MUSCLES OF THE SHOULDER AND ARM. 33 



9. The Deltoid Muscle. In the dog it is divided 

 into two separate muscles, not including the portion 

 of the levator humeri which corresponds to the clavo- 

 deltoid. 



a. The acromio-deltoid, the smaller division, takes 

 its origin from the acromion process, and is inserted 

 into a ridge along the outer surface of the proximal 

 third of the humerus. 



b. The spino-deltoid arises by a strong fascia from 

 the posterior border of the spine of the scapula along 

 most of its extent, and is inserted into the humerus to- 

 gether with the acromio-deltoid. 



Cut through the bellies of the deltoid 7miscles, and 

 reflect the cut ends. 



10. The Infra-spinatus Muscle fills up the infra- 

 spinous fossa from which it arises. Its fibres end 

 in a strong tendon which is inserted into the great 

 tuberosity of the humerus just below the insertion of 

 the supra-spinatus. 



11. The Teres Minor is seen just posterior to the 

 infra-spinatus near its insertion. When dissected out 

 it will be found to arise along the posterior margin of 

 the scapula by a tendinous expansion lying beneath 

 the infra-spinatus. It is inserted into the humerus 

 below the great tuberosity. 



12. The Latissimus Dorsi is a very large muscle 

 arising by a fascia from the spinous processes of the 

 lumbar and posteroir thoracic vertebrae. From this 

 origin the fibres form a wide sheet of muscle lying on 

 the dorso-lateral wall of the thorax, and converge 

 anteriorly toward the humerus. It ends in a tendinous 

 expansion which fuses with the sheath of the triceps 

 medius and teres major, through which its action on 

 the humerus takes place. Just before its insertion the 



