278 SPECIAL PHYSIOLOGY. 



It appears probable, however, that their difference in function, that is, 

 whether they convey the effects of sensory impressions to the gray 

 matter of the cord, thence to be conducted to the brain, or whether 

 they convey the effects of stimuli to the gray matter of the cord, thence 

 to be reflected on to the efferent reflex nerves, depends upon the nature 

 of their connections with the cells of the gray matter. Neither can 

 any structural difference be found amongst the efferent fibres of the 

 anterior roots ; and it is presumable, therefore, that the particular ef- 

 ferent fibres, concerned in voluntary movements, and in reflex involun- 

 tary movements, owe their special mode of action to the fact, that the 

 former descend continuously from the cerebrum (that is, from the cor- 

 pora striata), whilst the latter probably originate in connection with 

 the nerve cells of the gray matter of the cord. It appears, moreover, 

 both from experiment and from observation in cases of disease, that 

 the posterior white columns of the cord, and the contiguous gray mat- 

 ter, the nerve cells of which are peculiar, may have some special rela- 

 tion to the reflex function. The various instances in which the spinal 

 cord acts as a reflex nervous centre, and some further points concern- 

 ing these reflex movements, will be hereafter considered. 



It remains to be noticed, that the spinal cord exercises, probably by 

 a continuous reflex action, a permanent influence on the muscles, upon 

 which the so-called tonicity, or tonic state, of those organs, depends ; 

 for when, in an animal, the cord is destroyed, or carefully removed 

 from the spinal canal, all the muscles become atonic or flabby, and the 

 habitually contracted condition of the sphincters is lost. The rigor 

 martis, nevertheless, takes place as usual. This tonicity, or tension, 

 is supposed to depend on a slight but constant exercise of a stimulus, 

 originating in the cord, upon the entire muscular system. It does not 

 depend on the brain, for decapitated animals will retain their position ; 

 but the limbs immediately become flaccid and fall asunder, when the 

 cord also is destroyed. It is from loss of this tonicity in the paralyzed 

 muscles of the face, that, independently of the will, the features are 

 drawn over to the opposite side by the still healthy muscles. That 

 this 'effect does not depend on loss of the contractility in the paralyzed 

 muscle, is shown by the fact that the muscle will still contract, on the 

 application of a direct stimulus. 



Irritation of the cord of a recently decapitated animal increases 

 considerably the force of the heart's beats ; it also accelerates the con- 

 tractions of the intestines, ureter, and bladder, and even of the small 

 arteries. These phenomena indicate a partial dependence of those 

 organs on the spinal cord, through the intervention of the sympathetic 

 nerves, which constitute the only channel of communication between 

 them and the cord. 



Partial extirpation of the cord from animals is borne for a very long 

 time, and, in birds especially, is followed by loss of sensibility and 

 mobility in the corresponding parts of the body. Complete removal 

 of the cord, in mammalia, causes death, only after one or two days, 

 provided hemorrhage be guarded against. Partial destruction of the 

 cord by a wire is much more speedily fatal, death occurring in a few 

 hours, and more quickly, the nearer the injury to the cervical region ; 



