570 THE VASCULAR SYSTEMS 



valves which are shortly afterward pushed aside during the next succeeding 

 auricular contraction. 



It will be observed that each set of chambers contracts and relaxes alternately. 

 However, the time occupied by the auricular contraction is short, while that occu- 

 pied by the relaxation is long in comparison with the time occupied by the similar 

 events in the ventricles. From the beginning of the ventricular diastole until the 

 beginning of the next auricular systole the entire heart is at rest the so-called 

 common pause of the heart's chambers. The average duration of a cardiac cycle 

 is about -fa of a second, which may be divided, approximately, among its different 

 phases as follows: 



Auricular systole, fa. Auricular diastole, -fa. 



Ventricular systole, ^5-. Ventricular diastole, -f$. 



Common pause ^. 



As mentioned before, the contraction process, which begins at the mouths of 

 the great veins, spreads with great rapidity over the auricles, and then to and 

 over the ventricles. A slight delay is experienced by the contraction wave in 

 passing from the auricles to the ventricles. This is due in all probability to the 

 circumstance that the contraction wave must be conducted along a narrow bridge 

 of muscle tissue which is more or less embryonic in character. This bridge of mus- 

 cle tissue connecting the auricles with the ventricles, morphologically and physio- 

 logically, is the auriculoventricular bundle of His (page 567). In accordance with 

 this view, the origin and conduction of the contraction process is a property of 

 the muscle tissue, and the theory which embodies this view is known as the myo- 

 genic theory of the heart beat. Inasmuch as nerve cells and their axones are 

 found in many parts of the heart, the auriculoventricular bundle included, it is 

 thought by some that the origin and conduction of the stimulus giving rise to 

 the contraction process is a property of the nerve tissue. The theory embodying 

 this view is known as the neurogenic theory of the heart beat. It must be said, 

 however, that many facts tend to demonstrate that the myogenic theory is the cor- 

 rect one. Whatever be the tissue in which the heart beat originates, the cause of 

 the contraction must be sought in the heart itself; for the mammalian heart can be 

 made to beat for a relatively long time when completely isolated from the body of 

 the animal. 



The cardiac muscle, therefore, does not depend for its contractions on the dis- 

 charge of nerve impulses by the central nerve system, although the latter, through 

 the extracardiac nerves, exercises a regulative influence on the heart's action, 

 increasing or decreasing the rate or force of the heart beats in accordance with 

 the physiological needs of the body. (G. Bachmann.) 



Surface Form. In order to show the extent of the heart in relation to the front of the thorax, 

 draw a line from a point in the second left intercostal space, 4.5 cm. (nearly two inches) from 

 the median line to the upper border of the third right costal cartilage, 3 cm. (one inch and a 

 quarter) from the median line. This represents the base line or upper limit of the organ. Take 

 a point in the fifth left intercostal space 8 cm. from the median line (about three and a quarter 

 inches) ; this represents the apex of the heart. Draw a line from this apex point, with a slight 

 convexity downward, to the junction of the seventh right costal cartilage to the sternum 2.5 cm. 

 from the median line. This represents the lower limit of the heart. Join the right extremity 

 of the first line that is, the base line with the right extremity of this line that is, to the seventh 

 right chondrosternal joint with a slight curve outward, so that it projects about 3.5 cm. (one 

 inch and a half) from the mid-line of the sternum. Lastly, join the left extremity of the base 

 line and the apex point by a line curved slightly to the left. 



The position of the various orifices is as follows viz., the pulmonary orifice is situated in 

 the upper angle formed by the articulation of the third left costal cartilage with the sternum; 

 the aortic orifice is a little below and internal to this, behind the left border of the sternum, 

 close to the articulation of the third left costal cartilage to this bone. The left auriculoventricular 



