614 



HEART. 



contraction of the auricle must force an ad- 

 ditional quantity into the ventricle, and this, 

 though small in quantity, may be quite suffi- 

 cient to excite the ventricles to contraction, 

 when the irritability is not too much impaired.* 

 It is only in this manner, taken along with the 

 greater irritability of the internal surface over 

 the external, that we can explain the observa- 

 tion made by Dr. Knox in the course of his 

 experiments upon the irritability of the heart 

 in fishes, where, when the irritability was nearly 

 exhausted, contractions excited in the auricle 

 were sometimes followed by contractions of the 

 ventricle, when irritation of the outer surface of 

 the ventricle itself produced no efiect.f Cer- 

 tainly, under ordinary circumstances, this regu- 

 larity of the heart, so necessary for the proper 

 performance of its functions, is a marked fea- 

 ture in its action ; but that it is not either ne- 

 cessarily connected with its structure or vital 

 properties, but depends solely on the manner 

 in which its stimulant, the blood, is applied, is 

 proved by various facts. 1st. The movements 

 of the auricles and ventricles generally cease at 

 different times after death ; and though the 

 auricles much more frequently continue to con- 

 tract after the ventricles, yet several accurate 

 experimenters have observed the left auricle 

 become quiescent before its corresponding 

 ventricle. J 2dly. When the movements of 

 the ventricle have ceased, while the auricles 

 continue to contract, the ventricle may generally 

 be excited to vigorous contractions by the ap- 

 plication of a powerful stimulus. 3dly. When 

 the irritability of the heart becomes somewhat 

 languid, two, three, or sometimes six or seven 

 contractions of the auricle may take place be- 

 fore the ventricles are roused to contraction ; 

 the evident deduction from which is, that the 



* When the heirt has ceased to contract, it may 

 frequently be called into pretty vigorous action by 

 opening one of the large veins, and blowing some 

 air into its cavities. 



t I have repeatedly attempted to ascertain if the 

 circumstances here described as sometimes occurring 

 in the cold-blooded animals could be observed in 

 the warm-blooded animals, but without success. 

 In one experiment upon the heart of a rabbit, after 

 all the movements of the ventricles had ceased, 

 but where they could still be readily excited by the 

 applifiition of a stimulant, we were convinced that 

 contraction of the auricle, when excited by stimu- 

 lation applied to itself alone, was sometimes fol- 

 lowed by contaction of the ventricle even after 

 the ventricle had been slit open. Hut in subsequent 

 experiments upon dogs, we ascertained a source of 

 fallacy which we had overlooked in the other expe- 

 riment, for we found that a slight movement of 

 the heart on the surface upon which it rests, such 

 as that caused by a very gentle pull at the large 

 arteries, and not exceeding the effects produced by 

 the contraction of the auricle, was, in some of 

 these cases, sufficient to excite contractions of the 

 ventricles. 



f In one experiment upon a cat, I distinctly ob- 

 served the rijilit ventricle occasionally pulsate 

 twice for each pulsation of the auricle. In another 

 experiment, I distinctly observed the contractions 

 of the ventricles precede those of the auricles, 

 when the contractility of the heart had become en- 

 feebled. In this case, the pause in the heart's 

 action occuricd after the contraction of the auri- 

 cles. 



contractions of the ventricles do not neces- 

 sarily follow those of the auricles, unless 

 the contractions of the auricles occasion the 

 application of a stimulant to the inner sur- 

 face of the ventricles sufficient to excite them 

 to contraction. 4thly. The movements of the 

 ventricles and auricles will go on in the same 

 manner, though detached from each other 

 by the knife. 5thly. If we were allowed to 

 argue from final causes in negative cases, we 

 could easily shew that a peculiar endow- 

 ment, such as we are contending against, 

 would not be of the slightest advantage in se- 

 curing the regularity and constancy of the 

 heart's movements. Jt appears, then, quite un- 

 philosophical to call in the agency of some un- 

 known and indefinite principle for the produc- 

 tion of these periodic movements, as they have 

 been called, of the different chambers of the 

 heart, when they can be satisfactorily referred 

 to the laws which regulate muscular contracti- 

 lity in other parts of the body. We have here 

 a beautiful example of the manner in which 

 nature produces adaptation of means to an end, 

 not by the creation of new properties, which 

 we, in our ignorance, sometimes erroneously 

 attribute to her, but by the employment of 

 those already in use in the performance of other 

 functions, only modified to accommodate them 

 to the circumstances under which they are 

 placed. 



Sounds of the heart. On applying the ear 

 over the region of the heart, two distinct 

 sounds are heard accompanying its contraction. 

 Though the existence of such sounds seems to 

 have been known to Harvey,* who compares 

 them to the noise made by the passage of fluids 

 along the cesophagus of a horse when drinking, 

 yet, as is well known, it is to Laennec that we 

 owe the first accurate description of the charac- 

 ter of these sounds, the order of their succes- 

 sion, and the manner in which they may here- 

 after be made available for the important pur- 

 poses of the diagnosis of the diseases of the 

 heart. 



The first of these sounds is dull and pro- 

 longed ; the second, which follows closely upon 

 tht: tirst, is sharp and quick, and is likened by 

 Laennec to the flapping of a valve, or the lap- 

 ping of a dog. After the second sound a pause 

 ensues, at the end of which the sounds are 

 again heard. These three the first sound, the 

 second sound, and the pause occur in the 

 same uniform order, and when included along 

 with the movements of the heart, to which they 

 owe their origin, have received the term rhythm 

 of the heart. As the dull prolonged sound is 

 synchronous with the impulse of the heart, and 

 consequently with the contraction of its ventri- 

 cles, Laennec attributed this sound to the con- 

 traction of the ventricles. The second sound, 

 which is synchronous with the diastole of the 

 ventricles, he supposed must depend upon the 

 systole of the auricles ; and to this he was 

 naturally led by the supposition that their con- 

 traction must also produce some sound. From 

 the weight of Laennec's authority, this opinion 



* Op. cit. cap. v. 



