490 



TESTING INDIVIDUAL MUSCLES. 



Comparative. Comparing the absolute muscular force of different muscles, even in different 

 animals, it is usual to calculate it with reference to that of a square centimetre. The mean 

 transverse section of a muscle is obtained by dividing its volume by its length. The volume is 

 equal to the absolute weight of the muscles divided by its specific gravity = 1058. The absolute 

 muscular force for 1 O centimetre of a frog's muscle = 2*8 to 3 kilos. [6*6 lbs.] (J. Rosen- 

 thal) ; for I D centimetre of human muscle = 7 to 8 [Hcnke and Knorz), or even 9 to 10 kilos. 

 [20 to 23 lbs.] (Korster, HaugMon). Insects can perform an extraordinary amount of work 

 an insect can drag along sixty-seven times its body-weight ; a horse scarcely three times its 

 own weight 



5. During tetanus, when a weight is kept suspended, no work is done as long as 

 the weight is suspended, but of course work is done in the act of lifting the load. 

 To produce tetanus, successive stimuli are required, the muscular metabolism is in- 

 creased, and fatigue rapidly occurs. The potential energy in this case is converted 

 into heat ( 302). When a muscle is stimulated with a maximal stimulus, it can- 

 not lift so great a weight with one contraction as when it is stimulated tetanically 

 (Hermann). The energy evolved, even during tetanus, is greater the more frequent 

 the stimulation, at least up to 100 stimuli per second (Bernstein). 



II. Medium Stimuli. If a muscle be caused to contract by stimuli of moderate 

 strength, i.e., such as do not cause a maximal contraction, there are two possibilities: 

 Either the feeble stimulus is kept constant whilst the load is varied, in which 

 case the amount of work done follows the same law as obtains for maximal stimula- 

 tion ; or, the load may be kept the same, whilst the strength of the stimulus is 

 varied. In the latter case, Fick observed that the height to which the load was 

 lifted increased in a direct ratio with the strength of the stimulus. 



The stimulus which causes a muscle to contract must reach a certain strength or intensity 

 before it becomes effective, i.e., the "liminal intensity" of the stimulus, but this is independ- 

 ent of the weight applied to the muscle. With minimal stimuli, a small weight is raised 

 higher than a large one, but as the stimulus is increased, the contractions also increase in a 

 larger ratio with an increased load (v. Kries). 



The blood-stream within the muscles of an intact body is increased during 

 muscular activity. The blood-vessels of the muscle dilate, so that the amount of 

 blood flowing through them is increased (Luthvig and Sczelkow). At the time that 

 the motor fibres are excited, so also are the vaso-dilator fibres, which lie in the 

 same nervous channels ( 294, II.). [Gaskell found that faradisation of the nerve 

 of the mylohyoid muscle of the frog not only caused tetanus of the muscle, but 

 also dilatation of its blood-vessels.] 



Testing Individual Muscles. In estimating the absolute force of the individual muscles or 

 groups of muscles in man, we must always pay particular attention to the physical relations, 



i.e., to the arrangement of the levers, direction 

 of the traction, degree of shortening, &c. ( 306). 

 Dynamometer. The absolute force of certain 

 groups of muscles is very conveniently and 

 practically ascertained by means of a dyna- 

 mometer (fig. 339). This instrument is very 

 useful for testing the difference between the 

 power of the two arms in cases of paralysis. 

 The patient grasps the instrument in his hand 

 and an index registers the force exerted. 

 Quetelet has estimated the force of certain 

 muscles the pressure of both hands of a man 

 to be = 70 kilos. ; while by pulling he can move 

 double this weight. The force of the female hand is one-third less. A man can carry more 

 than double his own weight ; a woman about the half of this. Boys can carry about one-third 

 more than girls. [Very convenient dynamometers are made by Salter of Birmingham, both 

 for testing the strength of pull and squeeze ; in testing the former, the instrument is held as 

 an archer holds his bow when in the act of drawing it, and the strength of pull is given by an 

 index ; in the latter another form of the instrument is used. Large numbers of observations 

 were made by means of these instruments by Francis Galton at the Health Exhibition, 1885.] 



Amount of Work Daily. In estimating the work done by a man, we have to consider, not 

 only the amount of work done at any one moment, but how often, time after time, he can 



Fig. 339. 

 Dynamometer of Mathieu. 



