THE THORAX 1363 



It is bounded in front by the middle, and behind by the prevertebral layer. Its contents are — 

 larynx, trachea, oesophagus, thyreoid, carotid sheath, glands; and below, brachial plexus, sub- 

 clavian artery, and abundant loose cellular tissue for the movements of the neck. Suppuration is 

 somewhat rarer here; but either pus or growths, if oonfined in this space, may have baneful 

 effects, from pressure, or from their tendencj' to travel behind the sternum. (4) This space 

 between the prevertebral layer in front and muscles behind, is very limited. Retropharyngeal 

 abscess forms here, and the dyspnoea it causes is thus explained. The origin of such abscesses 

 is chiefly twofold, either in one of the highest deep cervical nodes, e. g., from infection of the 

 naso-pharynx (p. 717), or from disease of the upper cervical vertebrae. In the former cases 

 fStiles, Chiene) the suppuration will be in front of the prevertebral fascia, pointing toward the 

 pharynx; in the latter behind the above fascia, spreading laterally, behind the carotid sheath. 

 In making his incision, now along the posterior border of the sterno-mastoid, the surgeon should 

 keep close to the transverse processes of the vertebrae, to avoidfopening the visceral compartment 

 and infecting the structures in it. (B) The deep cervical fascia gives sheaths or canals to 

 certain veins which perforate it, e. g., the external jugular. These are thus kept patent, and a 

 ready passage of blood ensured from the head and neck. Further, this fact accounts for the 

 readiness with which air may enter veins, in operations low down in the neck. The carotid 

 sheath is another and different instance. (C) It helps to resist atmospheric pressure. (D) 

 Hilton's suggestion as to its action on the pericardium has already been mentioned. 



The lymphatic nodes of the head and neck have already been described. 

 (See Section VI, Lymphatic System.) 



THE THORAX 



The bony landmarks of the thorax vnl\ be discussed first, followed bj'' the 

 structures of the thoracic wall, the lungs and pleura, and finally the heart and 

 pericardium. 



Bony landmarks. — The top of the sternum corresponds (in inspiration) to the 

 fibro-cartilage between the second and third thoracic vertebrse, and is distant 

 about 6.2 cm. (2| in.) from the spine. In the newborn child it corresponds to 

 the middle of the first thoracic vertebra (Symington). If traced downward, the 

 subcutaneous sternum presents a ridge (sternal angle of Louis) opposite to the 

 junction of the manubrium and bod}^ and the second costal cartilages on either 

 side; this ridge usually corresponds to the disc between the fourth and fifth 

 thoracic vertebrae. At the lower extremity of the sternum the xiphoid cartilage 

 usually retires from the surface, presenting the depression of the epigastric angle 

 or 'pit of the stomach.' This is opposite to the seventh costal cartilages and the 

 expanded upper end of the recti, and corresponds to the tenth thoracic vertebra 

 behind. 



Parts behind manubrium. — There is little or no lung behind the first bone of 

 the sternum, the space being occupied bj^ the trachea and large vessels, as follows; 



The left innominate vein crosses behind the sternum just below its upper border. Next 

 come the great primary branches of the aortic arch. Deeper still is the trachea, dividing into 

 its two bronchi opposite to the junction of the first and second bones of the sternum. Deepest 

 of all is the oesophagus. About 2.5 cm. (1 in.) below the upper border of the sternum is the high- 

 est part of the aortic arch, lying on the bifurcation of the trachea. (Holden.) (Fig. 1104). 



Sterno -clavicular joint. — The expanded end of the clavicle and the lack of 

 proportion between this and the sternal facet, on which largely depends the 

 mobility of this, the onl}^ joint that ties the upper extremity closely to the trunk, 

 can be easily made out through the skin. Its strength, considerable when the 

 rarity of dislocation compared with fracture of the clavicle is considered, depends 

 mainly on its ligaments, the buffer-bond meniscus, the costo-clavicular ligament, 

 which checks excessive upward and backward movements, and the fact that the 

 elastic support of the first rib comes into play in strong depression of the shoulder 

 as in carrying a weight. The relative weakness of the anterior ligament deter- 

 mines the greater frequency of anterior dislocation of the clavicle at this joint. 



Behind the joint lie, on the right side, the innominate artery, right innominate vein, and 

 pleura; on the left, the left innominate vein, the left carotid, and the pleura. 



Acromio -clavicular joint. — On tracing the clavicle laterally, it is found to 

 rise somewhat to its articulation with the acromion. This joint has very httle 

 mobility, and owes its protection to the strong conoid and trapezoid ligaments 

 hard by. Owdng to the way in which the joint-surfaces are bevelled, that of the 

 clavicle looking obliquely downward, and resting upon the acromion, it is an 

 upward displacement, of the clavicle which usually takes place. 



Ribs. — In counting these, the position of the second is denoted by the trans- 



