I098 A MANUAL OF ANATOMY 



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

 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 muscle 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 nerve then courses behind the brachial 

 plexus and the first part of the axillary artery to the axillary surface 

 of the serratus magnus, which it supplies. 



The Nerve to the Subclavius. — This small branch arises from the 

 front of the upper trunk, its fibres being derived from the fifth cer- 

 vical. It descends over the third part of the subclavian artery, 

 and, passing behind the clavicle, it enters the subclavius muscle on 

 its deep aspect. This nerve sometimes communicates with the 

 phrenic nerve. 



The Suprascapular Nerve. — ^This is a large nerve which arises from 

 the back of the upper trunk, its fibres being derived from the fifth 

 and sixth cervical. It is directed downwards, outwards, and back- 

 wards beneath the trapezius and posterior belly of the omo-hyoid 

 to the upper border of the scapula, on approaching which it gets 

 in company with the suprascapular artery. It is distributed to 

 the supraspinatus and infraspinatus muscles and shoulder-joint. 



For the infraclavicular branches of the brachial plexus, see p. 314. 



Anterior Triangle. — ^This triangle is situated in front of the sterno- 

 cleido-mastoid muscle, and its base is directed upwards. 



Boundaries — Anterior. — The middle line of the neck, that is to 

 say, a line extending from the chin to the upper border of the 



