THE IXTERXAL CAROTID ARTERY. 565 



POSTERIOR TRIANGLE OF THE XEOK. 



The posterior triangle is bounded, in front, by the Sterno-mastoid muscle; 

 behind, by the anterior margin of the Trapezius ; its base corresponds to the 

 upper border of the clavicle; its apex, to the occiput. The space is crossed, 

 about an inch above the clavicle, by the posterior belly of the Omo-hyoid, which 

 divides it unequally into two, an upper or occipital and a lower or subclavian 

 triangle. 



The Occipital, the larger of the two posterior triangles, is bounded, in front, 

 by the Sterno-mastoid ; behind, by the Trapezius ; below, by the Omo-hyoid. Its 

 floor is formed from above downward by the Splenius capitis, 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, 

 superficial and deep fascine, 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 intermediate 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 across the clavicular margin of the space are the 

 suprascapular vessels, and traversing its upper angle in the same 1 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 whicn 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 foundVin the 

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

 magnus. 



The Internal Carotid Artery. I J ' 



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. In its cervical portion it occasionally presents 

 one or two flexures near the base of the skull, whilst through the rest of its extent 

 it describes a double curvature which resembles the italic letter s placed horizon- 

 tally. These curvatures most probably diminish the velocity of the current of 



