440 PRINCIPLES OF GENERAL PHYSIOLOGY 



opened, auxotonic, that is, it contracts against an increasing resistance, as the 

 arterial pressure rises. 



There are several forms of experimental twitch, " arrested," " inertia," and so 

 on, which do not concern us here. 



The external work done is clearly the weight raised multiplied by the height 

 to which it is raised. Although no external work is done in maintaining the 

 weight at this height, it is a familiar fact that fatigue results and the metabolism 

 and heat of the muscle show a considerable consumption of energy. This point 

 will be returned to in a later page. 



When we remember that, with zero load and maximum height of twitch, no 

 external work is done, nor when the load is so great that the muscle cannot 

 shorten, a little consideration will show that there must be a load of a certain 

 magnitude with which the maximal work is done ; this is found experimentally to 

 be the case. 



When the muscle is allowed to relax again with the weight still on, this weight 

 falls to its original position, so that no permanent work is done. To enable a 

 muscle to perform actual external work, Fick devised the " Arbeitsammler " or 

 " work collector" in which, by a system of catches, the weight is taken off the 

 muscle at the height of contraction, so that the weight does not fall again ; but 

 when the muscle makes a further contraction, it catches the rim of the wheel on 

 whose axis the weight is suspended, and raises it by a further amount and so 

 on (see Fick's book, 1882, pp. 139-143). 



Blix (1891, p. 306) has described a muscle indicator, which draws a curve 

 whose ordinates are lengths of the muscle and whose abscissa 1 are corresponding 

 tensions. The area of the curve is thus the work done. Similarly, in the 

 indicator of the steam engine, the co-ordinates of the curve are pressures and 

 volumes in the cylinder. 



In order to measure the work done by an animal or man for the purpose of 

 metabolism experiments, some form of bicycle mechanism or treadmill is generally 

 used. A brake is applied so that the amount of work can be varied and determined. 

 The brake may be f rictional, as in the simple but accurate pattern of C. J. Martin 

 (1914), or it may be in the form of a dynamo, as in the earlier apparatus of 

 Atwater and Benedict (see Atwater, 1904), or again as Foucault currents, produced 

 in a copper disc rotated between the poles of an electro-magnet, whose magnetising 

 current can be varied (see Krogh's article, 1913). 



TETANUS 



When a second stimulus is applied to a skeletal muscle before the muscle has 

 returned to its original length, the contraction due to this second stimulus starts 

 from the level at which the first contraction is at the time, and so on for subsequent 

 stimuli. Thus a summation is produced, by which the extent of contraction is 

 much greater than can be brought about by a single stimulus, however strong. 

 But each stimulus produces less increase than its predecessor, so that, after a 

 certain number have been applied, no further increase in height results ; a constant 

 level only is maintained. This is known as " tetanic contraction." 



An effect of the same kind is produced by reflex or voluntary contraction of 

 .skeletal muscle. A series of disturbances at the rate of 50 per second, in the 

 median nerve of man, is sent out from the centre (Piper, 1912, p. 98). This value 

 was obtained by leading off the muscles of the forearm to a string galvanometer. 

 It is practically the same in the case of all the muscles tested, namely, 40 per 

 second in the quadriceps femoris, 60 in the masseter. The shortest voluntary 

 movements always consist of at least three or four waves. It is interesting that 

 the frequency of these waves in the tortoise is a linear function of />'///'/v////v, 

 and, indeed, through the wide range from 4 to 40. Put in other words, we may 

 say that it is directly proportional to the absolute temperature, just as the simplest 

 physical phenomena, such as the volume of a gas, the osmotic pressure of solutions, 



