10 THE ANATOMT OF THK HORSE. 



it as it is being exposed by the dissector of the back. It arises by an 

 aponeurotic tendon from the series of vertebral spines, beginning about 

 the 4th dorsal, and extending backwards to the last lumbar. This 

 tendon is succeeded by a thick muscular portion, which contracts and 

 passes in between the limb and the trunk, where it will afterwards be 

 followed to its insertion into the internal tubercle of the humerus. Its 

 anterior fibres will be noticed to play over the dorsal angle and cartilage 

 of prolongation of the scapula. 



Action. — It is a flexor and an inward-rotator of the shoulder-joint. 



The Ehomboidetjs (Plate 4), like the trapezius, comprises a cervical 

 and a dorsal portion. The cervical part is an elongated, narrow muscle, 

 which extends as far forward as the axis, and arises from the funicu- 

 lar part of the ligamentum nuchse. Its fibres take a very oblique 

 direction downwards and backwards, and are inserted into the anterior 

 part of the cartilage of prolongation on its inner surface, being there 

 confounded with the insertion of the levator anguli scapulse. The 

 dorsal portion consists of fibres which arise from the anterior dorsal 

 spines, and pass in a nearly vertical direction to be inserted into 

 the inner surface of the cartilage of prolongation, behind the fibres of 

 the cervical division. It will be recollected that the nerve to these 

 muscles passes from the 6th cervical nerve, and reaches its destination 

 by traversing the levator anguli scapulse. 



Action. — To pull the scapula upwards and forwards on the chest-wall. 



The Mastoido-humeralis, or Levator Humeri (Plate 4). This 

 muscle, in the greatest part of its extent, is found in the head and 

 neck, where it takes its origin from the mastoid crest and the trans- 

 verse processes of the first four cervical vertebrae ; but attention must 

 here be given to its insertion, which is into the external lip of the musculo- 

 spiral groove, after covering the shoulder-joint. It receives here some 

 branches from the circumflex nerve. 



Action. — It is an extensor and inward-rotator of the sho alder-joint. 

 When the limb is fixed, it bends the neck laterally. 



Directions. — The limb may now be detached from the trunk by 

 severing the attachment of the rhomboideus, serratus magnus, levator 

 anguli scapulae, mastoido-humeralis, and latissimus dorsi, the last being 

 cut where it plays over the angle of the scapula. Pieces of clean cloth 

 saturated with some preservative solution should be placed on the outer 

 aspect of the shoulder where the skin has been removed, while the 

 dissector proceeds to examine the structures over the inner surface of 

 the scapula and humerus. 



INNER ASPECT OP THE SHOULDER AND ARM. 



Directions. — The dissector should now identify the terminal portions 

 of the muscles already dissected, and cut them off within an inch or 



