THE SKELETON 45 
than at present. This conclusion is strengthened by the fact 
that the eighth rib not infrequently reaches the sternum even in 
adults. 
Eight sternal ribs are found in the lower Apes (which may 
have as many as ten), and may occur in the higher Apes, with 
the exception of the Orang. It is certain that in all Mammals 
those ribs which have their ventral ends in any way attached to 
one another were once connected with the sternum. 
On the other hand, the union of only six ribs with the 
sternum is not rare in Man; and the existence of this condition 
is a clear indication of the gradual degeneration (shortening) of 
the thoracic skeleton and sternum. In such cases the distal 
end of the xiphisternum may bear two lateral prongs, which 
correspond with the sternal ends of the seventh pair of ribs. 
There are certain considerations which confirm the statement 
above made that the process of degeneration at the upper end of 
the thorax is slower than that at the lower end, to which latter, 
indeed, no limits of variation can be foreseen. We have first 
the rhythmic respiratory mechanism, which is so closely connected 
anatomically and topographically with the complete ribs; and, 
second, the attachment to this part of the thorax of the 
musculature of the shoulder girdle (I refer especially to the 
serratus magnus and the pectoralis major). [These muscles 
under certain conditions play an important part in effecting the 
movements of respiration], and in order to secure a sufficient 
range of activity they must necessarily be inserted into a certain 
number of fixed points. Such poits are supplied by the bony 
framework formed by the seven upper pairs of ribs, the sternum, 
and the clavicles; and as long as these muscles remain indis- 
pensable, the bones named cannot well degenerate further. 
We have here a striking example of the important reciprocal 
relation and close interdependence existing between the various 
organs and systems which, so to speak, hold each other in 
check. 
We learn both from Ontogeny and Comparative Anatomy 
that the sternum (which is first formed by the fusion of a couple 
of sternal bands) consisted, in the ancestors of Man, of a row of 
* [Cunningham and Robinson have recorded the existence of an eighth sternal 
rib on one or both sides in 20 per cent of (seventy) subjects examined (Nature, vol. 
Xxxix. p. 248, and Jour. Anat. and Phys., vol. xxiv. p. 127). In the unilateral 
condition it was found to be dextral in eight out of nine examples ; and Cunningham 
suggests that this may be a reversionary feature, associated with the greater use of 
the right fore-limb. ] 
