M. MCTOPECTOEALIS. 239 



Lift the cepbalic Iborder at its middle and relax the parts so as to 

 permit dissection entad of the muscle from one border to the other ; 

 then transect. 



In reflecting the mesal end of the muscle, cut an artery and 

 nerve which emerge from the subjacent ental layer, and note that, 

 near the meson, the cephalic border curves cephalad. The ental 

 surface of the lateral part of the muscle is often so closely joined 

 with the subjacent muscle that there is danger of cutting fibers. 

 The tracer should be used in tearing the connective tissue untU the 

 bone is reached. 



§ 650. Origin. — From a median raphe common to it and its 

 platetrope ; the caudal half or three fifths of the raphe is attached 

 to the prestemal keel, the remainder is continuous with the line of 

 union of the caudal portion of the MM. sterno-mastoidei. 



Insertion. — The middle of the length of the line of insertion cor- 

 responds closely with the middle of the length of the humerus, but 

 the insertion includes rather more than a third of the length of the 

 bone. Its distal end is almost in line with the middle of the distal 

 end of the bone, but its proximal end is nearly midway between 

 the ventral and cephalic aspects. The caudal border of the line of 

 insertion is well defined, but the cephalic is not so clearly separable 

 from the insertions ot the spino-deltoideus SinA. hracMalis, and a 

 strong fascia sometimes extends proximad from the border of the 

 muscle toward the trochiter. 



LAMINA ENTALIS, DV. CBPHALICA. 



§ 651. Synonymy. — " Le second chef du large pectoral, sa partie antirieure," S.-D., 

 A, II, 343 ; pectoralis, part 5, " subclavicular part," Miv., B, 147. 



Figures. — Sternal end (73, left side) ; humeral end, reflected (73, left side) ; insertion 

 area (68, 69). 



General Description. — Narrower and thicker than the ectal 

 layer ; 15-18 mm. wide ; from the presternum and rapTie to the 

 proximal fourth of the cephalic side of the humerus. 



Dissection. — The artery and nerve mentioned (§ 649) as passing 

 from the ental to the ectal layer of the ectopectoralis usually pene- 

 trate the former through a narrow interval about one third of the 

 distance from the meson to the humerus. This interval usually 

 marks the line of separation between the cephalic and caudal divi- 

 sion of the ental lamina of the ectopectoralis. Mesad of the inter- 

 val, the plane of separation is at a right angle with the surface of 



