MUSCLES OF THE TRUNK 57 



pericardium. The hinder two-thirds of the 

 muscle pass dorsal to the tendon of the obliquus 

 externus and are inserted, Like it, into the linea 

 alba. 



2. Muscles of the back. 



Pin out tJie frog on its belly : remove the skin, and clean the 

 muscles in order. 



i. The depressor mandibulse is a broad triangular 

 muscle which arises from the fascia covering the 

 dorsal surface of the suprascapula : the fibres run 

 downwards behind the tympanic membrane, and 

 converge to be inserted into the angle of the 

 lower jaw. The muscle by its contraction opens 

 the mouth. 



ii. The cucullaris is a small oblong muscle which, 

 arising from the exoccipital near the middle line, 

 runs backwards and outwards, and is inserted 

 into the dorsal border of the suprascapula. 



iii. The latissimus dorsi is a triangular muscle lying 

 behind the depressor mandibulae : it arises from 

 the fascia dorsalis just behind the shoulder girdle, 

 its origin being partly covered by the obliquus 

 externus. The fibres run forwards and outwards, 

 converging to be inserted by a long tendon into 

 the deltoid ridge of the humerus. 



Dissect away the depressor mandibulse and latissimus dorsi 

 from their origins, and turn them down. 



iv. The infraspinatus arises from the dorsal surface 

 of the suprascapula, partly overlapped by the 

 latissimus dorsi : it runs outwards to be inserted 

 into the deltoid ridge of the humerus : its action 

 is to elevate the arm. 



Lift up the suprascapula and note the muscles attaching it to 

 the body, viz., 



v. The retrahens scapulae, behind, 

 vi. The levator anguli scapulas, in front. 



Remove the suprascapula on one side, and clean tJ>e, median 

 longitudinal muscles of the back. 



