116 OF VITAL MOTION. 



contractions. In the structure of the coats of the 

 auricles^ also, there is reason for the same conclusion. 

 This is different from that of the ventricle, for though 

 truly muscular, yet it is that unstriped variety which 

 is more closely allied to what is found in the coats of 

 the veins, than in voluntary muscle and the muscle of 

 the ventricle. Now, in veins there is an absence of 

 any rapid rhythmic changes, though there is a capacity 

 for slower and more prolonged movements, and hence 

 it may be doubted whether there are any rapid changes 

 in the auricles. It is the same, also, in the coats of 

 the alimentary canal, and hence another and con- 

 firmatory reason for the same conclusion. In relation 

 to structure, indeed, the two cavities of the heart are 

 widely different : on the one hand, the ventricles are 

 composed of muscle which, from its striped character 

 and close analogy to voluntary muscle, may be sup- 

 posed to be extremely apt to rapid and energetic 

 movements, especially under the agency of nervous 

 influence; and on the other hand, the auricles, like 

 the coats of the veins and alimentary canal, are com- 

 posed of the unstriped variety of muscle, and hence 

 we may infer that their movements, like those of the 

 veins and intestines, are slow and not likely to respond 

 with readiness to a nervous stimulus. 



In this manner, therefore, the problem of the heart's 

 action may be greatly simplified, and we may find 

 some reason why the auricle should contract at a time 



