THE SKELETON 39 



depression of the heart. It is, moreover, evident that the 

 change undergone by the heart and diaphragm, due to the 

 forcing of the former out of the median plane and the shifting 

 of its longitudinal axis towards the ventral and left side of the 

 body, must again react upon the form and limitations of the 

 pleural cavities. 



Slight changes in the limitation of the pleural cavities occur 

 also in the lower Mammals ; but how far these may be related 

 to each other, or in any way to those occurring in the Primates, 

 is not very clear. The original causes of the changes are 

 very various, but their close dependence upon the skeleton is 

 evident. 



The tendency towards a gradual diminution in the number 

 of ribs, previously referred to, requires further consideration. 



The presence of free ribs, as is well known, distinguishes the 

 thoracic vertebrae of the adult from those of the cervical and 

 lumbar regions. The limits of the thoracic region, however, are 

 liable to variation, akin to that already described as occurring in the 

 lumbar and sacral regions. Twelve pairs of free ribs are present 

 normally in Man, as in the Orang, but a comparison with other 

 (and chiefly lower) Vertebrates points to the earlier existence of 

 a larger number. This view is supported by Ontogeny, as well 

 as by the occasional occurrence of so-called supernumerary ribs. 

 These are less frequently found at the upper than at the lower 

 end of the thorax ; and in either case, the thirteenth rib is 

 subject to great variation both in form and size. For example, 

 a thirteenth rib at the lower end of the human thorax may vary 

 'in length from 2 to 14 cm. ; but thirteen is the normal number 

 of ribs in the Gorilla and the Chimpanzee, and Hylobates has 

 thirteen or fourteen. Where a free rib is borne by the seventh 

 cervical vertebra, the number of these vertebrae naturally appears 

 to be reduced to six. Where a thirteenth rib occurs in the 

 thorax, the lumbar vertebrae similarly appear to be reduced to 

 four unless the embryonic forward shifting of the pelvis has 

 been arrested at the twenty-sixth pre-sacral vertebra, as is not 

 unfrequent under these circumstances, for it has been observed 

 that the thirteenth rib, which always appears in the embryo, i 

 jbegins to degenerate as soon as the twenty-fifth pre-sacral vertebra | 

 jis incorporated in the sacrum. 



We have further evidence that Man has inherited more than 

 twelve pairs of free ribs, in the fact that reduced ribs are found 

 in the embryo, not only in connection with the first but with all 



