PHYSIOLOGY. 



the veins. Lastly, the p'assage of the 

 blood from the arteries into the veins, 

 may be proved by the direct testimony of 

 the senses in living- animals. The use of 

 the microscope affords this proof in the 

 transparent parts of cold blooded animals, 

 as the mesentery and web of the foot in 

 frogs, the tail of fishes, &.c. 



The motions of any part of the heart, 

 considered singly and individually, consist 

 in a constant series of alternate contrac- 

 tions and dilatations ; or, as they are tech- 

 nically named, alternate states of systole 

 and diastole. The contractions take place 

 as in other muscles, the dilating cause 

 consists in the forcible entrance of blood 

 into the cavity. The auricles and ventri- 

 cles, when viewed in relation to each 

 other, are successively contracted and di- 

 lated ; the corresponding parts acting at 

 the same time on both sides of the heart. 

 Thus, when the auricles contract, in or- 

 der to expel the blood which they have 

 received from the system at large, and 

 from the lungs, the ventricles are relaxed, 

 and therefore in a state fit for receiving 

 this blood. When, in the following mo- 

 ment, the recently filled ventricles con- 

 tract, in order to urge forwards the blood 

 into the two arterial trunks, the auricles 

 are relaxed, and become immediately dis- 

 tended by the current of venous Wood. 



The action of the heart, and of the ves- 

 sels connected with it, may therefore be 

 distributed into successive periods. In 

 the first of these, the ven?s cavae and pni- 

 monary veins pour their blood into the 

 two auricles, and thus cause a diastole of 

 these cavities. The systole of the auricles 

 tran-mits the blood into the ventricles in 

 the second period ; and these latter ca- 

 vities expel their contents into the arteries 

 in the third portion of time. Thus the ac- 

 tion of the veins takes place at the same 

 point of time with that o t the ventricles ; 

 and the contraction of the auricles is syn- 

 chronous with that of the arteries. 



The systole of the ventricles, which is 

 supposed to occupy one third of the time 

 of the whole pulsation of the heart, is ac- 

 complished by an approximation of the 

 sides of the cavities to the middle parti- 

 tion, and of the apex to the basis of the 

 heart. The whole viscus by this means 

 becomes shorter and more obtuse. The 

 well known fact of the heart striking 

 against the left breast in its contraction, 

 may seem on the first glance to refute 

 this account of the systole of the ventri- 

 cles. But, on a further examination, it 

 can have no such effect ; since the phe- 

 nomenon in question depends on two 

 causes amply sufficient to produce, the 



effect. The swelling of the auricles, 

 which are at the back of the heart, and 

 particularly of the left auricle, which is 

 interposed between the spine and the base 

 of the left ventricle, necessarily causes 

 the point of the heart to advance towards 

 the side ; and this motion may be imitat- 

 ed in the dead body by injecting or in- 

 flating the muscles. The other cause 

 consists in the connection of the large 

 arteries, particularly of the aorta, vvitli 

 the base of the heart. A curved and flexi- 

 ble tube, when suddenly distended, be- 

 comes in some measure straightened. 

 Thus, when the blood is impelled into the 

 aorta, the curve of that vessel approach- 

 es more nearly to a straight line. Its 

 posterior end being firmly attached to 

 the vertebras is immovable ; to its anterior 

 and moveable part is fixed the heart, 

 which, by the straightening of the vessel, 

 is obliged to describe a portion of a cir- 

 cle, in doing which, the apex strikes 

 against the side. These two circumstan- 

 ces occur simultaneously; the venous 

 blood rushes into the auricles at the same 

 time that the ventricle fills the aorta. 

 The impulse of the blood expelled by the 

 aortic ventricle is felt in the whole arte- 

 rial system ; and it produces in all arte- 

 ries, which come within the sphere of the 

 touch, and which have an area of not less 

 than one-sixth of a line in diameter, an 

 obvious and perceptible effect, called the 

 pulse, which is a real state of diastole of 

 the artery, and which is ascertained to 

 correspond exactly, and to be perfectly 

 synchronous with the systole of the heart. 

 The number of pulsations in a given space 

 ot time varies infinitely in healthy per- 

 sons. Age is the chtef cause of these 

 varieties; but other circumstances, which 

 constitute the peculiar state of health of 

 each individual, have considerable effect; 

 so that no standard can be settled which 

 shall prove generally correct. The fol- 

 lowing numbers afford, we believe, as 

 near an approximation as can be expect- 

 ed amidst so much uncertainty ; they will 

 sevve at least as a comparative view in 

 subjects of different ages. The heart of 

 an infant, sleeping tranquilly, performs in 

 the first days of existence about 140 pul- 

 sations in a minute ; at the end of the 

 first year the pulsations are, in the same 

 space of time, 124. 

 At the end of the second year 110 

 Third and following years 96 



Seventh and following 86 



Time of puberty 80 



Manhood 75 



Sixtieth year 65 



The pulsations of the heart proceed ia 



