224 THE BODY AT WORK 



territories appertaining to the several nuclei. In heart-muscle, 

 on the other hand, nuclear division has been followed by cell- 

 division ; but minute protoplasmic bridges are left between the 

 cells. The whole of the heart-substance is thus in structural 

 continuity. The cells are not invested with sarcolemma. As 

 the result of this arrangement, an impulse started in one part of 

 the heart spreads over the whole, with certain limitations as to 

 the directions in which it is able to travel, whereas in voluntary 

 muscle a separate impulse must be delivered to each fibre. 

 The wave of contraction commences in the great veins, the 

 venae cavae and pulmonary veins, near their junction with the 

 heart, spreads from cell to cell throughout the auricles, and 

 onwards down the ventricles to the apex of the heart. The 

 substance of the heart has not, however, a homogeneous appear- 

 ance. Its cells are collected into fascicles, which lie in various 

 planes and cross the axis of the heart at various angles. In a 

 boiled sheep's heart it is easy to separate one fascicle from 

 another, and to distinguish the sheets into which the fascicles 

 are collected. The four valves of the heart lie in almost the same 

 plane. They are supported by a fibrous plate divided into four 

 rings (Fig. 11). Most of the fascicles are attached to this plate, 

 though some which encircle the auricles are independent of it. 

 With one or with both ends attached to the plate, fascicles loop 

 over the auricles. They run down the ventricles with a twist 

 from right to left. Those on the surface turn in at the apex of 

 the heart, and run up the inner surface of the ventricles. Some 

 of them go to form the free columns which are found on the inner 

 surface of the ventricles, pointing towards the valves musculi 

 papillares. The fibrous plate which supports the valves cuts 

 off almost all of the muscle which makes the walls of the 

 auricles from that which constitutes the ventricular walls ; but 

 a thin sheet is continued from the inner surface of the auricles 

 down the inter ventricular septum. To a considerable extent 

 the walls of the two auricles and of the two ventricles are 

 respectively continuous, insuring synchronous contraction. 



The arrangement of the fascicles accounts for the changes in 

 form which the heart undergoes when it contracts. Systole 

 commences in the cardiac ends of the venae cavae and pulmonary 

 veins. They empty the last of their blood into the auricles, 

 and close to prevent regurgitation, their mouths not being 



