THE CIRCULATION. 293 



The average arterial pressure, in the dog and other animals of similar 

 size, is equivalent to a column of mercury 150 millimetres in height. 

 But while, in such an instrument, connected with the arterial system, 

 the mercurial column indicates on the whole an average pressure, it 

 also exhibits two series of oscillations ; showing a fluctuation in the 

 degree of pressure, owing to two different causes. 



One of these oscillations is synchronous with respiration. At every 

 inspiration, the level of the mercury falls somewhat, with every expira- 

 tion it rises. As the movement of inspiration consists in an expansion 

 of the chest cavity, its effect is to diminish the pressure on the heart 

 and great blood-vessels, and consequently to lower in a similar degree 

 the tension of the whole arterial system. In expiration, on the other 

 hand, the thoracic walls return to their former position, and the pressure 

 on the organs within the chest is reestablished. These changes are 

 indicated by corresponding variations in the height of the mercurial 

 column. The oscillations due to this cause, however, are not uniform, 

 but vary according to the condition of the respiratory movements. 

 When respiration is active and labored, they may reach the extent 

 of 30 millimetres ; when it is quiet, as in an animal deeply etherized, 

 they may be nearly imperceptible. 



The remaining oscillation is more uniform, and is due to the cardiac 

 pulsations. It consists of comparatively rapid undulations of the mer- 

 curial column, simultaneous with the movements of the heart. At 

 every ventricular contraction the mercury rises 12 or 15 millimetres, 

 and at every relaxation falls to its previous level. The instrument 

 thus indicates the intermitting pressure of the heart's action ; and has 

 accordingly received the name of the car diameter. As the average 

 height of the column in the cardiometer is 150 millimetres, and as it 

 varies by 15 millimetres under the influence of the cardiac pulsations, 

 it appears that each contraction of the heart is superior in force to the 

 resistance of the arteries by about one-tenth ; and the arterial system 

 is, therefore, kept full, and the arterial tension maintained, notwith- 

 standing the constant discharge of blood into the capillaries. 



Rapidity of the Arterial Current. The blood moves in the arteries 

 more rapidly than in any other part of the vascular system. Its exact 

 rate varies according to the situation of the vessel and the period of the 

 pulsation ; being greatest in the immediate neighborhood of the heart, 

 and diminishing from this point outward. The division of the arterial 

 trunks into branches and ramifications increases their surface of con- 

 tact with the blood ; and the increased adhesion produced by this con- 

 tact retards the current, which is accordingly slower in the small 

 arteries than in those of large or medium size. In the smallest arteries, 

 as seen under the microscope in the transparent tissues, the partial 

 adhesion of the blood to the vascular wall, and the greater rapidity 

 of its flow in the axis of the vessel are readily perceptible. The con- 

 sistency of the circulating fluid, however, and the smoothness of the 

 internal surface of the arteries, are such that this obstacle to the move- 



