50 A MANUAL OF VETERINARY PHYSIOLOGY 



sodium chloride which, in a strength of 0-5 to o-6 per cent., is 

 mainly responsible for the normal osmotic pressure of the blood, 

 and though single-handed it is not capable of maintaining the 

 heart-beat, nor of furnishing the tissues with all the inorganic 

 material they require, nevertheless it is more effective in this 

 respect than any other single salt. The presence of calcium, 

 though the amount in blood is only small, appears to be necessary 

 to contraction ; at any rate, the addition of calcium to the ' fed ' 

 heart, or its application to the isolated strip, not only brings on 

 contractions, but increases the length of time during which they 

 are continued. The physiological effect of potassium appears 

 to be connected with relaxation rather than with contraction. 



Facts such as the above suggest that the heart is capable of 

 generating in its own substance a stimulus to contraction, and 

 the hypothesis of an inner stimulus, though it takes us no nearer 

 to a conception of its nature, assists the imagination. It is not 

 considered that the inner stimulus is represented by the inorganic 

 salts, but rather that in the presence of these it is capable of 

 doing its work. 



The Physiological Properties of Cardiac Muscle differ from 

 those of skeletal muscle. The stimulation of skeletal muscle 

 with a weak current causes a weak contraction, with a strong 

 current a more powerful contraction ; but the heart muscle 

 behaves differently : the amount of contraction it exhibits under 

 stimulation is the full amount which it is at the time capable 

 of exhibiting, no matter whether the current be a weak or a 

 strong one. This remarkable difference has been explained by 

 the fact that in skeletal muscle the fibres are isolated from each 

 other, while in heart muscle they form a continuous sheet, and 

 the excitation travels from fibre to fibre. 



Another distinction from skeletal muscle is the reaction to 

 repeated electrical stimulation. If a rapid series of induction 

 shocks be passed into skeletal muscle, it is thrown into a condition 

 known as tetanus ; but heart muscle cannot be tetanised, for the 

 reason that electrical stimuli applied to it during the period of 

 its contraction produce no effect whatever. It is only during 

 diastole that a response to stimulation is obtained. 



The non-irritable nature of the heart muscle during its con- 

 traction is spoken of as the refractory period, and the response 

 to stimulation applied during diastole is described as an extra 

 contraction. Having executed this contraction out of its regular 

 sequence, the succeeding pause is longer than usual, by which 

 means the heart picks up its rhythm as though it had never 

 been disturbed. The pause is known as a compensating pause. 



The heart muscle stands by itself in being non-irritable during 

 its period of actual contraction ; neither skeletal nor plain muscle 



