DISSECTION OF THE ANTERIOR LIMB. 6 



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 the 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 siirface 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 fii'st four ribs, and from the immediately 

 subjacent lateral surface of the sternum ; and, being carried upwards in 

 front of the supraspinatus nearly as far as the cei'vical 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 muscle 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- 

 j(;)int of the horse, the dissection of the space is attended with much 

 greater difiiculty 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 sufficient extent, it may be necessary to cut the mastoido-humeralis in 



