32 
Dr. Carson on the 
cartilage, muscular substance, and membrane. Behind, on 
each side, and in front, it is osseous. It is fenced by the spine 
behind, by the breast bone in front, and on each side by a tier 
of ribs, which are connected at one end to the spine by articu- 
lations, and at the other end to the breast bone by elastic 
cartilages. The intercostal muscles secure the spaces between 
the ribs. The ribs on one side with the corresponding ribs on 
the other, and with the intervening parts of the spine and 
breast bone, form the circumferences of so many circles. 
The circle formed by the junction of one rib with its corres- 
ponding rib on the other side, is largest at the bottom, till at 
the top it is reduced to so small a diameter, as to serve for 
little more than a passage for the windpipe and gullet. Strong 
muscles and tendons traversing each other from the upper 
rib on one side to that on the other, and to the vertebrae of 
the neck and to the clavicles, and connected by a union of 
membranes with the exterior surface of the tubes, to which 
they afford a passage, form the secure boundary of the chest 
at the top. The form of the chest is evidently conical. The 
floor or base of this cone is formed by a thin circular plate, 
partly fleshy and partly membranous, but every where flexi- 
ble and yielding, with its rim all around firmly concreted 
with the shell of the chest. It is called the diaphragm or 
midriff. It cuts the trunk of the body transversely, and 
separates the cavity of the chest just described from that of 
the belly. One condition of this flooring is worthy of parti- 
cular notice. Its area is more extensive than that of the 
largest transverse section of the chest. In consequence of 
this, it admits of motion to a considerable extent ; and, for 
reasons which will be afterwards explained, assumes in the 
