THE CIRCULATION. 285 



ventricle. It is also momentary in duration, the line, after reaching its 

 maximum elevation, immediately descending nearly to its former level. 

 Then follows a series of undulations, due to the vibration of the auriculo- 

 ventricular valves, already closed by the contraction of the ventricle. 

 The pressure in the relaxed auricle then slowly increases by the influx 

 of blood from the great veins, until the time arrives for a second auric- 

 ular contraction, when it suddenly rises to a maximum and again falls 

 as before. The ventricular contraction, which follows almost immedi- 

 ately that of the auricle, is much more powerful, but requires a little 

 longer interval to arrive at its maximum, and its entire duration is at 

 least three times as long as that of the auricle. It will be seen that 

 during the slow filling of the auricle with blood, a similar partial 

 increase of pressure takes place in the ventricle ; and the maximum 

 pressure in the auricle corresponds in time, with a momentary undula- 

 tion in the ventricle, immediately followed by the extreme rise of 

 pressure, due to the ventricular contraction. ' 



The exploration of the left cavities of the heart by these means, is 

 much more difficult than that of the right, but Marey succeeded, in 

 several experiments, in introducing pressure-tubes simultaneously into 

 the right auricle and ventricle, through the jugular vein, and into the 

 left ventricle through the carotid artery, and in ascertaining the com- 

 parative pressure in these cavities, as measured in millimetres of the 

 mercurial column. In one instance the force indicated by the different 

 pressure-gauges was as follows : 



PRESSURE OF BLOOD ix THE 



Eight Auricle 2.5 mm. 



Right Ventricle 25.0 " 



Left Ventricle 128.0 



The relation in force between the two ventricles varied somewhat in 

 different animals, according to their bodily condition and the state of 

 the circulation ; but taking all the observations together, the force of 

 pressure in the left ventricle was in general from three to five times 

 greater than in the right. 



The pressure to which the blood is subjected in the ventricles is 

 therefore much greater than in the auricles ; and it is prevented from 

 reacting in a backward direction by the closure of the auriculo-ventricu- 

 lar valves. The force of the right ventricle is expended on the blood 

 in its passage through the pulmonary artery and the pulmonary capil- 

 laries ; while that of the left ventricle is sufficient for its propulsion 

 through the general arterial system. 



The Arterial Circulation. 



The arteries are a system of branching tubes, which commence with 

 the aorta and ramify throughout the body, distributing the blood to 

 the peripheral vascular organs. They consist of three principal coats, 

 namely, an inner coat, composed of thin elastic lamin* lined with 

 flattened epithelium cells ; a middle coat, containing elastic tissue and 



