and from the lateral furrow on its superior 
and inferior surfaces. Proceeding from this furrow, its roots are 
referrible to, and can be traced from, the central column above and 
below. The fasciculi from the superior surface are few, and com¬ 
paratively large, and they unite and form, or pass through a 
ganglion; those from the inferior surface are smaller and more 
numerous—(they err who speak of any definite number—that 
varies in different nerves and different subjects); they are more 
superficial, more easily lacerable ; they converge, unite, join the 
other branch beyond the ganglion, and then, escaping from the 
•dura mater, we have the perfect compound nerve of sensation and 
motion. Having pierced the dura mater, it emerges from the 
spinal cavity through a foramen in the anterior part of the atlas; 
and no sooner is its escape accomplished, than it divides into two 
main trunks, which subdivide and ramify in every direction, and 
