Next in order, a number of conglobate glands, and a plexus of anastomosing veins, principally from the scapular region, come into view ; these latter usually communicate with the external jugular, or with the subclavian vein. Areolar tissue which presents a laminated arrangement encloses these glands and super ficial vessels, and isolates them from the deeper-seated parts.
These structures being removed, the sub clavian artery appears to lie within a second triangle of smaller dimensions, bounded inter nally by the scalenus anticus muscle, externally and superiorly by the omo-hyoid muscle, and inferiorly by the first rib; this bone represents the base of the triangle, and over it the artery is seen to pass. At this depth, two collateral arterial branches of considerable size cross the supra-clavicular space, the one, the trans versalis colli, above, the other, the supra scapular, below the level of this portion of the subclavian artery ; the latter is placed under cover of the clavicle, and in contact with the front of the subclavian vein. As the supra scapular artery pursues its course towards the shoulder, it crosses in front of the subclavian artery and of the brachial plexus of nerves. Here likewise the clavicle and the subclavius muscle constitute additional anterior relations of the subclavian artery, now near its termi nation.
The nervous bundle of the brachial plexus is parallel to the subclavian artery in its third stage, and lies superior and external to the vessel ; in its descent the lower division of the plexus overhangs the artery, and one or two of the branches (anterior thoracic) cross the anterior surface of the artery, and some times even encircle it in a nervous loop.
The anterior relations of the third stage of the subclavian artery may therefore be thus arranged :— I. Integument, superficial cutaneous nerves, platysma, fascia.
2. Areolar tissue in layers, glands, external jugular vein, an intricate plexus of smaller veins.
3. Anterior thoracic branches of the bra chial plexus, the subclavian vein, supra-sca pular artery, clavicle, and subclavius muscle.
Anomalies in the origin of the subclavian ar teries..-1. The right subclavian artery some times arises separately from the arch of the aorta, in which case there is no arteria inno minata ; the branches that arise from the arch of the aorta are then four in number, but con siderable variety has been observed in the relation which the right subclavian bears to the other three branches ; thus, a. It may occupy the usual position of the innominate artery, being the first in order of the branches of the arch of the aorta ; its re lations within the thorax will then correspond with those assigned to the vessel whose place it comes to occupy.
b. It may be the second in numerical order of the branches of the arch, arising after the right carotid artery, behind which it subse quently passes to arrive at its proper position in the neck.
c. It may arise after the two carotids as the third branch of the arch ; or, d. It may be the last branch of the aorta, and occupy the usual situation of the left sub clavian artery. Of the varieties already men tioned, this is the most frequently met with, and, according to the statistics of Pro fessor Quain, it occurs once in every 250 examinations.
e. Sometimes (but much more rarely) this vessel arises below the arch, from the thoracic aorta, and its position may be so low, that it will furnish some of the upper intercostal arteries.
The course of the artery, when it thus arises from the left of the arch, is very remarkable ; it crosses in front of the spinal column, either behind the cesophagus, or between that tube and the trachea, and necessarily passes across the neck behind all the other branches given off from the arch of the aorta. When thus ab normally situated behind the cesophagus, it has been accidentally wounded by a foreign body which had first transfixed that tube. A remarkable example of this occurrence is mentioned by Mr. Kirby, in the 2d vol. of the Dublin Hospital Reports.
The irregularity in question, of the right subclavian artery, was regarded by Dr. Bay ford as the cause of difficult deglutition, in a case which had been accurately observed for many years, and this new disease, as he con sidered it, he quaintly termed " Dyspba,gia lusoria." * In those instances, where the right sub clavian artery has been found to deviate thus strangely from its usual course, the inferior laryngeal nerve presented a remarkable change of direction, depending no doubt on the altered course of the artery ; in all the instances which were noted, the nerve was given off from the pneumogastric, higher up than usual, and passed directly to the larynx, so as not to he entitled to the name of "recurrent." Dr. Hart, who first directed attention to this fact, has thus clearly explained the connection be tween the unusual position of the artery and the altered direction of the nerve : "In the ear lier periods of the existence of the foetus, the rudiment of the head appears as a small pro jection from the upper and anterior part of the trunk, the neck not being yet developed. The larynx at this time is placed behind the ascending portion of the arch of the aorta ; while the brain, as it then exists, is situated so low, as to rest on the thymus gland and front of that vessel. Hence it is that the inferior laryngeal nerves pass back to the larynx, se parated by the ascending aorta, the left going through its arch, while the right goes below the arteria innominata.