THE SKELETON 43 



dition, however, the internal and external intercostal muscles 

 between this cervical and the first thoracic rib are well developed 

 in cases like that above figured ; indeed this is so even when 

 (as occasionally happens) the fibrous connecting band is wanting 

 (Leboucq). The sternal portion of the rib is as a rule very 

 weakly developed, sometimes free, sometimes partly fused with 

 the first thoracic rib. The vertebral end varies much in form, 

 size, and articulation upon the vertebral column ; and further, its 

 relations to the first thoracic rib may, as Leboucq has shown, vary 

 greatly. It may either be altogether fused with the latter, merely 

 loosely attached to it by connective tissue, or actually articulated 

 with it. In the first case, the first thoracic rib appears forked 

 at its vertebral end, and this (according to P. J. van Beneden) 

 is the rule in many Cetaceans. 



Apart, however, from such cases as these, a further proof of 

 the former existence of cervical ribs in Mammals is derived from 

 the study of the adult Edentata. Among these, Cholcepus has 

 normally only six cervical vertebrae [defined as those destitute of 

 free ribs]. 1 Bradypus infuscatus and B. tridactylus illustrate 

 the other extreme, possessing normally nine such vertebra? ; 

 while B. cuculliger has either eight or nine. In the latter 

 cases the upper end of the thorax has undergone greater reduction 

 than in any other Mammal. 



The fact that in Man the first thoracic rib is probably 

 beginning to degenerate, 2 and is at the present time in process 

 of atrophy, is established by the not infrequent recurrence of 

 undoubted cases of its abortive development. Such have been 

 recorded by Struthers, Grosse, Hunauld, Gruber, Turner, Leboucq, 

 and others (cf. Fig. 27, B). The description given above of 

 the seventh cervical rib might, in these cases, be applied to the first 

 thoracic. Nevertheless, I believe, for reasons to be given later, 

 that should reduction at the upper end of the thorax advance, 

 it will do so far more slowly than at the lower, or indeed that 

 it may even be arrested for an indefinite period (cf. p. 45). 3 



1 A similar numerical reduction of the cervical vertebra occurs also in the 

 Manatee [but there is reason for believing that it is in that animal due to the 

 excalation of at least the body of one of these, and not to the assumption of thoracic 

 characters by the last of the series.] 



2 I should like here to raise the question whether this tendency to reduction at 

 the upper end of the thorax may not be a determining factor in the degeneration so 

 frequently found to be commencing at the top of the lungs ? (cf. infra). 



3 It is interesting here to note that ventrally to the transverse process of the sixth 

 cervical vertebra, there often arises, on either side, a projection, which might be 

 claimed as a vestigial structure, since in most Mammals it stands out prominently 



