284 FASCLE AND MUSCLES OF THE HORSE 
pectoral muscles. It is directed obliquely backward and downward, and crosses 
the lower part of the first three intercostal spaces. 
Origin.—The lateral surface of the first rib, below the scalenus. 
Insertion.—The cartilage of the fourth rib. The aponeurosis usually joins the 
rectus abdominis. It may reach the fifth rib or the sternum. 
Cariniform cartilage 
First rib 
: 7 _ Internal thoracic vessels 
Sternal ligament 
Internal intercostal muscle 
Costal part of 
diaphragm Sternal Transversus 
(cut edge) part of abdominis 
diaphragm (cut) 
Fic. 274.—Dissection or. Firoor or THORAX OF Horse. 
The ribs have been sawn off near their sternal ends and the diaphragm and transversus abdominis cut off close to their } 
attachment. 
1, Eighth rib; 2, 3, cartilages of ninth and tenth ribs; 4, xiphoid cartilage; 5, apex of pericardium. 
Action.—It may assist in inspiration or concur with the rectus abdominis. 
Relations.—Superficially, the deep pectoral muscles; deeply, the intercostal 
muscles and the ribs. 
Blood-supply.—Internal and external thoracic arteries. 
Nerve-supply.—Intercostal nerves. 
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