912 PRINCIPAL SIR WILLIAM TURNER ON 
THORAX. 
The bony walls consisted of 14 dorsal vertebrae, 14 pairs of ribs, and the single 
seoment of the sternum. Owing to the marked curvatures of the great majority of 
the ribs, the side walls of the chest arched outwards so as to enclose in a great chamber 
the heart, the pair of lungs, and other subordinate viscera. From the relatively 
feeble curve of the 1st pair of ribs, the thoracic inlet was laterally compressed and 
somewhat ovoid in form (fig. 20, D1). Through their sternal attachment they could be 
fixed in inspiration by the action of the intercostal and other muscles attached to them. 
Starting from the first pair, the intercostals attached to the ribs behind could elevate 
S 
Fic, 20.—1st and 6th dorsi-costal segments. 
and rotate outwards their respective bones, so as enormously to increase the thoracic 
cavity laterally and dorsi-ventrally, whilst the diminution of the arch of the diaphragm 
would increase its capacity in the antero-posterior direction. The absence of a fixed 
attachment to bone of their ventral ends enabled these ribs on opposite sides to be 
drawn much further asunder than was their relative position during expiration, which 
also would materially contribute to the increase in size laterally of the thoracic cavity. 
If in man, where seven pairs of ribs articulate ventrally with the sternum, the ex- 
pansion of the lungs and of the thoracic capacity is three or even four times greater at 
the end than at the beginning of a full inspiration, there can be little doubt that in 
the large Cetacea with a single-segmented sternum the expansion of the lungs and 
chest-wall will be proportionally larger. It is through the thoracic construction, 
therefore, that the great whales during inspiration are enabled so to distend the air-cells 
