334 HEALTH AND DISEASE 



Action. When the horse occupies a standing position, this muscle, 

 with its fellow on the opposite side, supports the trunk as if in a sling 

 (fig. 44, Vol. I). When it acts from the chest it pulls down the posterior 

 angle of the scapula and moves the point of the shoulder upward and 

 forward. When the limb is fixed, it pulls the ribs upward and forward, 

 and by enlarging the chest becomes a muscle of inspiration. 



External Intercostal Muscles. These form a series of thin, flat 

 muscles occupying the spaces between the ribs from near the spine 

 downward to their inferior extremities. The fibres take an oblique 

 direction from before downward and backward. Each muscle originates 

 from the posterior border of one rib and is inserted in the anterior 

 border of the rib succeeding it. 



Action. To draw the ribs upward and forward and assist in in- 

 spiration. 



Internal Intercostal Muscles. Placed beneath the muscles last 

 described. These also occupy the spaces between the ribs, and in addition 

 extend between the sternal cartilages below. They resemble the exter- 

 nal intercostals in their general form, but their fibres take an opposite 

 course, viz. downward and forward, so that the two sets cross each other 

 like the lines of the letter X. 



Origin. From the anterior borders of the ribs. 



Insertion. Into the posterior borders of the ribs in front. 



Action. To assist in expiration. 



Levatores Costarum. These are situated beneath the longissimus 

 dorsi, and form a long series of small flat muscular slips passing downward 

 and backward from the spine to the superior part of the ribs. 



Origin. They arise from the transverse processes of the dorsal 

 vertebrae. 



Insertion. Into the outer surface of the rib behind the vertebra from 

 which each respectively arises. 



Action. To draw the ribs forward and assist in inspiration. 



Triangularis Sterni. A flat muscle situated on the floor of the 

 chest from one extremity to the other. 



Origin. From the superior surface of the sternum. 



Insertion. By a series of slips into the cartilages of the ribs, from 

 the 2nd to the 8th. 



Action. To assist in expiration. 



Lateralis Sterni. A thin, narrow, flat muscle placed on the outer 

 part of the chest in front. 



Origin. From the external surface of the first rib. 



Insertion. From its origin it passes obliquely downward and back- 



