PERICLAVICULAR SUPERNUMERARY MUSCLES 61 



mastoid and trapezius may. find their explanation in the per- 

 sistence of portions of this same muscular plane and their sec- 

 ondary fusion with the muscles named near their clavicular 

 attachment. 



Thus QuAiN ("Anat. of the Arteries," p. i86) describes a 

 muscular fasciculus which detaches itself from the anterior bor- 

 der of the trapezius and joins the sterno-cleido-mastoid, passing" 

 above the subclavian artery, Testut quotes this instance as 

 representing a first stage in the fusion of the two muscles. 



Davies-Colley encountered in several cases a distinct mus- 

 cular fasciculus which left the anterior border of the trapezius, 

 crossed the subclavian triangle diagonally and inserted into the 

 clavicle underneath the sterno-cleido-mastoid. In some in- 

 stances the descending branches of the superficial cervical 

 plexus were placed behind this fasciculus. 



In the case of the muscle here reported the supra-clavicular 

 nerves descended to the thorax between the clavicle and the 

 abnormal muscle. The connection of the two extremities of 

 the muscle with the deep surface of the clavicular head of the 

 sterno-cleido-mastoid and the trapezius was very intimate. A 

 firm fibrous fusion of its sheath with the fascia of these muscles 

 made a complete exposure from behind and careful dissection 

 necessary in order to demonstrate the independence of the mus- 

 cular fibres and their true insertion into the clavicle. When first 

 encountered in situ from in front the case was reported as a 

 muscular band joining the anterior edge of the trapezius to the 

 deep surface of the clavicular head of the sterno-cleido-mastoid. 

 It is conceiv^able that further reduction of the intermediate por- 

 tion of the typical sterno-chcndro-scapularis, with loss of the 

 secondary clavicular attachment seen in our case of siipT'ciclavic- 

 ularis propriiis posterior might leave a muscular fasciculus ap- 

 parently extending between the deep surfaces of trapezius and 

 sterno-cleido-mastoid, and thus give rise to the reports of partial 

 fusion of these muscles. 



