558 THE NERVOUS SYSTEM 



is the same in kind, though it exceeds in degree, that condition of the voluntary 

 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, 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. 



The Ano-spinal or Defecation Center. The mode of action of the ano- 

 spinal center appears to be this: The mucous membrane of the rectum is 

 stimulated by the presence of feces or of gas in the large bowel. The stim- 

 ulus passes up by the afferent nerves of the hemorrhoidal and inferior mesen- 

 teric plexuses to the center situated in the lumbar enlargement of the cord, 

 and is reflected through the pudendal plexus to the anal sphincter, and to the 

 muscular tissue in the wall of the lower bowel. In this way there is produced 

 a relaxation of the first and a contraction of the second, and expulsion of the 

 contents of the bowel follows. The center in the spinal cord is partially 

 under the control of the will, so that its action may be either inhibited or 

 augmented. The action may be helped by the abdominal muscles, which 

 are voluntary muscles, but are also stimulated to contract by reflex action. 



The Vesico-spinal or Micturition Center. The vesico-spinal center acts 

 .in a very similar way to that of the ano- spinal. The center is also in the 

 lumbar enlargement of the cord. It is stimulated to action reflexly by the 

 presence of urine in the bladder. The action may be voluntary and is excited 

 by the sensation of distention of the bladder by the urine. The sensory fibers 

 concerned are the posterior roots of the lower sacral nerves. The action of 

 the spinal center is double, or it may be supposed that the center consists of 

 two parts, one of which is usually in action and maintains the tone of the 

 sphincter, and the other which causes contraction of the bladder and other 



