140 THE CIRCULATION. 



regular series the successive contractions of the several 

 muscles of the heart. The mode in which ganglia thus 

 act as centres and co-ordinators of nervous power will be 

 described in the chapter on the NERVOUS SYSTEM ; and 

 it will appear probable that the chief peculiarity of the 

 heart, in this respect, is due to the nurober of its ganglia, 

 and the apparently equal power which they all exercise ; 

 so that there is no one part of the heart whose action, more 

 than another's, determines the actions of the rest. Thus, 

 if the heart of a reptile be bisected, the rhythmic, suc- 

 cessive actions of auricle and ventricle will go on in both 

 halves: we therefore cannot say that the action of the 

 right side determines or regulates that of the left, or vice 

 versa ; and we must suppose that when they act together 

 in the perfect heart, it is because they are both, as it were, 

 set to the same time. Neither can we say that the auricles 

 determine the action of the ventricles; for, if they are 

 separated, they will both contract and dilate in regular, 

 though not necessarily similar, succession. A fact pointed 

 out by Mr. Maiden shows how the several portions of each 

 cavity are similarly adjusted to act alike, yet independently 

 of each other. If a point of the surface of the ventricle 

 of a turtle's or frog's heart be irritated, it will immediately 

 contract, and very quickly afterwards all the rest of the 

 ventricle will contract; but, at the close of this general 

 contraction, the part that was irritated and contracted first, 

 is slightly distended or pouched out, showing that it was 

 adjusted to contract in, and for only, a certain time, and 

 that therefore as it began to contract first, so it began to 

 dilate first. 



Mr. Paget, however, has shown that the cause of the 

 rhythmic motion does not exist equally in all parts of the 

 heart. If, for example, the cut-out heart of a tortoise be 

 divided into two pieces, one comprising the auricles and the 

 base of the ventricle, the other comprising the rest of the 

 ventricle, the former will continue to act rhythmically, the 



