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 hsemal 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 by 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 stei'num pulls with great power, an epiphysis is ossified and 

 developed to a great length. Thus the rib appears to follow the 

 same general law 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- 



