of the Skull and the Skeleton. 353 
and supported on the lower part of the centrum. In the pec- 
toral region, where the viscera first enlarge, it becomes a little 
longer, and by the enlargement of the organs has its articula- 
tion forced higher up the centrum. In the back, where the 
viscera are at their maximum, it is found that the ribs are 
longest, and that they are entirely attached to the neural arches. 
In the tail these hemal arches ultimately disappear, and there 
the vessels dwindle almost to nothing. Here there appears to 
be an incontestable demonstration that as the internal pressure 
increases so do the bones lengthen, and so do they give way be- 
fore it, changing their articular place; and when the pressure 
becomes reduced in the tail, the arch dwindles to two lateral 
eminences, and at last is utterly lost. In other words, it is de- 
ducible from observation that the development of the ribs de- 
pends on the pressure to which the base of the centrum is sub- 
jected hy the vessels, counteracted, of course, by pressure from 
the outer muscles and media. This, indeed, we are led to ex- 
pect from the fact that the ribs are not developed in relation to 
the same function in animals where the lungs are rudimentary. 
Thus the frog has no ribs. And thus it is found that caries of 
the ribs is often associated with disease of the lungs; while the 
deformity of the chest called ectopia cordis consists in a partial 
or complete absence of the sternum and ribs with more or less 
deficiency in the pericardium, pleura, heart, and lungs. In ser- 
pents the ribs are functionally innumerable limbs. The rib in 
many animals terminates at its head in an epiphysis, which arti- 
culates with another epiphysis on the neural arch; while at its 
distal end, in birds, where the tension of the pectoral muscles on 
the sternum pulls with great power, an epiphysis is ossified and 
developed to a great length. Thus the rib appears to follow the 
same general Jaw as other bones; for the distention of the 
thorax, both by growth and muscles and function in breathing, 
performs the office of ever-acting muscles, while other muscles, 
and the skin, and the atmosphere act as a great opposing power. 
And in accordance with the same general law which produces 
the simple ribs, it is found that between their distal ends there 
is usually developed a common epiphysis, called the sternal arc. 
In Plesiosaurus and animals where the exterior force acting on 
them was not great, they are arranged one behind another like 
the rounds of a ladder; but in Saurornia and birds, where they 
came to give attachment to an enormous overgrowth of the pec- 
toral muscles, all are cemented together and modified into a 
sternum, the greater muscular force having produced a larger 
amount of ossification. The epiphyses of ribs appear only to be 
developed when the costal girdle is large and somewhat complete. 
And therefore, while cervical ribs may well be regarded as epi- 
