Experiment regarding the Thigh-bone. 9.ol 



pressure diminished below three inches of mercury, when the 

 head of the thigh-bone began to sink gradually, and to the 

 amount of half an incli, which was as far as was permitted by 

 the capsule. For the capsular membrane forms a ring round 

 the neck of the head of the thigh-bone, which is smaller than 

 the circumference of the head. This ring, therefore, allows 

 the sinking down of the head ; but it prevents it from escaping 

 altogether : that is, it protects the hip-joint from dislocation. 



After the head of the thigh-bone, in consequence of the rare- 

 fjiction of the air, had fallen eight lines (as far as it was per- 

 mitted by the capsular ring) we allowed the air to re-enter the 

 receiver. Immediately the fallen head of the thigh-bone, with 

 its attached weights, rose, and was elevated rapidly in a palpa- 

 ble manner, — apparently of its own accord, but in reality in 

 consequence of the pressure of the air, which had entered the 

 receiver but not the socket, — until it reached the cover of the 

 receiver, or, in other words, had returned to its original height. 

 This experiment was repeated several times, by alternately ex- 

 hausting and readmitting air to the receiver, and always with 

 the same result. 



The experiment was next performed after the capsular ring 

 had been cut transversely across, and the head of the thigh- 

 bone had been even entirely taken out of the socket, yet it was 

 again pressed in with violence. In fact, the air thus forced 

 into the socket became so much compressed that the head of 

 the thigh-bone was again suspended by the pressure of the air 

 from withont. (Our previous experiments proved that the hip- 

 joint may be regarded as an air-pump, out of which the piston 

 does not fall by its own weight, even when some air is left. 

 'J'his air becomes very much rarefied whenever the piston falls 

 in the slightest degree. We bored a fine hole through the ball 

 of the socket, and immediately closed it, opening it again im- 

 mediately, to further the object of our experiment, but we 

 could not prevent some air remaining behind in the hole after 

 the closing of it, whence it might be distiibuted, as out of the 

 space in the air-pump where air remains behind, without our 

 experiment being essentially disturbed.) By the diminution of 

 the pressure of the air, the head of the thigh-bone, on this oc- 

 casion, fell out entirely and more speedily, while previously 

 when the pressure of the air was undiminished, the weight o 



