218 OUTLINES OF EQUINE ANATOMY. 



teres internus, and runs between this muscle and tlie caput 

 magnum of triceps extensor brachii, to the posterior angle 

 of the scapula ; and the external thoracic artery, which 

 runs backwards to the lateral j^art of the thorax, under 

 latissimus dorsi. Inferiorly the humeral artery runs from 

 the axillary artery towards the elbow-joint, sending off 

 several constant branches, and finally breaking up into the 

 tdnar, spiral, and radial branches : the ulnar runs to break 

 up about the point of the ulna, sending an important 

 branch down the limb beneath ulnaris accessorius ; the 

 spiral twists round the front of the humerus ; the radial 

 runs along the internal surface of the limb. 



I have thus particularised these arteries, since with each 

 of these runs a nerve from the axillary plexus, which, 

 being formed by branches from the sixth, seventh, and 

 eighth cervical nerves, and the first dorsal (with a sym- 

 pathetic branch), passes between the long and short heads 

 of scalenus, and breaks up into the humeral thoracic, ex- 

 ternal thoracic, anterior and ^posterior scajoular, tdnar, spiral^ 

 and radial nerves. 



By division of the pectorals we have exposed the lateralis 

 sterni, running in a downward and backward direction 

 from the posterior border of the lower third of the first 

 rib, to be attached below the inferior extremity of the fourth 

 sterno-costal cartilage to the lateral part of the sternum. 

 This muscle being removed, we have the intercostales 

 interni as continued between the sterno-costal cartilages 

 exposed to view; they run in a downward and forward 

 direction, obliquely from the anterior border of one carti- 

 lage to the posterior border of the one immediately in 

 front of it. Posteriorly they are covered by the anterior 

 extremity of rectus abdominis, which is anteriorly attached 

 to the external surface of the three or four posterior sternal 

 cartilages of the true ribs. 



Having examined these structures, we remove the 

 aponeurotic layer covering serratus magnus, and thus 

 expose its inferior attachments. It will be found to arise 

 from the transverse processes of the five posterior cervical 

 vertebrae, and from the external surface of the eight true 

 ribs, having four digitations posteriorly with obliquus 

 abdominis externus ; from these points its fibres, forming 

 a large fleshy slab of muscle, converge towards the superior 

 margin of the venter surface of the scapula, into which they 



