IN THE HUMAN FRAME. 
63 
projections obliquely:* that is, in their natural position, they 
bend or slope from the place of articulation downwards. 
But the basis upon which they rest at this end being fixed, 
the consequence of the obliquity, or the inclination down¬ 
wards, is, that when they come to move, whatever pulls 
the ribs upwards, necessarily, at the same time, draws 
them out; and that, whilst the ribs are brought to a right 
angle with the spine behind, the sternum, or part of the 
chest to which they are attached in front, is thrust forward. 
The simple action, the' efore, of the elevating muscles does 
the business; whereas, if the ribs had been articulated with 
the bodies of the vertebrae at right angles, the cavity of the 
thorax could never have been farther enlarged by a change 
of their position. If each rib had been a rigid bone, ar¬ 
ticulated at both ends to fixed bases, the whole chest had 
been immoveable. Keill has observed, that the breastbone, 
in an easy inspiration, is thrust out one-tenth of an inch: 
and he calculates that this, added to what is gained to the 
space within the chest by the flattening or descent of the 
diaphragm, leaves room for forty-two cubic inches of air to 
enter at every drawing-in of the breath. When there is a 
necessity for a deeper and more laborious inspiration, the 
enlargement of the capacity of the chest may be so increas¬ 
ed by effort, as that the lungs may be distended with seventy 
or a hundred such cubic inches, f The thorax, says Schel- 
hammer, forms a kind of bellows, such as never have been, 
nor probably will be, made by any artificer.J 
* For the mode of articulation of the ribs with the vertebrae, see Plate 
IX. Fig. 1 and 2. 
t Anat. p. 229. 
t The thorax, or chest, is composed of bones and cartilages, so dis¬ 
posed as to sustain and protect the most vital parts, the heart and lungs, 
and to turn and twist with perfect facility in every motion of the body; 
and to be in incessant motion in the act of respiration, without a moment’s 
interval during a whole life. In anatomical description, the thorax is 
formed of the vertebral column, or spine, on the back part, the ribs on 
either side, and the breastbone, or sternum, on the fore part. But the 
thing most to be admired is the manner in which these bones are united, 
and especially the manner in which the ribs are joined to the breastbone, 
by the interposition of cartilages, or gristle, of a substance softer than 
bone, and more elastic and yielding. By this quality they are fitted for 
protecting the chest against the effects of violence, and even for sustaining 
life after the muscular power of respiration has become too feeble to con¬ 
tinue without this support. 
If the ribs were complete circles, formed of bone, and extending from 
the spine to the breastbone, life would be endangered by any accidental 
fracture; and even the rubs and jolts to which the human frame is con 
tinually exposed, would be too much for their delicate and brittle texture 
