142 LOCOMOTION. [CHAP. vi. 



assumes about the middle of the articular surface, where the base of 

 the wedge is turned in the opposite direction to that which it occu- 

 pies either at the upper or the lower part of the articular surface.* 



These provisions for the strength and security of the sacrum are 

 of great importance to the general mechanism of the pelvis, whether 

 we regard it as a bony girdle constructed for the transmission of 

 the weight of the trunk to the thigh-bones, or as an osseous cavity 

 destined to contain and protect certain important viscera. 



Viewing the pelvis in the former light, we must notice the thick- 

 ening of the iliac bones along either side of its upper outlet. The 

 groinings, thus formed, terminate opposite the acetabula, and trans- 

 mit the superincumbent weight, which they share with the sacrum, 

 to each of those cavities, whence it is again transferred to the heads 

 of the thigh-bones : they are formed of dense compact substance, 

 which contrasts strikingly with the thin lamellated structure of the 

 surrounding osseous tissue. 



The obliquity of the pelvis has a two-fold object : first, with 

 reference to the weight from above : and, secondly, with respect to 

 concussions transmitted upwards by the lower limbs, in leaps, or 

 other rapid movements. In both instances, the shock is distributed 

 over a greater extent of surface, and is participated in by a greater 

 number of joints, than if the pelvis were placed directly beneath the 

 spine ; for it is obvious, that, were the axis of the pelvis verti< 

 and the femora placed perpendicularly under it, the weight from 

 above would bear its chief force upon the sacrum, and the concus- 

 sion from below would be felt in the hip-joints alone. 



In progression, the whole pelvis receives the concussions, whei 

 they proceed from above or from below. Hence the ossa pubis 

 united by an intervening elastic fibre-cartilage ; and any disturban< 

 of this joint during pregnancy, or in the act of parturition, occasioi 

 great difficulty to the patient in walking, or even in maintaining the 

 erect posture. 



Of the Thorax. The thorax is a conoidal cavity, slightly flal 

 tened on its anterior aspect. It is constructed with obvious refer- 

 ence to lightness, elasticity, and mobility ; all these qualities bein^ 

 requisite for its adaptation to the ever-varying movements of tl 

 organs it contains. 



The walls of the chest are formed behind by the dorsal vertebi 

 to which twelve ribs are articulated on each side ; seven of the* 

 the true ribs, are connected to either margin of the sternum 



* We refer for further details on this subject to Mr. Ward's excellent worl 

 p. 256. 



