86 TEXT-BOOK OF PHYSIOLOGY. 



stimuli per second. In other words, thelvoluntary tetanus is also the 

 result of a discontinuous stimulation! The number of stimuli trans- 

 mitted to a muscle has been estimated by the employment of the 

 graphic method to vary from 8 to 13 per second, 10 being about the 

 average. Unless the contraction process of human muscle differs 

 from that of frogs, it is difficult, however, to see how 10 stimulations 

 per second can give rise to even an incomplete tetanic contraction. 



Muscle Sound. Providing a muscle be kept in a state of tension 

 during its contraction, the intermittent variations in tension cause 

 the muscle to emit an audible sound. This so-called muscle-sound or 

 tone is an evidence that the stimulation of the- muscle is not continu- 

 ous, but discontinuous. If the muscle is tetanized by induction 

 shocks, the pitch of the tone will correspond with the number of 

 stimuli. A voluntary contraction is attended by a tone having a 

 vibration frequency of about 36 to 40 per second, which is regarded 

 as the first overtone of the muscle tone, which would have a vibration 

 frequency in consequence of from 18 to 20 per second. This was 

 formerly regarded as an indication of the rate of stimulation of volun- 

 tary contraction. This view, however, is no longer sustained. 



CHEMIC PHENOMENA. 



The chemic changes which underlie the transformation of energy 

 in the living muscle even when in a state of rest are active and com- 

 plex, though but little is known as to their exact character. As shown 

 by an analysis of the blood flowing to and from the resting muscle, 

 it has, while flowing through the capillaries, lost oxygen and gained 

 carbon dioxid.\\The amount of oxygen absorbed by the muscle 

 (9 per cent.) is greater than the amount of carbon dioxid (6.7 per 

 cent.) given off .^ Notwithstanding the relation of the oxygen ab- 

 sorbed to the carbon dioxid produced, there is no parallelism between 

 these two processes, as the carbon dioxid will be given off in the 

 absence of oxygen or in an atmosphere of nitrogen. 



In the active or contracting muscle all the chemic changes are 

 increased, as shown both by an increased absorption of oxygen and 

 an increased production of carbon dioxid, though the ratio existing 

 between them differs considerably from that of the resting muscle. 

 Thus, according to Ludwig, an active muscle absorbs 12.26 per 

 cent, of oxygen and gives off 10.8 per cent, carbon dioxid. During 

 the activity of a muscle its tissue changes from a neutral to an acid 

 reaction, from the development of sarcolactic acid and possibly 

 phosphoric acid. The degree of the acidity depends to some extent 

 on the duration of the contraction periods. Chemic analysis of a 

 tetanized muscle shows that it contains less glycogen than a resting 

 muscle", and that it contains a larger_amomitpf water. Coincident 



