PROPERTIES OF THE HEART MUSCLE. 573 



auricle (first ligature of Stannius) the auricle and ventricle ceaso 

 to beat. In this quiescent condition a slight mechanical stimulus 

 to the ventricle causes it to beat and its contraction is immediately 

 followed by that of the auricle. A similar reversed rhythm may be 

 obtained from the mammalian heart under suitable conditions. 

 Such an experiment makes it most probable that the contraction 

 is propagated from one chamber to the other directly through 

 the muscular connections. It is not possible at present to conceive 

 that a definite mechanism of neurons should work thus in either 

 direction. 



3. There is much probable proof that the heart muscle tissue 

 possesses the property of automatic rhythmical contractions. Ex- 

 periments, initiated by Gaskell and since extended by numerous 

 observers, show that in the cold-blooded animals strips of heart 

 muscle taken from various parts of the heart will under proper 

 conditions develop rhythmical contractions. It is very improbable 

 that each of these strips, no matter how made, contains its own 

 resident nerve cells or nerve tissue which act as a motor center. 

 These results seem to demonstrate an inherent property of rhythm- 

 icity in cardiac muscle, whether or not this rhythmicity is directly 

 responsible for the normal beat. 



4. It has been shown that in the embryo chick the heart pul- 

 sates normally before the nerve cells have grown into it. More 

 recently it has been demonstrated that portions of the heart- 

 muscle may be removed from the embryo and be kept alive in a 

 blood-plasma medium*. Isolated pieces of this kind continue to 

 beat and to undergo multiplication and differentiation, giving rise 

 to isolated muscle-cells which exhibit rhythmic contractility. 

 This result indicates very clearly that the embryonic heart muscle 

 is capable of automatic contractility. It is possible of course 

 that at a later stage of development this property may be lost by 

 the muscle, but it is perhaps more probable that the property is 

 retained in some degree throughout life, the greatest power of 

 rhythmicity being exhibited by the least differentiated tissue in 

 the venous end of the heart, the sino-auricular node, in which the 

 heart beat originates. 



Automaticity of the Heart. As was said above, the ques- 

 tion of the cause or causes of the automatic rhythmical con- 

 tractions must be sought for whether the phenomenon turns out to 

 be a property of the muscular tissue or of the nervous tissue of the 

 heart. When we say that a given tissue is automatic we mean 

 that the stimuli which excite it to activity arise within the tissue 

 itself, and are not brought to it through extrinsic nerves. In the 

 heart, therefore, we assume that a stimulus is continually being 

 * Burrows, "Science," 36, 90, 1912. 



