CHAP. IX. t DECISION. 235 tion, and then to cease breathing; but he thinks that Sir C. Bell's explanation is erroneous. He maintains that arrested respiration retards the circulation of the blood, of which I believe there is no doubt, and he ad- duces some curious evidence from the structure of the lower animals, showing, on the one hand, that a retarded circulation is necessary for prolonged muscular exertion, and, on the other hand, that a rapid circulation is neces- sary for rapid movements. According to this view, when we commence any great exertion, we close our mouths and stop breathing, in order to retard the circulation of the blood. Gratiolet sums up the subject by saying,. " C'est la la vraie theorie de Peffort eontinu;" but how far this theory is admitted by other physiologists I do not know. Dr. Piderit accounts15 for the firm closure of the mouth during strong muscular exertion, on the principle that the influence of the will spreads to other muscles be- sides those necessarily brought into action in making any particular exertion; and it is natural that the muscles of respiration and of the mouth, from being so habit- ually used, should be especially liable to be thus acted on. It appears to me that there probably is some truth in this view, for we are apt to press the teeth hard to- gether during violent exertion, and this is not requisite to prevent expiration, whilst the muscles of the chest are strongly contracted. Lastly, when a man has to perform some delicate and difficult operation, not requiring the exertion of any strength, he nevertheless generally closes his mouth and ceases for a time to breathe; but he acts thus in order that the movements of his chest may not disturb those of his arms. A person, for instance, whilst threading a 1B 'Mimik und Physiognomik,' s. 79.