MUSCLES OF THE TRUNK. 67 



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 the 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 cucuUaris 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 mandibulae and latissimus dorsi 

 from their origins, and twn them down. 



iv. The infraspinatus arises from the dorsal surface of 

 the suprascapula, partly overlapped by the 

 latissimus dorsi : it runs outw^ards to be inserted 

 into the deltoid ridge of the humerus : its action 

 is to elevate the arm. 



Lift up the siqjrascajmla and note the muscles attaching it to 

 the body, viz. : 



V. The retrahens scapulae, behind, 

 vi. The levator anguli scapulae, in front. 

 Remove the suprascapula on one side, and clean the median 

 longitudinal muscles of the bach. 



vii. The extensor dorsi communis is a longitudinal mass 

 of muscle arising from the urostyle, and running 



