M. SPINO-TBAJPJUZIUS. 213 



thickens slightly as it crosses the vertebral border of the scapula, 

 and ends upon the fascia covering the M. infraspinatus. 



Continue to lift the ventro-caudal border, and dissect up the 

 middle part of the muscle as far cephalad as possible ; then puU it 

 caudad, and at the same time dorsad or ventrad. This will indicate 

 the cephalic border, which is much shorter than the other, and 

 extends latero-ventrad from a point between the 1st and 4th tho- 

 racic neural spines. 



The dorsal half of the cephalic border is muscular, and separable 

 without much difficulty from the adjacent caudal border of the M. 

 acromio-trapezius. Opposite, or slightly dorsad of, the vertebral 

 border of the scapula, the border of the M. spino-trapezius becomes 

 tendinous, thin and indistinct, so that its true limits are best ascer- 

 tained by pulling upon the muscular portion. It is also overlapped 

 to some extent by the M. acromio-trapezius. 



The cephalic border of the muscle may easily be traced, entad 

 of the tendon of the M. acromio-trapezius^ to a point 8-10 mm. 

 cephalad of the mesoscapula, and about the same distance from the 

 nearest part of the border of the scapula. Here it terminates upon 

 the fascia covering the J/, supraspinatus. 



The muscle should now be transected (§ 586), and the ental 

 surface cleared of fat, especially near the attachments. 



§ 612. Origin. — From the tips and interspinous ligaments of most 

 or all of the thoracic neural spines. The attachment of the cephalic 

 border may be at any point between the 1st and 4th spine, and that 

 of the ventro-caudal border at any point between the 11th and 13th. 

 The origin of the caudal 2-3 cm. is by a triangular tendon, the lat- 

 eral angle of which is 1 cm. from the meson. The rest of the muscle 

 arises by fleshy fibers. Opposite the cephalic 3 or 4 spines there 

 are sometimes slight intervals filled with loose connective tissue ; 

 opposite the others, the corresponding intervals, when they exist, 

 are occupied by a firm fascia which practically renders the attach- 

 ment continuous across the spines. 



Insertion. — Along a curved s-shaped line obliquely crossing the 

 mesoscapula (Fig. 44). The cephalic two thirds of the insertion is 

 by a thin tendon 1-2 cm. long and 1-1.5 cm. wide, which is attached 

 to the fascia upon the ectal aspect of the supraspinatus, and to the 

 mesoscapular tuberosity which it crosses very obliquely. Here it 

 joins the caudal and usually fleshy third of the insertion, which 

 extends upon the fascia covering the infraspinatus at an angle of 



