BASE OP THE NECK. 57 



will at once be seen, might essentially complicate the ope- 

 ration of tracheotomy. 



The COMMON CAROTID ARTERIES will be seen to have a 

 different origin, the right being from the innominata and 

 the left directly from the aorta ; the right is consequently 

 shorter than the left by the length of the innominata. 

 Separated at first only by the width of the trachea, they 

 diverge as they ascend, their relations, except at their ori- 

 gin, being the same on the two sides (p. 42). The right 

 carotid occasionally arises directly from the aorta. 



The SUBCLAVIAN ARTERY differs, not only in origin, but 

 in direction, on the two sides of the body, the right coming 

 from the innominata and the left from .the aorta direct ; the 

 right, moreover, is shorter than the left by the length of the 

 innominata. The left subclavian ascends horizontally, and 

 then turning suddenly, forms a right or an obtuse angle, 

 while the right describes a gradual and regular curve. 

 On both sides the artery passes beneath the scalenus anti- 

 cus muscle, and rests upon the first rib, which is slightly 

 grooved at the point where it reposes. The scalenus mus- 

 cle separates the artery from its vein, which lies in front 

 of the muscle ; behind the artery are the scalenus posticus 

 muscle, and the branches of the brachial plexus of nerves, 

 which in a measure surround it. At the lower border of 

 the first rib the subclavian becomes the axillary artery. 

 The right subclavian has been observed to spring from the 

 aorta direct, and is then most frequently the last in order 

 of the primary aortic trunks ; in such a case it crosses the 

 neck obliquely, in front of the vertebral column and be- 

 neath the oesophagus, to regain its usual position. When 

 this anomaly exists the carotid arteries not unfrequently 

 spring from a common trunk, very short, and showing some 

 marks of a tendency to divide. The left subclavian may 

 arise in common with the left carotid. 



The student should explore with his finger the tubercle 

 on the first rib into which the scalenus anticus muscle is 

 inserted, this being the guide to the position of the sub- 

 clavian artery, when the surgeon desires to place a ligature 

 upon it. He should also note the relations of the artery 

 to the subclavian vein, the brachial plexus of nerves, the 

 scalenus muscle, the clavicle, and the rib. 



At this period of the dissection may be observed the 

 variable height to which the pleural cavity sometimes ex- 

 tends above the first rib ; a prolongation of two or even 



