DISSECTION OF THE ANTERIOR LIMB. 3 



It ascends in the groove between the anterior superficial pectoral and 

 the mastoido-humeralis. »In the inner third of this groove it lies on the 

 anterior deep pectoral, in company with a branch of the inferior cervical 

 artery. It empties itself into the jugular about two inches from the 

 lower end of that vessel. 



Deep Pectoral Muscle. — This consists of two distinct divisions, which 

 may be distinguished as the anterior deep pectoral and the posterior 

 deep pectoral. 



The Posterior Deep Pectoral (Pectoralis magnus of Percivall) (Plate 

 2) is a muscle of large size ; and its posterior part, being subcutaneous, 

 was visible before reflection of the superficial pectoral. It arises from 

 the abdominal tunic covering the external oblique and straight muscles 

 of the abdomen ; from the tips of the cartilages of the 5th, 6th, 7th, 

 and 8th ribs ; and from the immediately subjacent lateral surface of the 

 sternum. It is inserted into the inner tuberosity of the humerus, into 

 the tendon of origin of the biceps, and into the fascia which retains that 

 muscle in the bicipital groove. By its deep face the muscle serves to- 

 bound the axillary space ; while its upper border is closely united to 

 the panniculus, and bordered by the subcutaneous thoracic nerve and 

 vessels. 



The Anterior Deep Pectoral (Pectoralis parvus of Percivall) (Plate- 

 2) arises from the cartilages of the first four ribs, and from the immedi- 

 ately subjacent lateral surface of the sternum ; and, being carried up- 

 wards in front of the supraspinatus nearly as far as the cervical angle 

 of the scapula, it is somewhat loosely inserted into the fascia covering 

 the last-named muscle. This insertion is concealed by the mastoido- 

 humeralis, and will be better seen in the dissection of the muscles on 

 the outer surface of the scapula (Plate 4). The deep face of the muscle 

 forms part of the inferior boundary of the axilla. 



Action. — The two divisions of the deep pectoral have the same action, 

 which is to pull the shoulder-joint, and thus the whole limb, backwards. 

 When the limbs are fixed the muscle may to some extent act as a 

 musqle of inspiration. 



Directions — The deep pectoral muscles are now to be severed care- 

 fully about midway between their origin and insertion, and the cut 

 portions are to be turned outwards and inwards. Their nerves, which 

 come from the brachial plexus, will be found entering their deep face ; 

 and care is to be taken of the external thoracic and inferior cervical 

 arteries. By the reflection of these muscles the axilla is exposed. 

 Owing to the limited power of abduction at the shoulder-joint of the 

 horse, the dissection of the space is attended with much greater 

 difficulty than in man. The best method of procedure is as follows : — 

 All the pectoral muscles having been cut across, the limb is to be 

 forcibly separated from the chest-wall ; and, to permit this to a 



