331 



3) The fusion of the ribs in front of the metasternum. 



4) The attachment of the rib to the sternum by fibrous tissue. 



5) The attachment of the rib to the proserial rib. 



The case of the metasternum is somewhat complicated by the 

 degeneration taking place in the course of the sternal bars. The ex- 

 planation however seems to be that the ventral portion of the 9th rib 

 (or occasionally 8th and 9th ribs) has failed while the dorsal portion 

 still remains attached to the mesosternum forming the metasternum. 



The failure of the 8th rib and sometimes even of the 7th to take 

 part in the formation of the sternum does not usually interfere with 

 the continuity of the meso- and metasternum but when any of the higher 

 ribs fails as in three cases described above the metasternum becomes 

 separated from the mesosternum. The general conclusions to be drawn 

 then are: 



That the caudal end of the thorax is degenerating as shown by 

 the diminution in the number of ribs which unite with the sternum 

 in man and the higher Primates as compared with the lower Monkeys. 



That the 8th rib has undergone so much degeneration that it 

 rarely joins in the sternum and falls short of the middle line but that 

 when degeneration has been partly arrested it reaches the middle line 

 without becoming incorporated in or joining with the sternum. 



That a similar degeneration is atfecting the 7th rib which usually 

 is joined with or incorporated in the mesosternum — but which like 

 the 8th may not join or be incorporated in the mesosternum but meet 

 its fellow of the opposite side. 



Again, a further change brings the 7th as in a few cases into the 

 same position as the 8th normally occupies — namely it fails to reach 

 the middle line and terminates by a secondary connection with the 6th. 



References. 



1) Hyrtl, quoted by Cunningham. 



2) CuNNiNaHAM, The occasional 8th true rib in man etc. Journ. of 

 Anat. and Physiol., Vol. 24, 1890. 



3) Collective Investigation Committee 1895/96. Journ. of Anat. and 

 Physiol., Vol. 31, 1896. 



4) Tredgold, Variations of ribs in the primates with especial reference 

 to the number of sternal ribs in man. Joui-n. of Anat. and Physiol., 

 Vol. 31. 



5) Pateeson, The sternum : its early development in man and mammals. 

 Journ. Anat. and Physiol., Vol. 35, 1901; also B. M. J., Sept. 13, 

 1902. 



6) Keith, The extent to which the post, segments of the body have 



