GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 143 



activity, greatly increases the heat-production. For this reason isometric 

 contractions, that is, those in which the muscle works against a resistance 

 which is sufficient to prevent it from shortening, are accompanied by a 

 greater liberation of heat than isotonic contractions, in which the contracting 

 muscle raises a constant weight. As the weight is increased, both the 

 amount of heat developed and the work are increased, but the liberation of 

 heat reaches its maximum and begins to decline sooner than the amount of 

 work — i.e., with large weights the muscle works more economically; simi- 

 larly, as the muscle is weakened by fatigue the heat-production lessens sooner 

 than the work. 



Muscle-tonus and Chemical Tonus. — During waking hours, the cells 

 of the central nervous system are continually under the influence of a shower 

 of weak nervous impulses, coming from the sensory organs all over the body ; l 

 moreover, activity of brain-cells, especially emotional forms of activity, leads 

 to an overflow of nervous impulses to the spinal cord and an increased irrita- 

 bility, or, if stronger, excitation of motor nerve-cells. If, when one is quietly 

 sitting and reading, he turns his attention to the sensory impressions which 

 are coming at every moment from all over the body to the brain, notes the 

 temperature of different parts of the skin, the pressure of the clothes, etc., 

 upon different parts, the light reflected from neighboring objects, and the slight 

 sounds about him, he will recognize that the central nervous system is all the 

 time subject to a vast number of excitations, which, because of their very 

 repetition, are ordinarily disregarded by the mind, but which are, nevertheless, 

 all the time influencing the nerve-cells. The efl'ect of this multitude of affer- 

 ent stimuli, in spite of their feebleness, is to cause the motor cells of the curd 

 to continually send delicate motor stimuli to the muscles. These cause the 

 muscle to keep in the state of slight but continued contraction which gives the 

 tension peculiar to waking hours, and which is called muscle-tonus. That 

 such a tension exists is made evident by the change in attitude which occurs 

 when the relaxation accompanying sleep comes on. The effect of brain activ- 

 ity to cause muscular tension is, likewise, most easily recognized by observing 

 the relaxation of the muscles which occurs when mental excitement ceases. 



Muscle-tonus, like every form of muscular contraction, is the result of chem- 

 ical change, and the liberation of energy. But little of this energy leaves the 

 body as mechanical energy, most of it being given off as heat. 



This view is by no means universally accepted, and many physiologists 

 believe in a production of heat by the muscles, as a result of nervous influences, 

 independent of contraction. It is thought that a condition of slight but con- 

 tinuous chemical activity resulting in the production, of heat may lie maintained 

 in the muscles by intermittent but frequent reflex excitations, a condition which 

 has been called chemical tonus. 2 Thai the chemical activity of muscles is kept 



1 Rrondgeest: Archiv fur Anatomie und Physiologie, 1860, S. 703; Hermann, Ibid., L861, 

 S. 350. 



2 Koehrig und Zuntz: Pfliiger'a Archie, 1871, lid. iv ; Piliiger: Pjluger'a Archie, 1878, xviii. 

 S. 247. 



