220 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



contraction or tonus. When the nerves controlling any such sot of muscles 

 are cut, or their central connections injured, the muscles at first relax. 



If a frog be hung up vertically alter removing the brain, the cord remain- 

 ing intact, it is found that the legs are slightly flexed at the hip and knee. 

 If now the sciatic nerve be cut upon one side, the leg on the side of the 

 section hangs straighter, indicating that the muscles have relaxed a little as 

 the result of the section of the nerve; if, in the same animal, the smaller 

 arteries in the web of the foot be examined both before and after the section, 

 it is found that after the section they have increased in diameter. Con- 

 versely, artificial stimulation of the peripheral stump causes a contraction 

 of the vessels, but it is not possible in so rough a way to imitate the tonic 

 contraction of the skeletal muscles. 



It is inferred from these experiments that normally there pass from the 

 central system along some of the nerve-fibres impulses which tend to keep 

 the muscles iu a state of slight contraction. Destruction of the entire cord 

 abolishes all outgoing impulses, and produces a complete relaxation of these 

 muscles. 



Though the intensity of these outgoing impulses is normally always small, 

 yet it is subject to significant variations. The difference between the tone 

 of the muscles of an athlete in prime condition and those of a patient recover- 

 ing from a prolonged and exhausting illness is easily recognized, and this 

 difference is in a large measure due to the difference in the intensity of the 

 impulses passing out of the cord. Among the insane, too, the variations in 

 this tonic condition follow in a marked way the nutritive changes in the 

 central system, and both facial and bodily expression have a value as an 

 index of the strength and variability of those impulses on which the tone of 

 the skeletal muscles depends. Indeed, so wide in the insane is the variation 

 thus brought about that when the expressions of an individual at one time in 

 a phase of mental exaltation, and at another in that of mental depression, arc 

 compared, it appears hardly possible that they can be those of the same person. 



This continuous outflow of impulses from the central system is indicated 

 also by the continuous changes within the glands, and the variations in these 

 metabolic processes according to the activities of the central system. 



Rigor Mortis. — Even in the very act of dying the influence of these 

 impulses can be again traced. The death of the central nerve-tissues being 

 expressed as a chemical change, causes impulses to pass down the efferent 

 nerves, and these impulses modify those chemical changes which, in the 

 muscles of a frog's leg, for example, lead to rigor mortis. It thus happens 

 that a frog suddenly killed and then left until the onset of rigor, will under 

 ordinary circumstances show rigor at about the same time in both legs. If, 

 however, the sciatic nerve on one side be cut immediately after the death of 

 the animal, the beginning of rigor in that leg is much delayed, thus showing 

 that the nervous connection is an important factor in modifying the time of 

 this occurrence (Hermann). 



The Nervous Background. — We return now to the conditions which 



