THE SPINAL CORD.—GENERAL. 495 
is both constant and essential to their well-being; and in such 
a case as that now considered it may be that a certain degree 
of tonus is normal to a healthy muscle in its natural surround- 
ings in the body. 
There is now considerable evidence in favor of placing cer- 
tain centers presiding over the lower functions, as micturition, 
defecation, erection of penis, etc.,in the spinal cord of mam- 
mals, especially its lower part—which centers, if they be not 
automatic, are not reflex in the usual sense; but their considera- 
tion is better attempted in connection with the treatment of 
the physiology of the parts over which they preside. 
SPECIAL CONSIDERATIONS. 
Comparative—Among invertebrates there is, of course, no 
spinal cord, but each segment of the animal is enervated by a 
special ganglion (or ganglia) with associated nerves. Never- 
theless, these are all so connected that there is a co-ordination, 
though not so pronounced as in the vertebrate, in which the 
actual structural bonds are infinitely more numerous, and the 
functional ones still more so. From this result possibilities to 
the vertebrate unknown to lower forms; at the same time, in- 
dependent life and action of parts are necessarily much greater 
among invertebrates, as evidenced especially by the renewal of 
the whole animal from a single segment in many groups, as in 
certain divisions of worms, etc. ; 
It also follows from the same facts that a vertebrated ani- 
mal must suffer far more from injury, in consequence of this 
greater dependence of one part on another; a thousand things 
may disturb that balance on which its well-being, indeed, its 
very life hangs. It is noticeable, moreover, that, as animals 
occupy a higher place in the organic scale, their nervous sys- 
tem becomes more concentrated; ganglia seem to have been 
fused together, and that extreme massing seen in the spinal 
cord and brain of vertebrates is foreshadowed. In the chapters 
on the brain numerous illustrations of the nervous system in 
lower forms will be found. 
The fact that the brain and cord arise from the same germ 
layer, and up to a certain point are developed almost precisely 
alike, is full of significance for physiology as well as morphol- 
ogy. That original deep-lying connection is never lost, though 
functional differentiation keeps pace with later morphological 
differentiation. But even among vertebrates the spinal cord 
