502 THE BLOOD-VASCULAR SYSTEM. 



front, by the Sterno-raastoid ; behind, by the Trapezius ; below, by the ( hno- 

 hyoid. Its floor is formed from above downward by the Splenius, Levator anguli 

 scapulae, and the middle and posterior Scaleni muscles. It is covered by the 

 integument, the Platysma below, the superficial and deep fasciae ; the spinal acces- 

 sory nerve is directed obliquely across the space from the Sterno-mastoid, 

 which it pierces, to the under surface of the Trapezius ; below, the descending 

 branches of the cervical plexus and the transversalis colli artery and vein cross 

 the space. A chain of lymphatic glands is also found running along the pos- 

 terior border of the Sterno-mastoid, from the mastoid process to the root of the 

 neck. 



The Subclavian, the smaller of the two posterior triangles, is bounded, above, 

 by the posterior belly of the Omo-hyoid ; below, by the clavicle, its base, directed 

 forward, being formed by the Sterno-mastoid. The size of the subclavian trian- 

 gle varies according to the extent of attachment of the clavicular portion of the 

 Sterno-mastoid and Trapezius muscles, and also according to the height at which 

 the Omo-hyoid crosses the neck above the clavicle. Its height also varies much 

 according to the position of the arm, being much diminished by raising the limb, 

 on account of the ascent of the clavicle, and increased by drawing the arm down- 

 ward, when that bone is depressed. This space is covered by the integument, 

 the Platysma, the superficial and deep fasciae, and crossed by the descending 

 branches of the cervical plexus. Just above the level of the clavicle the third 

 portion of the subclavian artery curves outward and downward from the outer 

 margin of the Scalenus anticus, across the first rib, to the axilla. Sometimes this 

 vessel rises as high as an inch and a half above the clavicle, or to any point inter- 

 mediate between this and its usual level. Occasionally it passes in front of the 

 Scalenus anticus or pierces the fibres of that muscle. The subclavian vein lies 

 behind the clavicle, and is usually not seen in this space : but it occasionally rises 

 as high up as the artery, and has even been seen to pass with that vessel behind 

 the Scalenus anticus. The brachial plexus of nerves lies above the artery, and 

 in close contact with it. Passing transversely behind the clavicle are the 

 suprascapular vessels, and traversing its upper angle in the same direction, the 

 transversalis colli artery and vein. The external jugular vein runs vertically 

 downward behind the posterior border of the Sterno-mastoid, to terminate in the 

 subclavian vein ; it receives the transverse cervical and suprascapular veins, which 

 occasionally form a plexus in front of the artery, and a small vein which crosses 

 the clavicle from the cephalic. The small nerve to the Subclavius muscle also 

 crosses this triangle about its middle. A lymphatic gland is also found in the 

 space. Its floor is formed by the first rib with the first digitation of the Serratus 

 magnus. 



The Internal Carotid Artery. 



The internal carotid artery supplies the anterior part of the brain, the eye, 

 and its appendages, and sends branches to the forehead and nose. Its size in 

 the adult is equal to that of the external carotid, though in the child it is larger 

 than that vessel. It is remarkable for the number of curvatures that it presents 

 in different parts of its course. It occasionally has one or two flexures near the 

 base of the skull, whilst in its passage through the carotid canal and along the side 

 of the body of the sphenoid bone it describes a double curvature which resembles 

 the italic letter s placed horizontally. These curvatures most probably diminish the 

 velocity of the current of blood, by increasing the extent of surface over which it 

 moves and adding to the amount of impediment produced from friction. 



In considering the course and relations of this vessel it may be conveniently 

 divided into four portions : a cervical, petrous, cavernous, and cerebral. 



Cervical Portion. This portion of the internal carotid commences at the bifur- 

 cation of the common carotid, opposite the upper border of the thyroid cartilage, 

 and runs perpendicularly upward, in front of the transverse processes of the three 

 upper cervical vertebrae, to the carotid canal in the petrous portion of the temporal 

 bone. It is superficial at its commencement, being contained in the superior 



