PLATE IV. 



With the case here recorded it is possible to present the entire 

 group of retro-clavicular supernumerary muscles in their mutual re- 

 lationship and in their common reference to the potential parent 

 muscular system of the mammalian sterno-chondro-scapularis. Sche- 

 matically this can be done as indicated in the plate IV. 



Fig. I represents the typical mammalian sterno-chondro-scapularis, 

 encountered as already stated in the human subject, as an occasional 

 variation. 



Fig. 2 corresponds to the retro clavicular muscle as described by 

 Weber and Tait : the " yJ/. sierno-chondro-clavicularis posterior'''' 

 results from the sterno-chondro-scapularis by loss of the scapular at- 

 tachment and transference of the distal insertion to the clavicle. 



Fig. 3 may be taken as illustrating Hyrtl's variations of the usual 

 M. sterno-clavicularis in which the sternal attachment of the slip is 

 beginning to show a tendency to migration laterad towards the 

 clavicle. 



Fig. 4 shows a theoretical combination between the sterno-clavicu- 

 laris posterior and the occasional scapulo-clavicularis or coraco-clav- 

 icularis. These two muscular slips would appear respectively as the 

 proximal and distal segments of a typical sterno-chondro-scapularis 

 whose intermediate portion had disappeared. 



In Fig. 5 is shown the reverse condition, illustrated by the case 

 reported in this paper. In place of the disappearance of the inter- 

 mediate segment of the sterno-chondro-scapularis, which produced 

 in Fig. 4 two muscles, a sterno-clavicularis and a scapulo-clavicu- 

 laris, we have in our instance the converse of this. The central 

 part of the typical sterno-chondro-scapularis persists, while the loss 

 of the proximal sternal and distal scapular attachment leaves us with 

 a retro-clavicular muscle, fixed at both extremities to the clavicle, 

 and hence properly designated as the " supraclavicularis proprius 

 posterior." This muscle, as well as the remaining members of the 

 retro-clavicular groups are, therefore, to be regarded as myotypical 

 reversions, in the sense that they represent the occasional develop- 

 ment of portions of a common ancestral mammalian muscular plane, 

 which in many living forms finds its expression in the sterno-chondro- 

 scapularis. 



(64) 



