THE THOKAX. 1395 



the middle of the superior border of the clavicle and the inferior part of the posterior 

 border of the sterno-mastpid muscle. The most important guides to the vessel are 

 the posterior belly of the omo-hyoid, the lateral border of the scalenus anterior, and 

 the scalene tubercle of the first rib. The close relation of the vessel to the lowest 

 trunk of the brachial plexus and to the cervical pleura must be kept in mind. In 

 the rare instances in which a cervical rib is present the subclavian artery lies either 

 in front of it, or arches above it, according to the degree of development of the rib. 

 The subclavian vein lies below, and anterior to, the artery, altogether under cover of 

 the clavicle. 



Entering the posterior triangle, from behind the lateral border of the scalenus 

 anterior, are the trunks of the brachial plexus. They lie upon the scalenus medius, 

 and can be felt, through the skin, immediately above and behind the third part 

 of the subelavian artery. The anterior ramus of the fifth cervical nerve supplies 

 the rhomboids, the abductors and lateral rotators of the arm, and the flexors 

 and supinators of the forearm ; that of the sixth the serratus anterior, the adductors 

 and medial rotators of the arm, and the extensors and pronators of the forearm ; 

 that of the seventh the flexors and extensors of the wrist ; that of the eighth the 

 flexors and extensors of the fingers ; that of the first thoracic all the small muscles 

 of the hand. The carotid tubercle lies between the anterior rami of the sixth 

 and seventh cervical nerves. The fifth and sixth cervical nerves are those which 

 suffer most when the* plexus is injured by forcible depression of the shoulder 

 while the head is bent to the opposite side, such as occurs, for instance, in the 

 " Obstetrical Paralyses " of Duchenue. 



To expose the trunks of the brachial plexus an incision is made from the 

 junction of the middle and inferior thirds of the posterior border of the sterno- 

 mastoid downwards and laterally to the junction of the lateral and intermediate 

 thirds of the clavicle. 



The relation of the lowest trunk of the brachial plexus to the first rib is 

 important in relation to those forms of brachial neuritis in which the motor 

 and sensory symptoms indicate pressure on the anterior ramus of the first 

 thoracic nerve. Wood Jones has shown that the sulcus for the subclavian artery 

 on the superior surface of the first rib frequently lodges the lowest trunk of the 

 brachial plexus as well as the artery, and that the more the first thoracic nerve 

 contributed to the plexus the deeper is the sulcus. In two cases the writer has 

 cured the neuralgia and the partial paralysis of the intrinsic muscles of the hand 

 supplied by the first thoracic nerve by removing the portion of the first rib con- 

 taining the " sulcus nervi brachialis." The symptoms were due to the portion of 

 the first thoracic nerve which goes to join the brachial plexus being stretched and 

 pressed upon by the first rib as it crosses its inner edge to join the eighth cervical 

 nerve. Although similar symptoms may be produced by the first thoracic nerve 

 being stretched across a cervical rib, the surgeon must not expect to find this 

 anomaly in all cases ; and when a skiagram has been obtained, care must be taken 

 not to mistake a well-developed posterior tubercle of the transverse process of the 

 seventh cervical vertebra for a foreshortened view of a rudimentary cervical rib. 



In the median line of the neck posteriorly is the nuchal furrow, at the bottom of 

 which are the cervical spines and the ligamentum nuchae. At the superior part of 

 the furrow, about two inches below the external occipital protuberance, is the large 

 spine of the epistropheus, which can be distinctly felt ; a line drawn from it laterally 

 and slightly upwards to the transverse process of the atlas corresponds to the position 

 of the inferior oblique muscle and, therefore, to the inferior margin of the sub-occipital 

 triangle. The course of the deep part of the greater occipital nerve may be mapped 

 out by drawing a line from the centre of the above-mentioned line to a point one 

 inch lateral to the external occipital protuberance. In the floor of the sub- 

 occipital triangle is the posterior arch of the atlas upon which the vertebral artery 

 lies. 



THE THOEAX. 



For the convenience of topographical description, clinicians, by the use of 

 vertical and transverse lines, have arbitrarily divided the surface of the chest into 



89 a 



