700 THE NEEVOUS SYSTEM. 



MORPHOLOGY OF THE CERVICAL PLEXUS. 



The characteristic feature of the cervical plexus is the combination of parts of adjacent nerves 

 into compound nerve-trunks by the formation of series of loops. The result of the formation of 

 these loops is that parts (particularly cutaneous areas) are supplied by branches of more than one 

 spinal nerve. 



A. Cutaneous Distribution. By the combinations of the nerves into loops the discrimination 

 of the elements in the upper cervical nerves, corresponding to the lateral and anterior rami 

 of a typical thoracic nerve, is made a matter of some difficulty. The second, third, and 

 fourth nerves, through the cervical plexus, supply an area of skin extending, laterally, from the 

 side of the head to the shoulder ; anteriorly, from the face to the level of the third rib. The higher 

 nerves supply the upper region (second and third) ; the lower nerves supply the lower region 

 (third and fourth). It is not possible to compare the individual nerves strictly with the lateral 

 and anterior rami of a thoracic nerve. A line drawn from the ear to the middle of the 

 clavicle separates, however, a lateral from an anterior cutaneous area ; and certain of the 

 cutaneous nerves fall naturally into one of these two categories. The nerves homologous with 

 anterior rami of intercostal nerves are the n. cutaneus colli and the anterior branches of the 

 supra-clavicular series ; those homologous with lateral branches are the smaller occipital and 

 posterior supra -clavicular branches. The great auricular and middle supra-clavicular branches 

 are mixed nerves, comprising elements belonging to both sets. 



B. Muscular Distribution. The nerves from the cervical plexus supplying muscles are 

 simpler in their arrangement. They are not generally in the form of loops, and they are easily 

 separated into lateral and anterior series. The lateral nerves comprise the branches to the 

 rectus capitis lateralis, sterno-mastoid, trapezius, levator scapulae. The nerves in the anterior 

 series are those to the longus capitis, rectus capitis anterior, the hyoid muscles, and the 

 diaphragm. 



It is noteworthy that the last-named muscles genio-hyoid, thyreo-hyoid, sterno-hyoid, omo- 

 hyoid, sterno-thyreoid, and diaphragm are continuously supplied by branches from the first five 

 cervical nerves : the higher muscles by the higher nerves ; the lower muscles by the lower nerves. 



PLEXUS BRACHIALIS. 



The [Brachial Plexus is formed by the anterior rami of the fifth, sixth, seventh, 

 and eighth cervical nerves, along with the greater part of the first thoracic nerve. 

 In some cases a slender branch of the fourth cervical nerve is also engaged ; and 

 the second thoracic nerve also, in all cases, contributes to the innervation of 

 the arm, through the intercosto-brachial (O.T. intercosto-humeral) nerve. In many 

 cases it contributes also directly to the plexus, by an intra-thoracic communication 

 with the first thoracic nerve. 



Position of the Plexus. The nerves forming the brachial plexus appear in 

 the posterior triangle of the neck between the scalenus anterior and scalenus 

 medius muscles; the^ plexus is formpri in .close relation to the subclavian and 

 axHHry"ar leri.es ; the nerves emanating from it accompany h&_a^^tojfa^xtta,, 

 whence they aredistributed to the shoulder-aa4~upfier limb. 



Communications with the Sympathetic. The lower four cervical nerves communicate 

 with the cervical portion of the sympathetic by means of gray rami communicantes. 

 Two branches arise from the middle cervical ganglion, and join the anterior rami 

 of the fifth and sixth nerves. Two arising from the inferior cervical ganglion join the 

 seventh and eighth nerves. They reach the nerves either by piercing the prevertebral 

 muscles or by passing round the border of the scalenus anterior muscle. 



Composition of the Brachial Plexus. In an analysis of the brachial plexus 

 four stages may be always seen : 



(1) The undivided nerves. 



(2) The separation of the nerves into anterior and posterior trunks ; ancj^k^ 



formation of three primary cords. 



(3) The formation of three secondary cords lateral, medial, and posterior. 



(4) The origin of the nerves of distribution. 



(1) The undivided nerves have only a very short independent course at the side 

 of the neck, after passing between the scalene muscles. 



(2) Three primary cords are formed almost immediately after the undivided j 

 nerves enter the posterior triangle : the first cord is formed by the union of the 

 fifth and sixth nerves ; the second, by the seventh nerve alone ; and the third, j 

 by the union of the eighth cervical and first thoracic nerves. While these cords 

 are being formed, a division occurs in each of the nerves, into anterior and j 

 posterior trunks. The anterior and posterior trunks of the fifth, sixth, and seventh 



