THE THORAX 363 



here the artery divides, forming the external and internal carotid 

 arteries at about the level of the upper border of the thyroid 

 cartilage, or "Adam's apple." 



Surgical note. The carotid is called the triangle of election because, since 

 the vessels are near the surface, the surgeon would naturally choose, or elect, 

 this place for the operation of ligation. In the muscular triangle the vessels 

 are more deeply placed and covered by the lower portion of the sterno-mas- 

 toid. Ligation of the artery would be done here only under necessity, so it 

 is called the triangle of necessity. 



Occipital triangle. The occipital artery and nerve run through 

 this triangle. 



Subclavian triangle. Most important structures are sub- 

 clavian artery and vein, brachial plexus, and phrenic nerve. 



Clinical note. Pressure in this triangle, close to the clavicle, will be felt 

 by the nerves of the brachial plexus. Pressure downward and backward close 

 to the sterno-mastoid will compress the subclavian artery against the first 

 rib. Its pulsation is plainly felt. 



Submaxillary triangle. This is a small space marked off from the carotid 

 by the digastric muscle. It contains the submaxillary gland and external 

 maxillary artery. 



THE THORAX AND THORACIC VISCERA 



The bony thorax is narrow above and broad below, but the 

 proportions are reversed in the completed human body by the 

 presence of the large muscles_which connect the upper extremity 

 with the thorax. 



Observe the transverse ridge on the sternum, marking the junc- 

 tion of the first and second pieces (the manubrium and the body). 

 The second rib joins the sternum at this ridge (Fig. 234). 



The boundaries of the completed thorax are the spinal column 

 at the back, the sternum in front, and the ribs at the sides, with the 

 intercostal muscles in the intercostal spaces and the diaphragm in 

 the floor. It is covered behind by the muscles of the back, while 

 the anterior serratus is on the side and the pectoral muscles are in 

 front. The shoulder blades are placed behind the thorax. 



The intercostal arteries and nerves are protected from injury by 

 their position under the borders of the ribs. A stab-wound would 

 have to be directed upward to reach them. 



All muscles which are attached to the ribs are muscles of res- 



