CARDIAC MUSCLE 87 



as may he observed in the fish or amphibian heart. In mammals, 

 ho\ve\er, there is a rich ca])inary network carried between the 

 fibers by the interpenetrating connective tissue. The syncytial 

 organization of the heart is probably directly concerned with the 

 rhythmical contractions so characteristic of it, but it is also essential 

 that a free circulation takes place regularly, for interferences in the 

 l)lo()(l flow aft'ects the normal rhythmical activity. Cardiac muscle 

 contractions are shorter in duration than the resting phase and 

 under normal conditions fatigue rarel}' occurs. Regeneration does 

 not seem to be possible in cardiac muscle and enlargements of the 

 heart occurring in some adult animals result from increase in the 

 size of the muscle fibers, not their number, or possibly from increase 

 in the connective tissue if present. 



Neurogenic and Myogenic Theories of the Heart-beat. ^Although 

 this ap])ears to be primarily a functional problem, histological dis- 

 coveries have had much to do with our knowdedge of the subject. 

 The heart of an elasmobranch is two-chambered ; there is one auricle 

 and one ventricle. The sinus venosus draining blood from the body 

 carries this blood into the auricle; from the latter, the blood passes 

 into the ventricle whose contraction drives the blood into circulation 

 through the body. In action, the sinus contracts first, then the 

 auricle, then the ventricle, then the bulbus arteriosus. In this 

 order, one after the other, repetition occurs rhythmically. It 

 appears that whatever the nature of the stimulus is, it begins in the 

 sinus wall. The amphibian heart has two auricles and one ven- 

 tricle; reptiles have two auricles and the beginning of two ventricles; 

 birds and mammals have two auricles and two ventricles. In each 

 case the old elasmobranch heart organization is represented roughly 

 by tissue at the junction of the vena cava with the right auricle, 

 and careful observation shows that the rhythmic contraction of 

 these higher hearts begins at this tissue and is followed by con- 

 traction of the auricles and ventricles. 



The heart is provided with branches of sympathetic nerves which 

 have a sensory function, and with branches of the tenth cranial 

 and the vagus over which the impulses regulate the speed of the 

 rhythmic beat. There is no evidence, however, that either set of 

 nerves is concerned with the origin and continuance of the rhythm. 

 Rhythmical contractions of so-called hearts of invertebrates are 

 eflfected by nerve impulses, and the occurrence of nerves in vertebrate 

 hearts suggested that the heart-beat was due directly to nerve stimuli. 

 However, this explanation does not apply to vertebrate hearts. 



