THE THORAX. 107 



peculiar to pregnancy. The pelvis, however, has upon the 

 spine, flexion, extension, lateral inclination, and rotation ; the 

 latter being performed by a series of very slight twists of the 

 lumbar vertebra upon each other. Like all other motions, it 

 is much extended by habit in early life. Of this I have seen 

 an instance, in an adult Indian, who, from infancy, had been 

 deprived entirely of the use of the lower extremities; but who, 

 by being seated in a large wooden bowl, with a round bottom, 

 and having his legs drawn up in a squatting position, could, by 

 alternate twists of the spine, with the assistance of a short staff 

 in each hand, move with surprising speed over a plain surface. 



SECT. VIII. OF THE THORAX. 



The thorax is the upper part of the trunk, and is formed by 

 the dorsal vertebra? behind, by the sternum in front, and by the 

 ribs with their cartilages at the intermediate spaces. It is of 

 a conoidal figure, flattened in front, somewhat concave behind, 

 and semi-cylindrical on the sides. The interior circumference 

 corresponds with the exterior, with the exception of the poste- 

 rior part, where, owing to the projection of the column of dor- 

 sal vertebra?, a partial septum exists which has a tendency to 

 divide it into two chambers. The superior part of the cone, 

 or its summit, is much smaller than the inferior part or the 

 base, and presents a very oblique cordiform foramen, much 

 lower in front than behind, owing to the superior margin of the 

 sternum being lower than the first dorsal vertebra. The base 

 of the thorax is a very large opening: its lateral and posterior 

 margins, formed by the ribs and their cartilages, present a 

 convexity downwards; but, in front, where the latter run up to 

 join the sternum, a large notch is formed between the carti- 

 lages of the opposite sides, into the apex of which notch the 

 third bone of the sternum projects. 



Of the Ribs. 



The ribs, costae, (cotes,) are twenty-four in number, twelve 

 on either side. Of the latter, the upper seven, in consequence 



