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. 1 



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 points 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 



1 [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.] 



