MUSCLES OF THE TRUNK. 297 



digitations at its inferior border, applied against the thoracic walls, and partly 

 concealed by the shoulder. 



Structure.— It is composed of divergent fleshy fibres, all of which converge 

 towards the superior extremity of the scapula, and are covered by a very strong 

 aponeurosis that gradually diminishes from above to below, and only adheres to 

 the muscle in its inferior part. 



Attachments.— \. To the external face of the eight sternal ribs. 2. To the 

 anterior triangular surface of the internal face of the scapula, behind the 

 angularis, with which it is confounded. 3. To the whole extent of the posterior 

 triangular surface of that bone. 



On reaching the scapula, the aponeurosis separates from the fleshy fibres, and 

 is inserted alone into the fibrous plane which covers the muscular fasciculi of the 

 subscapularis. 



i^eZaftows,— Outwardly, and through the medium of an abundant supply of 

 connective tissue, which facilitates the play of the limb against the lateral wall 

 of the thorax, to the subscapularis, supra-spinatus, adductor of the arm, latissimus 

 dorsi, and the mass of olecranian muscles ; inwardly, to the first seven external 

 intercostals, to the sides of the sternum, and to the anterior small serratus. Its 

 four posterior digitations cross the first five of the great oblique muscle of the 

 abdomen, and are covered by a prolongation of the abdominal tunic. 



Action. — With that of the opposite side, this muscle constitutes a vast brace 

 or girth on which the thorax rests when the animal is supported on its anterior 

 limbs ; it therefore acts, in relation to the trunk, as a suspensory ligament. 

 When it contracts, its fixed point being the thoracic walls, it pulls the superior 

 extremity of the scapula downward and backward, and causes this portion of the 

 limb to perform a swinging movement which carries the inferior angle upwards 

 and forwards. If the limb is the fixed point, then it raises the thorax between 

 the two anterior limbs, and assists in the respiratory movements by elevating 

 the ribs. 



2. External Intercostals (Figs. 161, 162). 



Situation — Form. — These muscles fill the spaces between the ribs, but do not 

 descend beyond their inferior extremities ; they, therefore, do not occupy the 

 intervals between the cartilages. They are flattened, fleshy bands, gradually 

 diminishing in thickness from above to below. 



StriKture — Attachments. — Each external intercostal muscle is composed of a 

 series of muscular fasciculi, intermixed with numerous aponeurotic fibres, both 

 of which pass obliquely backwards and downwards, from the posterior border of 

 the preceding to the external face of the succeeding rib. 



Relations. — Outwardly, to the different muscles applied against the thoracic 

 walls ; inwardly, to the internal intercostals. 



3. Internal Intercostals (Fig. 163, 16). 



These are placed at the internal face of the preceding, which they exactly 

 repeat with regard to their general form, but from which they differ in the 

 following points : — 



1. Very thick between the costal cartilages, these muscles are reduced at the 

 upper part of the intercostal spaces to a thin aponeurotic layer, supported only 

 by some fleshy fibres. They, therefore, gradually diminish in thickness from 

 below upwards. 



