398 PRINCIPLES OF GENERAL PHYSIOLOGY 



In the state of excitation there is evidence of increased permeability of the 

 muscle cell, evidence of a more direct nature than in the case of nerve. The 

 observations of Lillie (1911) on the larva of Arenicola have been already referred 

 to (pages 138 and 139). Some experiments in which substances such as bile suits. 

 saponin, and sodium oleate, which are known to make the cell membrane per- 

 meable, were found to cause quick, vigorous twitches of frog's muscle, are 

 reported in the same paper. 



The same fact is shown by the increased electrical conductivity of striated 

 muscle in a state of excitation, as in the experiments of M'Clendon (1912, 2). This 

 means that the membrane Incomes permeable to ions to which it was impermeable 

 while unexcited. The state of polarisation, in other words, ceases to exist. In 

 general, the remarks made above with respect to the similar change in nerve apply 

 also to muscle. 



A further fact which indicates increased permeability is that found by Siebeck 

 (1913). Potassium chloride enters more rapidly into excited muscles than into 

 resting ones. 



If the change of semipermeability into permeability is essential to the act of excitation, it 

 will readily be seen that, while this state lasts, there will be a "refractory period." There is 

 evidence also that the duration of the electrical change coincides very closely with that of 

 the refractory state, as would be expected to be the case if this electrical change were due 

 to the disappearance of the state of impermeability to the ions of one sign of charge, with the 

 consequent depolarisation at the membrane. 



In muscle, however, we find an additional factor, that of contraction, by which 

 energy is given out. Along with this, phenomena are shown by muscle which 

 nerve does not show. These will be treated of more fully in the next chapter ; 

 but there are four properties of muscle which are connected with this factor that 

 should be mentioned here. They are latent period, metabolism, heat production, 

 and fatigue. 



Latent Period. The state of excitation indicated by the electrical change, 

 commences at such a short interval after the application of a stimulus, that it is 

 difficult to be certain that they are not simultaneous. There is, on the contrary, 

 an interval which can easily be measured before the state of contraction begins. 

 If the electrical change were unknown, it would appear that nothing was taking 

 place in the latent period before contraction. 



It appears, then, that there is, in muscle, an extra mechanism superadded 

 to the simple excitation process, namely, that giving rise to the contractile effect. 



There is direct evidence that the propagated disturbance, with its electrical 

 change, can continue in muscle which has been treated in such a manner as to 

 show no trace of contraction. HartPs experiments are the most convincing. If 

 a part of a muscle be immersed in distilled water, it will be found to be incapable 

 of contraction when stimulated, although this waterlogged part will still conduct 

 a disturbance to the normal part. Noyons (1908 and 1910) has shown that 

 certain drugs will abolish the beats of the heart of the frog and tortoise, while 

 leaving the electrical change still strong* And Mines (1912, 2) has shown that 

 skeletal muscles of the ray, treated with a dilute solution of ether, completely lose 

 their power of contractile response to strong electrical stimuli, while retaining that 

 to acid, alkali, or potassium salts. Presumably, the loss to electric stimuli was due 

 to failure of conduction of a propagated disturbance. In the same paper, Mines 

 refers to his observations that the conduction of the excitation process in heart 

 muscle is arrested by trivalent cations, whereas the contractile process is not so 

 affected (see Fig. 172 below). 



Owing to the fact that this contractile process is one attended with the 

 performance of external work, we find phenomena not present in nerve con- 

 sumption of oxygen, giving off carbon dioxide, production of heat, fatigue, and so 

 on, all of which belong to the subject of the next chapter. 



In order, however, to throw light on the process of inhibition, it is necessary 

 to make use of the states of contraction and relaxation as indications of what has 

 happened. 



We have already seen that there is a distinction between striated, skeletal, 



