106 THE SUPERIOR EXTREMITY 



volar ulnar recurrent artery. The posterior branch pierces 

 the medial intermuscular septum, and will be seen, at a later 

 stage, in the posterior compartment of the arm. 



Dissection. Clean the biceps brachii. Clean the coraco- 

 brachialis; separate it carefully from the short head of the 

 biceps, and find the musculo-cutaneous nerve as it leaves the 

 lateral surface of the coraco-brachialis. Follow the musculo- 

 cutaneous nerve, between the biceps and the brachialis, to the 

 point where it emerges at the lateral border of the tendon of the 

 biceps and becomes the lateral cutaneous nerve of the forearm. 

 Clean the brachialis as far as the bend of the elbow, but do not 

 injure the lacertus fibrosus. 



Nervus Musculocutaneus. The musculo-cutaneous nerve 

 arises from the lateral cord of the brachial plexus, at the 

 lower border of the pectoralis minor. Inclining laterally, 

 it perforates the coraco-brachialis, and passes between the 

 biceps brachii and the brachialis. It proceeds obliquely 

 distally, between those muscles, until it reaches the bend 

 of the elbow, where it comes to the surface at the lateral 

 border of the tendon of the biceps brachii. From that 

 point onwards it has already been traced as the lateral 

 cutaneous nerve of the forearm (p. 69). 



In the arm the musculo-cutaneous nerve supplies branches 

 to three muscles of the region. The branch to the coraco- 

 brachialis is given off before the parent trunk enters the 

 substance of the muscle ; the branches to the biceps brachii 

 and brachialis issue from it as it lies between them. 



M. Coracobrachialis. The coraco-brachialis is an elongated 

 muscle, which takes origin from the tip of the coracoid 

 process, in conjunction with the short head of the biceps 

 brachii. It proceeds distally, along the medial margin of the 

 biceps brachii, and obtains insertion into a linear ridge 

 situated upon the medial aspect of the body of the humerus, 

 about its middle. 



M. Biceps Brachii. The biceps brachii muscle arises 

 from the scapula by two distinct heads of origin. The short 

 or medial head springs from the tip of the coracoid process in 

 conjunction with the coraco-brachialis (Fig. 24, p. 46). The 

 long or lateral head is a rounded tendon, which occupies the 

 intertubercular sulcus of the humerus. Its origin cannot be 

 studied at this stage of the dissection, because it is placed with- 

 in the capsule of the shoulder-joint, where it arises from an 

 impression on the scapula immediately above the glenoid cavity. 



