CIRCULATION. 



657 





kind referred to must be so difficult as to be 

 almost useless; indeed, it is very probable 

 that some have mistaken the contraction for 

 the dilatation, and we shall afterwards find 

 that the power of suction, exerted by the heart 

 on the blood, as measured by the force with 

 which the veins are emptied, is veiy small 

 indeed. It is clear that the blood driven on 

 from behind by a propelling power, or flowing 

 through parts which are pressed upon by 

 neighbouring organs, must enter the heart 

 more easily during the relaxation of the pa- 

 rietes of the ventricle than at any other period 

 during the heart's action, so as to give rise 

 to an appearance of suction, but direct expe- 

 riments make it sufficiently obvious that the 

 force of impulsion from behind is almost the 

 sole cause of the entrance of the blood from 

 the trunks of the great veins into the cavities 

 of the heart. 



In order to form an estimate of the time in 

 which a given quantity of blood may pass 

 through the heart, or of the time in which the 

 whole quantity of blood contained in the body 

 would take to pass through the heart, several 

 data are required which are not yet furnished 

 by accurate experiments. In the first place, 

 we must know the average quantity of blood 

 contained in the body, and, in the next place, 

 the quantity which is evacuated from the heart 

 at each stroke or systole of the ventricles. 



With regard to the first of these points, 

 a number of calculations have been made 

 which vary greatly in their results. Animals 

 have been bled to death by the section of the 

 larger bloodvessels, and the quantity of blood 

 lost has been measured. The quantity of 

 blood lost in this way seems to have varied 

 from l-10th to l-30th of the weight of the 

 whole body, and Dr. Moulins, who formed 

 his estimates from experiments of this kind, 

 rated the quantity of blood in the human body 

 at eight or nine Ibs. only, or l-20th of the 

 weight of an average sized man, taken at 150 

 or 160 Ibs. But it is obvious that when one of 

 the larger bloodvessels is opened, from the 

 suddenness of the flow, the animal faints or 

 dies before the whole or even a considerable 

 proportion of the blood has been lost; and it 

 has been ascertained from numerous obser- 

 vations, that when the blood flows more gra- 

 dually and from small vessels, as occurs in 

 hemorrhages from the nose, stomach, rectum, 

 or uterus, a proportionally much greater quan- 

 tity of blood may be lost than occasions death 

 in animals experimented upon by the section 

 of the larger arteries or veins. Instances are 

 on record in which from ten to twenty Ibs. 

 and even greater quantities of blood have 

 flowed from the human body within twenty- 

 four hours.* We feel inclined on these 

 grounds to coincide with the estimate formed 

 by Haller, that the blood forms about a fifth of 

 the weight of the body, or equals from twenty- 

 five to thirty Ibs. in a man of the average 

 weight of 150 Ibs. It is obvious that this 



* Sec Haller'.s Eleincma, and Kiill on the An. 

 Econ. 



must vary in different individuals from other 

 circumstances besides a difference of stature. 

 In the young, the quantity of blood is con- 

 sidered to be greatest. Of the whole of the 

 blood contained in the body, it is estimated by 

 Haller, and probably with accuracy, that four 

 parts are contained in the arterial and nine in 

 the venous system. 



In endeavouring to estimate the quantity 

 of blood which passes through the heart in a 

 given time, we must find the capacity of the 

 cavities of the heart, we must ascertain whe- 

 ther the cavities on the two sides are of the same 

 size, and, as it is almost impossible to measure 

 the quantity of blood evacuated from the heart 

 at each stroke, we must find to what e\tent 

 the ventricles empty themselves during their 

 systole. It is obvious that, so long as the 

 circulation is uniform and no local accumu- 

 lation of blood takes place, the same quantity 

 of blood must pass out of the ventricles into 

 the larger arteries which enters by the veins, 

 and for the same reasons, that the quantity of 

 blood passing through the right and left cavi- 

 ties of the heart must be exactly equal. The 

 circumstance that an equal quantity of blood 

 passes out of the right and left cavities of the 

 heart during their systole does not entitle us 

 to conclude that the capacity of the different 

 auricles and ventricles is the same, because 

 any one of them during its systole may be 

 more or less completely emptied than the rest, 

 and a regurgitation obviously takes place from 

 some of them, so that the whole blood which 

 they contain is not propelled in its onward 

 course. According to some anatomists the au- 

 ricles are larger in capacity than the ventricles, 

 probably in the proportion of three or two and 

 a half to two, and the auricles are by no means 

 completely emptied during their systole. An 

 opinion has very generally prevailed that the 

 cavities on the right side of the heart are some- 

 what larger than those on the left. There is no 

 doubt that in making measurements of the rela- 

 tive capacity of the two sides after death, it is 

 most frequently found so ; but it is obvious that 

 some have very much overrated the difference, 

 and there is much reason to believe that the 

 greater capacity of the right auricle and ven- 

 tricle depends in part on the accumulation of 

 blood which generally takes place in most 

 kinds of slow death in the pulmonary arteries, 

 and in part also upon the greater thinness and 

 consequent distensibility of the right ventricle. 

 In men dying suddenly, and in animals killed 

 purposely, in which the pulmonary artery is 

 opened so as to allow of the free egress of the 

 blood from the right side of the heart, the 

 capacity of this ventricle is not greater than 

 that of the left, and the proportions of the 

 capacity of the two sides of the heart usually 

 found after slow death are sometimes reversed 

 when a ligature is placed on the aorta and the 

 pulmonary artery is opened.* Most authors 

 seem to have agreed to follow the estimate of 

 the capacity of the ventricles given by Hales 

 in his Medical Statics. This author esti- 



* Sabatier. 



