THE UPPER LIMB 313 



by arranging it into four stages, namely, (i) nerve roots, (2) nerve 

 trunks, (3) divisions of nerve trunks, and (4) nerve cords. 



First Stage. — ^The nerves which form the plexus are the anterior 

 primary divisions of the fifth, sixth, seventh, and eighth cer\'ical, and 

 the greater part of that of the first thoracic. Superiorly the plexus 

 is reinforced by a small descending branch from the fourth cervical, 

 which joins the fifth, and interiorly it is occasionally reinforced by a 

 branch from the second thoracic, which joins the first. As regards 

 the first thoracic nerve, the part of it which does not join the plexus, 

 and which is of small size, enters the first intercostal space to become 

 the first intercostal nerve. The nerves, as they emerge at the side 

 of the neck, are placed between the scalenus anticus and scalenus 

 medius. 



Second Stage. — The fifth and sixth cervical nerves join at the outer 

 border of the scalenus anticus to form the upper trunk ; the seventh 

 cervical remains meanwhile single, and forms the middle trunk ; 

 and the eighth cervical and greater part of the first thoracic unite 

 between the scalene muscles to form the lower trunk. There are 

 thus three trunks — upper, middle, and lower. 



Third Stage. — A little above the clavicle each of the three tnmks 

 breaks up into an anterior and a posterior division. 



Fourth Stage. — ^The anterior divisions of the upper and middle 

 trunks unite to form the outer cord of the plexus ; the anterior 

 division of the lower trunk, which is of large size, forms the inner 

 cord ; and all three posterior divisions (that of the lower trunk being 

 of small size) unite to form the posterior cord. As a variety, the 

 anterior division of the middle trunk may subdivide into two 

 branches, one entering the outer cord and the other the inner. 

 There are thus three cords — outer, inner, and posterior. 



Branches of the Plexus. — ^The branches are conveniently divided 

 into two groups — supraclavicular, arising above the clavicle and 

 coming from nerve roots and nerve trunks ; and infraclavicular, 

 arising below the clavicle and coming from nerve cords. 



Supraclavicular Branches. — ^These are as follows : Muscular 

 branches from the four cervical nerves to the scalene muscles and 

 longus colli. 



One root of the phrenic nerve (inconstant) from the front of the 

 fifth cervical. 



The Nerve to the Rhomboids. — ^This branch arises from the 

 back of the fifth cervical close to, or along with, the highest root of 

 the posterior thoracic, and it takes a backward course through the 

 scalenus medius. 



The Posterior Thoracic Nerve or External Respiratory Nerve of 

 Bell. — This branch arises by three roots from the back of the fifth, 

 sixth, and seventh cervical nerves. The upper two roots pierce the 

 scalenus medius below the nerve to the rhomboids, either conjointly 

 or separately, whilst the lowest root passes in front of the scalenus 

 medius, and joins the trunk formed by the others near the first rib. 

 The ner\'e then courses behind the first part of the axillary artery, 



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