THE SPINAL CORD— GENERAL. 477 



tion of the muscles referred to, is, we are convinced, the «ase. 

 To say that it is either entirely automatic or purely reflex, or 

 that the whole of the facts would be covered even by any com- 

 bination of these two processes, would probably be unjustifiable. 

 The influence of the centers over the metabolism of parts 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. Neverthe- 

 less, 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. 



