PERICLAVICULAR SUPERNUMERARY MUSCLES 59 



stages, to the conditions found in our M. supraclavicidans 

 propriiis posterior. In Hyrtl's first recorded case the beginning 

 loss of the sternal attachment is signalized by the single median 

 tendon which connects the two muscles with the manubrium. 

 In his second variation the muscles have largely given up the 

 manubrial attachment and have fused into an inter-clavicular 

 muscle. Separation of the inter- clavicular muscle in the median 

 line into its original components, and further regression of each 

 laterad would lead to the arrangement described in Weber's 

 case, while Tait's case only differs in having an additional at- 

 tachment to the posterior margin at the first rib, which is likely 

 to be acquired in the course of migration. Lastly, in our in- 

 stance, the mesal extremity of the muscle has lost all con- 

 nection with the sternum, the episternal (inter-clavicular) 

 structures and the sterno-clavicular articulation and has ac- 

 quired a purely clavicular attachment. We may, therefore, be 

 justified in regarding the typical sterno-clavicularis as the ante- 

 cedent of the mesal extremity of the three supra- or retro- 

 clavicular muscles heretofore recorded, the condition presented 

 by our case being the final stage in a progressive migration of 

 the mesal tendon of the muscle from the sternum succes- 

 sively to the inter-clavicular ligament, the capsule of the sterno- 

 clavicular articulation and finally to the posterior surface of the 

 clavicle. 



Turning to the lateral termination of the muscle under con- 

 sideration and examining cognate variations in order to deter- 

 mine its significance, we have to consider in the first place, a 

 group of aberrant muscles extending between the upper border 

 of the scapula and the clavicle. 



The M. scapiilo-clavicularis , or coraco-claviadaris has in 

 several instances been observed to extend as a muscular slip 

 between the superior border of the scapula, or the transverse 

 ligament, or base of coracoid process, and the posterior border 

 or inferior surface of the clavicle, passing behind the clavicular 

 attachments of the trapezius and sterno-mastoid. 



Moreover, a human muscular .variation, described by Wood, 

 Gruber, Hellema, Curnow, Reid and Taylor, Shepherd, and 



