ACTION OF THE HEART. 



109 



semilunar valves are placed side by side around the arterial 

 orifice of each ventricle, so as to form three little pouches, 

 which can be thrown back and flattened by the blood pass- 

 ing out of the ventricle, but which belly out immediately 

 so as to prevent any return (6, fig. 34) . This will be again* 

 referred to immediately. 



The muscular fibres of the heart, unlike those of most 

 involuntary muscles, present a striated appearance under 

 the microscope. (See Chapter on Motion.) 



THE ACTION OF THE HEAET. 



The heart's action in propelling the blood consists in the 

 successive alternate contractions and dilatations of the mus- 

 cular walls of its two auricles and two ventricles. The 

 auricles contract simultaneously ; so do the ventricles ; their 

 dilatations also are severally simultaneous ; and the con- 

 tractions of the one pair of cavities are synchronous with 

 the dilatations of the other. 



The description of the action of the heart may best be 

 commenced at that period in each action which immediately 

 precedes the beat of the heart against the side of the chest, 

 and, by a very small interval more, precedes the pulse at 

 the wrist. For at this time the whole heart is in a passive 

 state, the walls of both auricles and ventricles are relaxed r 

 and their cavities are being dilated. The auricles are 

 gradually filling with blood flowing into them from the 

 veins : and a portion of this blood passes at once through 

 them into the ventricles, the opening between the cavity 

 of each auricle and that of its corresponding ventricle 

 being, during all the pause, free and patent. The auricles, 

 however, receiving more blood than at once passes through 

 them to the ventricles, become, near the end of the pause, 

 fully distended ; then, in the end of the pause, they con- 

 tract and empty their contents into the ventricles. The 

 contraction of the auricles is sudden and very quick ; it 

 commences at the entrance of the great veins into them, 



