134 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



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 effect of this multitude of affer- 

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

 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 musde-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 be maintained 

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

 has been called chemical tonus. 1 That the chemical activity of muscles is kept 

 up by small stimuli from the spinal cord is shown by the fact that if the nerves 

 be severed, or the nerve-ends be poisoned by curare, the muscle absorbs less 

 oxygen and gives off less carbon dioxide than when at rest under normal 

 conditions. 2 



The theory of a reflex chemical tonus independent of contraction implies 

 the existence of special nervous mechanisms for the exciting of chemical 

 changes in the muscles which shall result in the liberation of energy as heat, 

 independent of the change of form of the muscle. The question of the exist- 

 ence of special nervous mechanisms controlling heat-production heat-centres,, 

 as they are called will be considered in another part of this book. 



E. ELECTRICAL PHENOMENA IN MUSCLE AND NERVE. 

 The active muscle liberates three forms of energy : mechanical work, heat,. 

 and electricity. The active nerve makes no visible movements, gives off no 

 recognizable quantity of heat, but exhibits changes in electrical condition quite 



1 Roehrig und Zuntz: Pfliiger's Archiv, 1871, Bd. iv.; Pfliiger: Pftuger>s Archiv, 1878, xviiL 

 p. 247. 



2 Zuntz: Pftuger>s Archiv, 1876, xii. 522; Colasanti, Ibid., 1878, xvi. p. 57. 



