48 THE HEART 



former condition first, since it corresponds more closely to the 

 conditions under which the other forms of muscle are studied. 

 The heart may be reduced to this state of rest by four methods : 



(1) By placing it in physiological saline and waiting for it to 

 cease beating, the cardiac muscle being excitable for a variable 

 period after such stoppage. This method is more appropriate for 

 the hearts of warm-blooded than of cold-blooded animals, since 

 the latter may continue beating for days under these conditions. 



(2) By shutting off the part of the heart possessing the greatest 

 automatic power. This is done in the Stannius ligature experi- 

 ment, more usually on the frog's heart, in which a ligature is tied 

 round the sino -auricular junction. The same result can also be 

 obtained by cutting away the more automatic parts of the heart 

 in other words, by making a ventricle preparation only. 



(3) By stimulating the vagus most hearts can be reduced to 

 a standstill. 



(4) By certain drugs, such as muscarine, a like condition is 

 obtained. 



It must be stated, however, that under conditions 3 and 4 

 the normal properties of cardiac muscle are greatly altered, and 

 therefore the methods are unsuitable. Preference must therefore 

 be given to methods 1 and 2. 



The preparation being obtained, it is found that cardiac muscle, 

 like other forms of muscle, possesses the properties of excitability 

 and contractility. It responds to a single stimulus by a single 

 contraction. The stimulus may be either mechanical, thermal, 

 chemical, or electrical. The last is most commonly chosen, but it 

 is interesting to note with regard to the chemical stimuli that the 

 heart muscle responds to the stimuli for muscle but not to those 

 for nerve. Thus ammonia, dilute lime water, dilute mineral acids 

 when applied to the apex of a frog's ventricle excite contraction or 

 a number of contractions. These, according to Kuhne, do not 

 excite motor nerves. The potent nerve stimulant, glycerine, on 

 the other hand, fails to excite a single contraction from such a 

 preparation. 



Different parts of the heart possess different degrees of ex- 

 citability. This power of excitability, moreover, does not run 

 parallel with the degree of automatism possessed by the same 

 part of the heart. As an instance, the heart of the embryo chick 

 at three days possesses great automatic power, but only a small 



