88 THE MUSCLE TISSUES 



The embryo heart beats rhythmically before nerves ha\'e developed 

 in it, and when the heart of a cold-blooded vertebrate is removed 

 from its body, rhythmic beating may be continued for many days 

 if proper conditions are maintained. Also, small pieces of heart of 

 the chick or rabbit live, grow, and contract in tissue culture after 

 the ner\'es have degenerated. Evidence is still needed to prove the 

 neurogenic theory of the heart-beat. 



The myogenic theory, on the other hand, has much in its favor. 

 According to this idea, the stimulus arises in cardiac tissue and is 

 transmitted by this tissue to various contracting portions. The 

 question then arises as to whether impulses pass over ordinary 

 muscle fibers or whether there are special fibers for this function. 

 Such fibers have been found in mammals, where a small mass of 

 especially modified fibers occur at the junction of the superior vena 

 cava and the right auricle. These fibers are poor in fibrils but rich 

 in sarcoplasm; they are small, poorly striated, and form a network. 

 This mass is called the sino-auricular node and is the place where 

 the automatic rhythmic contractions begin. The impulses continue 

 over the auricle via a network of the same type of fibers, the so-called 

 Purkinje fibers, which become continuous with typical cardiac 

 fibers. The auricles are separated from the ventricles by rings of 

 connective tissue aroimd the openings between them and Purkinje 

 fibers converge there to form the auricvilar-ventricular node, a 

 second mass of modified cardiac fibers. This node is located near 

 the ventricles and Purkinje fibers extend on into the* ventricles. 

 Impulses beginning at the junction of the vena cava with the 

 auricles cause the latter to contract, and then the impulses continue 

 into the ventricles, causing a progressive contraction in them also. 

 The rest period between each cycle of contraction is longer than the 

 contraction period. The heart is refractory during this rest period 

 and it does not respond to stimuli. Its sensitivity to stimuli increases 

 toward the end of the rest period. If the im})ulses to the ventricles 

 are interfered with, the ventricles do not respond, or may set up an 

 independent rhythm of their own, in which case the heart as a whole 

 fails to function properly. 



SKELETAL MUSCLE. 



As the name implies, this type is associated with skeletal ])arts. 

 The unit of structure is the fiber, as it was in cardiac muscle, but 

 here the fibers are unbranching, elongated, multinucleated cylinders 

 of varying length. Such fibers likewise are formed from myoblasts 



