SUBCLAVIAX. 391 



branch of the basilar. This artery varies considerably in size, being sometimes 

 small, and occasionally so large that the posterior cerebral may be considered as 

 arising from the internal carotid rather than from the basilar. It is frequently 

 larger on one side than on the other. 



The anterior choroid is a small but constant branch which arises from the back 

 part of the internal carotid, near the posterior communicating artery. Passing 

 backwards and outwards, it enters the descending horn of the lateral ventricle, 

 beneath the edge of the middle lobe of the brain. It is distributed to the hippo- 

 campus major, corpus fimbriatum, and choroid plexus. 



AETEEIES OF THE UPPER EXTREMITY. 



The artery which supplies the upper extremity, continues as a single trunk from 

 its commencement, as far as the elbow ; but different portions of it have received 

 different names, according to the region through which it passes. Thus, that 

 part of the vessel which extends from its origin, as far as the outer border of 

 the first rib, is termed the subclavian ; beyond this point to the lower border of 

 the axilla, it is termed the axillary ; and from the lower margin of the axillary 

 space to the bend of the elbow, it is termed the brachial ; here, the single trunk 

 terminates by dividing into two branches, the radial and ulnar, an arrangement 

 precisely similar to what occurs in the lower limb. 



SUBCLAVIAN AETERIES. 



The Subclaviau Artery on the right side arises from the arteria innomiuata, 

 opposite the right sterno-clavicular articulation ; on the left side, it arises from the 

 arch of the aorta. It follows, therefore, that these two vessels must, in the first 

 part of their course, differ in their length, their direction, and in their relation 

 with neighboring parts. 



In order to facilitate the description of these vessels, more especially in a sur- 

 gical point of view, each subclavian artery has been divided into three parts. 

 The first portion, on the right side, ascends obliquely outwards, from the origin of 

 the vessel to the inner border of the Scalenus anticus. On the left side, it ascends 

 perpendicularly to the inner border of that muscle. The second part passes out- 

 wards, behind the Scalenus anticus ; and the third part passes from the outer 

 margin of that muscle, beneath the clavicle, to the lower border of the first rib, 

 where it becomes the axillary artery. The first portions of these two vessels differ 

 so much in their course, and in their relation with neighboring parts, that they 

 will be described separately. The second and third parts are precisely alike on 

 both sides. 



FIRST PART OF THE RIGHT SUBCLAVIAN ARTERY (figs. 205 and 207). 



It arises from the arteria innominata, opposite the right sterno-clavicular arti- 

 culation, passes upwards and outwards across the root of the neck, and terminates 

 at the inner margin of the Scalenus anticus muscle. In this part of its course, it 

 ascends a little above the clavicle, the extent to which it does so varying in different 

 cases. It is covered, in front, by the integument, superficial fascia, Platysma, deep 

 fascia, the clavicular origin of the Sterno-mastoid, the Sterno-hyoid and Sterno- 

 thyroid muscles, and another layer of the deep fascia. It is crossed by the 

 internal jugular and vertebral" veins, and by the pneumogastric, the cardiac 

 branches of the sympathetic, and phrenic nerves. Beneath, the artery is invested 

 by the pleura, and behind, it is separated by a cellular interval from the Longus 

 colli, the transverse process of the seventh cervical vertebra, and the sympathetic; 

 the recurrent laryngeal nerve winding around the lower and back part of this 

 vessel. The subclavian vein lies below the subclavian artery, immediately behind 

 the clavicle. 



