478 MOVEMENTS. 



or no great amount of force is required, is very different from 

 a muscular effort. For example, in ordinary progression there 

 is simply a movement produced by the action of the proper 

 muscles, almost without our consciousness, and this is unat- 

 tended with any modification in the circulation or respira- 

 tion; but if we attempt to lift a heavy weight, to jump, to 

 strike a powerful blow, or to make any vigorous effort, the 

 action is very different. In the latter instance, we prepare 

 for the muscular action by inflating the lungs, closing the 

 glottis, and contracting more or less forcibly the expiratory 

 muscles, so as to render the thorax rigid and unyielding ; 

 and by a concentrated effort of the will, the proper muscles 

 are then brought into action. 



This remarkable action of the muscles of the thorax 

 and abdomen, due to simple effort, and independent of 

 the particular muscular act that is to be accomplished, 

 compresses the contents of the rectum and bladder, and 

 obstructs very materially the venous circulation in the 

 large vessels. It is well known that hernia is frequently 

 produced in this way; the veins of the face and neck be- 

 come turgid ; the conjunctiva may become ecchymosed ; and 

 sometimes aneurismal sacs are ruptured. An effort of this 

 kind is generally of short duration, and cannot, indeed, be 

 prolonged beyond the time during which respiration can be 

 conveniently arrested. At its conclusion there is commonly 

 a prolonged expiration, which is audible and somew;hat vio- 

 lent at its commencement. 



There are degrees of effort which are not attended with 

 this powerful action of the muscles of the chest and abdo- 

 men, and in which the glottis is not completely closed ; and 

 an opening into the trachea or larynx, rendering immobility 

 of the thorax impossible, does not interfere with certain acts 

 that require considerable muscular power. If we examine a 

 dog with the glottis exposed, when he makes violent efforts 

 to escape, we can see that the opening is firmly closed. This 

 fact is indicated by Longet, and we have often observed it 



