38 LIGAMENTS AND MUSCLES OF THE HORSE. 



4. Posterior Deep Pectoral (pectoralis magnus). 



Origin. — From the cartilages of 5th to 8th ribs (inchisive), and 



from the adjacent lateral surface of the sternum : also from 



the abdominal tunic. 

 Insertiun. — To the inner trochanter of the humerus. 

 Action. — To draw the shoulder joint and fore-limb backwards. 



COSTAL REGION. 



1. Serratus AJaynns. 4, Levatores Costarum (17). 



2. Exte7mal Inter Gostalsill). 5. Triangularis 8ter7ii. 



3. Internal Intercostah (17). 6. Lateralis Sterrii. 



1. Serratus Magnus. 



, Origin — From the outer surface of the eight true ribs, and 

 from the transverse processes of the last five cervical 

 vertebra. 

 Insertion. — To two rough triangular surfaces on the upper part 



of the internal surface of the scapula. 

 Action. — The two muscles form a sling to support the trunk. 

 In contracting, it draws the upper extremity of the scapula 

 downwards and backwards ; or, if the limb is fixed, it may 

 act as a muscle of inspiration. 



2. External Intercostals. 



Each of these occupies an intercostal space, the fibres arising 

 from the anterio*' I'ib, and passing downwards and back- 

 wards to be attached to the posterior rib. 



Action. — To draw the ribs forwards and upwards, assisting in 

 inspiration. 



3. Internal Intercostals. 



The fibres of these arise from the posterior rib, and pass down- 

 wards and forwards to be attached to the anterior rib. 



Action. — Opposed to the preceding, and thus muscles of expira- 

 tion. 



