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. 
Shght 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 vertebree of the adult from those of the cervical and 
lumbar regions. The limits of the thoracic region, however, are 
hable to variation, akin te 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 vertebre naturally appears 
to be reduced to six. Where a thirteenth rib occurs in the 
thorax, the lumbar vertebre 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, 
begins to degenerate as soon as the twenty-fifth pre-sacral vertebra 
is 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 
