406 THEORIES OF MUSCULAR ACTIVITY. [BOOK I. 



and that this is largely due to the presence of oxygen is shewn in the 

 experiments of Ludwig and A. Schmidt already described (p. 380) 

 (Kronecker). But how far the accumulation of the products of con- 

 traction, or the defect of oxygen, and probably of other constituents of 

 normal muscle, can be trusted to explain the fatigue of muscles which 

 are still within the body, it is impossible to say. 



stenson's It is, well known 1 that ligature of the blood-vessels 



Experiment. supplying muscles is followed by paralysis and rigor 

 of the muscles: and the same consequence follows any stoppage 

 whatever of the blood-current. Renewal of the current, on the other 

 hand, is followed by a restoration of irritability provided that rigor 

 is not complete. There can be little doubt in this case, that the 

 paralysis is in part due to an interruption of the normal exchanges 

 between muscles and the blood, i. e. to the accumulation of carbon 

 dioxide and the acid of rigor, and to the defect of oxygen, etc. 



Among the mechanisms which contribute to the revival of fatigued 

 muscles or the prevention of exhaustion, must be mentioned the vaso- 

 motor nerves. Ludwig and Sczelkow 2 discovered that venous blood 

 flows more rapidly from contracting than from resting muscles, a 

 phenomenon which has been investigated by the pupils of Ludwig and 

 traced to the vaso-motor system. The dilatation of the blood-vessels of 

 muscle, upon which the accelerated outflow seems to depend, also 

 accompanies the reflex stimulation of muscles, and is said to be visible 

 under the microscope in the case of frog-muscles, even when the 

 circulation is stopped and the blood-pressure abolished 3 . 



It will be remembered that the first effect of subjecting a muscle 

 to a vacuum was to increase its irritability (p. 371) : and that the 

 same result followed the withdrawal of blood from muscles in the ex- 

 periments of Ludwig and A. Schmidt (p. 379), No explanation of 

 this has yet been given. 



SECT. 5. THE THEORY OF MUSCULAR ACTIVITY. 



The archives of Physiology contain many curious speculations as to 

 tha nature of muscular motion ; nor is this surprising when we reflect 

 that the power of self-movement must ever have appeared the chief 

 and most characteristic attribute of animals. Many of the hypotheses 

 are extremely fanciful, and most of them are incomplete, offering to 

 explain certain elements only of the complicated act of contraction. In- 

 deed few theories are burdened with a heavier task than that which 

 comprehends all the phenomena of a muscular act. It must explain 

 how nervous stimuli affect muscle, and how contraction is transmitted 



1 Steno, quoted by Schwammerdam (de Bespiratione, Leydeu, 1679, p. 61), and by 

 Haller (Elementa PhysioL, iv. p. 544. 1762). 



2 Ludwig and Sczelkow, Op. cit. See p. 375. 



3 See inter alia the researches of Gaskell, Studies from the PhysioL Lab. of Cambridge, 

 in. 1877 (Journal of Anat. and PhysioL Vol. XL); Journal of PhysioL, ed. by M. Foster, 

 Vol. i., 1878. 



