THE CIRCULATION OP THE BLOOD. 271 



The ventricles usually completely empty themselves of 

 blood and maintain their systolic contraction even after this 

 has been effected. The contraction of the heart, which really 

 begins in the great veins near their junction -with the auri- 

 cles (that do not fully empty themselves), is at once fol- 

 lowed up by the auricular and ventricular contraction, the 

 whole constituting one long peristaltic wave. Then follows 

 the cardiac pause, which is of longer duration than the entire 

 systole. 



When the heart contracts it hardens, owing to closing on a 

 non-compressible fluid dammed back within its walls by resist- 

 ance a f route. At the same time the hand placed on the chest- 

 walls over the heart is sensible of the cardiac impulse, owing 

 to what has just been mentioned. The systole of the chambers 

 of the heart gives rise to a first and a second sound, so called, 

 caused by several events combined, in which, however, the ten- 

 sion of the valves must take a prominent share. The work of 

 the heart is dependent on the quantity of blood it ejects and 

 the pressure against which it acts. The pulse is an elevation 

 of the arterial wall, occurring with each heart-beat, in conse- 

 quence of the passage of a wave over the general blood- stream. 

 There is a distention of the entii;e arterial system in every direc- 

 tion. The pulse travels with extreme velocity as compared with 

 the blood-current. The heart-beat varies in force, frequency, 

 duration, etc., and with age, sex, posture, and numerous other 

 circumstances. 



The whole of the circulatory system is regelated by the cen- 

 tral nervous system through nerves. There is in the medulla 

 oblongata a small collection of nerve-cells making up the 

 cardio-inhibitory center. This center, with varying degrees of 

 constancy, depending on the group of animals and the needs 

 of the organism, sends forth impulses (which modify the beat 

 of the heart in force and frequency) through the vagi nerves. 

 There are nerves of the sympathetic system with a center in 

 the cervical spinal cord, and possibly another in the medulla, 

 which are capable of originating either an acceleration of the 

 heart rhythm qr an increase of the force of the beat, or both 

 together, known as accelerators or augmentors. In the verte- 

 brates thus far examined the vagus is in reality a vago-sympa- 

 thetic nerve, containing inhibitory fibers proper, and sympa- 

 thetic, accelerator, or motor fibers. 



The inhibitory fibers can arrest, slow, or weaken the cardiac 



