1891.] On the Form of the 'thorax. 143 



V. " A simple Mode of Demonstrating how the Form of the 

 Thorax is partly determined by Gravitation." By T. P. 

 ANDERSON STUART, M.D., Professor of Physiology in the 

 University of Sydney, N.S.W. Communicated by Professor 

 SCHAFER, F.R.S. Received January 12, 1891. 



It is a well-known fact that the quadrupeds have the transverse 

 gectiou of the thorax elliptical with the long axis vertical. This form 

 of thorax, more or less, is possessed also by the human foetus. As 

 the erect posture is gradually assumed in the development of species 

 and of the human individual the ventro-dorsal and transverse 

 diameters approximate to each other, and then, the process continuing, 

 in the adult the transverse diameter exceeds the aiitero-posterior. 



That these aro the forms proper to the thorax when under the 

 influence of gravitation alone is seen by holding a hoop made of a 

 strip of ordinary crinoline steel ^ inch wide and about 6 feet long, so 

 that its plane is vertical ; its form is that of an ellipse. Now grasp 

 the hoop firmly between the fore-finger and thumb of one hand, and 

 gradually turn the internal face of the portion grasped till it looks 

 straight forwards. The front part of the hoop will, of course, be 

 lower, corresponding in some measure to the slope of the ribs, &c. 

 At the same time the diameters approximate to each other. Con- 

 tinue the tuining till the face that looked straight forwards looks 

 upwards and forwards, so that in fact the plane of the grasped 

 portion corresponds to that in which the lower dorsal region of the 

 vertebral column of man lies. The slope of the ribs is lessened, but 

 the interesting points are that the transverse diameter exceeds the 

 antero-posterior, and the exact curve and direction of the surface of 

 the lower ribs are reproduced. Then are seen the twist in the long 

 axis of the rib and likewise that great hollow on each side of the 

 \ertebral column which is so marked a feature in the human thorax. 



I do not overlook the fact that the conditions in the organism aie 

 not just the same as they are in this simple hoop ; but I think it will 

 be conceded that where there is a force so constant and so potent in 

 its action as is that of gravitation it will be yielded to by the 

 organism unless there be some good reason to the contrary. Now 

 there does not seem to me to be any such reason here, and it is 

 interesting to observe how closely the thorax of the animal follows 

 the lines of the hoop of steel when the conditions as to gravitation 

 are the same. 



I am thus led to suspect that gravitation has had a larger share 

 than is usually thought in moulding the form of the vertebrate thorax 

 both in health and disease. 



