194 LOCOMOTION. [CHAP. VII. 



circumstances prevent them. The heart beats independently of our 

 consciousness or controul ; but the respiratory action may be has- 

 tened, or retarded, at will, though not stopped. This voluntary 

 power is given because these muscles are required in various move- 

 ments of the body, either alone, or in aid of others ; they minister 

 to other functions besides that of respiration. The voluntary, or 

 irregular action, however, is entirely subordinate to the involuntary 

 and rhythmical. 



The rhythmical character of the respiratory act is to be ex- 

 plained by reference to the stimulus by which it is ordinarily 

 excited. This is an impression made on the internal surface of the 

 lungs by the deteriorated air, and recurs periodically from the 

 change induced in the inspired air by its contact with the blood in 

 the air-cells. 



Though the heart is in no respect under voluntary influence, yet 

 emotional and instinctive impulses easily affect it : its action is 

 throbbing, tumultuous, or feeble. These impulses act through the 

 cardiac nerves, which, if stimulated mechanically, will excite con- 

 tractions in a heart removed from the body, and which has almost 

 ceased to beat : but, under all circumstances, the action of the heart 

 is rhythmical. The cause of the rhythm it is exceedingly difficult 

 to resolve. This variety of contraction is coincident with periodic 

 distension of the cavities, and impressions on their lining membrane. 

 But it continues long after the heart is empty, and its nerves cut. 

 Hence, whatever share these circumstances may have in giving the 

 rhythmical character in the natural condition of the parts, they are 

 certainly not essential to each individual pulse. It is singular that 

 a mechanical stimulus applied once to the heart will often excite a 

 series of contractions after they had ceased, or modify the rhythm of 

 those previously existing ; its effects being thus prolonged through 

 many beats. 



In reviewing the actions of the voluntry muscles, we may remark 

 the following interesting circumstances : 



1 . As to Association of Movements. By the mechanical arrange- 

 ments of the muscles on the bony framework, and by the pecu- 

 liarity of their several nervous connexions, they are rendered capa- 

 ble of conspiring in those combined actions which produce the 

 various attitudes and general movements of the body. There are 

 few muscular actions indeed of an entirely solitary kind. In the 

 animation of the features under the passions, in articulation, in 

 deglutition, in respiration, and in numberless other cases, we have 

 examples of this association of many actions to the production of 



