SPINAL REFLEXES IN MAN AND MAMMALS 525 



When the cord is first cut, the shock is very great, the lower or isolated 

 portion of the cord remains for a time quite non-irritable. The vaso-motor and 

 thermogenic centers are cut off so that there is great vascular dilatation and 

 marked fall of temperature, the effects of which are likely to lead to death 

 unless the operated animal is carefully attended. But these effects are slow- 

 ly recovered from, and man, as well as lower mammals, soon regains the 

 vascular tone. The general tonus of the muscular system, which is lost at first, 

 is also regained. 



In this partially recovered condition man, and such animals as the cat, the 

 dog, and the monkey, perform certain of the low r er functions with a remarkable 

 degree of perfection. Of course these functions are under constant coordi- 

 native regulation and control in the normal animal, but experiments and ob- 

 servation have shown how much of such activity really is a primary function 

 of the cord. Of these activities the following may be especially mentioned: 

 Muscular tonus, general reflexes, the special reflexes of micturition, defecation, 

 erection and the sexual reflex, and parturition, some of which will be briefly 

 discussed. 



The Center of the Tone of Muscles. The tonic influence of the spinal cord 

 on the sphincter ani and sphincter urethrae will be presently mentioned. The 

 cord maintains these muscles in permanent tonic contraction. The condition 

 of the sphincters, however, is not altogether exceptional. Their contraction is 

 the same in kind, though it exceeds in degree, that condition of muscles which 

 has been called tone, a condition of slight contraction which they always 

 maintain during health. This tone of all the muscles of the trunk and limbs 

 depends on the spinal cord, just as does the contraction of the sphincters. 

 If an animal be killed by injury or removal of the brain, the muscles retain 

 their tenseness, but if the spinal cord be destroyed, the sphincter ani relaxes, 

 and all the muscles become loose, flabby, and atonic, remaining so till rigor 

 mortis commences. 



If an animal, such as the dog, be held off the ground in the erect position 

 assumed by the human body, when the trunk and hind limb muscles are not 

 in voluntary contraction the limbs will assume a normal pendular position. 

 In the pendular position the legs of a dog with cord severed hang more limp 

 and are more completely extended. The muscles of the former exhibit that 

 tone which keeps antagonistic muscles always slightly tense, the muscles of the 

 latter have lost the tenseness. 



Whether or not muscular tone is maintained through the constant sub- 

 minimal action of sensory nerve impulses on the tonic centers of the cord, or 

 whether these centers are automatic in their action, is a question that can be 

 answered only by inference. The probability is that tone is a reflex activity, 

 though it may be contributed to by the normal healthy nutritional condition 

 of the muscles themselves a condition which is itself dependent on the 

 trophic influence of the nerve cells of the cord and brain stem. 



