648 FUNCTIONS OF THE CEREBRO -SPINAL NERVOUS SYSTEM. 



remaining unaltered. 1 That an actual continuity of nerve-fibres, however, 

 is not requisite for the establishment of those connections between excitor 

 and motor nerves, in which the central organs take part, seems probable 

 from the fact, that under particular circumstances we find the influence of 

 such impressions radiating in every direction, and extending to nerves which 

 they do not ordinarily affect. Still there can be no doubt that the nerve- 

 force is disposed to pass in special tracks; and it seems probable, that whilst 

 some of these are originally marked out for the automatic movements, others 

 may be gradually worn in (so to speak) by the habitual action of the Will ; 

 and that thus, when a train of sequential actions primarily directed by the 

 Will has been once set in operation, it may continue without any further 

 influence from that source. 



512. Another manifestation of the independent power of the Spinal Cord 

 is seen in its influence on Muscular Tension. The various muscles of the body, 

 even when there is the most complete absence of effort, maintain in the 

 healthy state of the system a certain degree of firmness, by their antagonism 

 with each other ; and if any set of muscles be completely paralyzed, the 

 opposing muscles will draw the part on which they act out of its position of 

 repose; as is well seen in the distortion of the face which is characteristic of 

 paralysis of the facial nerve on one side. This condition has been desig- 

 nated as the tone of the Muscles; but this term renders it liable to be con- 

 founded with their tonic contraction,, which is also concerned in maintaining 

 their firmness, but which is a manifestation of the simple contractility of 

 their tissue, and is exhibited alike by the striated and the non-striated forms 

 of muscular fibre, but more especially by the latter. On the other hand, 

 the condition now alluded to, which may perhaps be appropriately termed 

 their tension, is the result of a moderate though continued excitement of that 

 contractility, through the nervous centres. It has been proved by Dr. M. 

 Hall that the Muscular Tension is dependent, not upon the influence of the 

 Brain, but upon that of the Spinal Cord, as the following experiments de- 

 monstrate : "Two Rabbits were taken: from one the head was removed; 

 from the other also the head was removed, and the spinal marrow was 

 cautiously destroyed with a sharp instrument : the limbs of the former re- 

 tained a certain degree of firmness and elasticity; those of the second were 

 perfectly lax." Again : "The limbs and tail of a decapitated Turtle pos- 

 sessed a certain degree of firmness or tone, recoiled on being drawn from 

 their position, and moved with energy on the application of a stimulus. On 

 withdrawing the spinal marrow gently out of its canal, all these phenomena 

 ceased. The limbs were no longer obedient to stimuli, and became perfectly 

 flaccid, having lost all their resilience. The sphincter lost its circular form and 

 contracted state, becoming lax, flaccid, and shapeless. The tail was flaccid, 

 and unmoved on the application of stimuli." 2 It is further remarked by 

 Messrs. Todd and Bowman, that "a decapitated frog will continue in the 

 sitting posture through the influence of the spinal cord; but immediately 

 this organ is removed, the limbs fall apart."- -This operation of the Spinal 

 Cord is doubtless but a peculiar manifestation of its ordinary reflex function. 

 A curious experiment by M. Brown-Sequard :! shows that this effect is not due 

 to loss of contractility in the muscles; for although, after division of the 

 Spinal Cord, there is a temporary diminution in their power, after a short 

 interval the muscles, when stimulated to contract, will not_ only raise an 

 equal but even a greater weight than before, the effect lasting in healthy 



1 See Dr. "Waller's important researches on the Reproduction of Nervous Sub- 

 stance, in Mailer's Archiv, 1852, Heft iv. 



2 Memoirs on the Nervous System, 1837, p. 93. 3 Comptes Rendus, 1847. 



