702 



THE NEEVOUS SYSTEM. 



important to realise the position of origin of certain nerves. The nerves to the 

 prevertebral muscles, the communication with the phrenic, the dorsal scapular, and 

 long thoracic nerves, arise from the anterior rami of the nerves involved in the 

 plexus. The supra-scapular and the nerve to the subclavius arise at the level of 

 formation of the secondary cords; and the anterior thoracic, subscapular, and 

 thoraco - dorsal nerves arise from the secondary cords, prior to their ultimate 

 subdivision into the nerves of distribution for the upper limb. 



Pars Supraclavicularis. The nerves derived from the plexus above the 

 level of the clavicle are, like the main trunks, divisible into two series : anterior 

 branches, arising from the front of the plexus ; posterior branches, arising from the 

 back of the plexus (Fig. 614, p. 701). 



Anterior Branches. 



1. Nerves to scalenus anterior and 



longus colli. 



2. Communicating nerve to join 



the phrenic nerve. 



3. Nerve to the subclavius muscle. 



Posterior Branches. 



1. Nerves to scalenus medius and 



scalenus posterior. 



2. Dorsal (posterior) scapular nerve. 



3. Long thoracic nerve. 



4. Supra-scapular nerve. 



INNER 



CORD 



The muscular twigs to the anterior scalene and longus colli muscles arise from 

 the lower four cervical nerves, as they emerge from the intervertebral foramina. 



The communicating branch to the phrenic nerve arises usually from the fifth 

 cervical nerve at the lateral border of the anterior scalene muscle. It is sometimes 



absent, and occasionally an additional root is 

 present from the sixth cervical nerve. In some 

 instances the nerve is replaced by a branch which 

 springs from, the nerve to the subclavius, and 

 passes medially behind the sterno-mastoid muscle 

 to join the phrenic at the inlet of the thorax. 



N. Subclavius. The nerve to the subclavius 

 is a slender nerve, which arises from the front 

 of the cord formed by the fifth and sixth cervical 

 nerves. It descends in the posterior triangle of 

 the neck over the third part of the subclavian 

 artery. It often communicates with the phrenic 

 nerve. 



The branches to the scalenus medius and scalenus 

 posterior, are small trunks which arise from the 

 lower four cervical nerves as they emerge from the 

 intervertebral foramina. 



N. Dorsalis Scapulae. The dorsal scapular 

 FIG. BISDIAGRAM OF THE ORIGIN AND nerve (- T posterior scapular or nerve to the 

 DISTRIBUTION OP THE NERVES TO THE rhomboids) arises from the back of the fifth 

 PECTORAL MUSCLES. cervical nerve, as it emerges from the interver- 



L.A.T, Lateral anterior thoracic nerve; tebral foramen. It appears in the posterior 



^ft$&5%ZMSti trian gk of the neok > afto pg fche scalen s 



plexus; ART, Axillary artery; CL, medius muscle. It is directed downwards, 

 Clavicle ; SCL, Subclavius muscle ; under cover of the levator scapulae and rhomboid 



$^8$ muscles > and alM >g the vertebral mar s in of the 



P.MA, Pectoralis major. scapula, to be distributed to the levator scapulae, 



rhomboideus minor, and rhomboideus major 

 muscles. It occasionally pierces the levator scapulae. 



N. Thoracalis Longus. The long thoracic nerve (O.T. posterior thoracic or 

 external respiratory nerve of Bell) arises by three roots, of which the middle one 

 is usually the largest, from the back of the fifth, sixth, and seventh nerves, as they 

 emerge from the intervertebral foramina. The nerve pierces the scalenus medius 

 as two trunks, of which the lower represents the contribution from the seventh 

 cervical nerve, and, descending along the. side of the neck behind the cords of the 

 brachial plexus, it enters the axilla between the superior edge of the serratus 



