308 DISSECTION OF THE THORAX. 



duct, the oesophagus with its nerves, the trachea, the splanchnic nerves at 

 the lower part, and some lymphatic glands. 



Dissection. The pleura and the fat are now to be cleaned from the side 

 of the pericardium. 



The root of the lung is to be dissected out by taking away the pleura 

 and the areolar tissue from the front and back, without injuring its several 

 component vessels. In this dissection the phrenic artery and nerve will 

 be found in front of the root, together with a few nerves (anterior pul- 

 monary) ; the last are best seen on the left side. Behind the root of the 

 lung is the vagus nerve, dividing into branches ; and arching above it is 

 the large azygos vein. 



For the present, the arch of the aorta and the small nerves on it may 

 be left untouched. 



The thymus body is a fretal organ, like the thyroid body, whose use is 

 unknown. It occupies chiefly the upper part of the thorax ; and it may 

 be best examined in a full-grown foatus. 



At birth it is about two inches in length, and is of a grayish color ; it 

 possesses two lobes of a conical form, which touch each other. Its upper 

 end is pointed and extends on the trachea as high as the thyroid body; and 

 the lower wider part, reaches as far as the fourth rib. In the thorax it 

 rests on the aortic arch and large vessels, on the left innominate vein, and 

 on the pericardium. 



In the adult all that remains of the thymus is a brownish rather firm 

 material in the interpleural space beneath the upper part of the sternum. 



In its perfect state it resembles much the thyroid body ; and a whitish 

 fluid, containing lymph-like corpuscles, escapes from it when it is cut. 



CONNECTIONS OF THE LUNGS. 



The lungs are two in number, and are contained in the cavity of the 

 thorax, one on each side of the spinal column. In these organs the blood 

 is changed in respiration. 



The lung is of a somewhat conical form, and takes its shape from the 

 space in which it is lodged. It is unattached, except at the inner part 

 where the vessels enter ; and it is covered by the bag of the pleura. Its 

 base and apex, borders and surfaces can be distinguished by differences in 

 form ; it is* divided also into lobes by fissures ; and it has a root formed 

 out of its vessels and nerves. 



The base of the lung is hollow in the centre and thin at the circumfer- 

 ence, and fits on the convexity of the diaphragm. Following the shape of 

 that muscle, it is sloped obliquely from before backwards, and reaches in 

 consequence much lower posteriorly than anteriorly. Its position with 

 respect to the wall of the thorax may be ascertained externally by taking 

 the level of the diaphragm as a guide (p. 306) ; and it will be a rib's 

 breadth lower in front on the left, than on the right side (fig. 97). The 

 apex is rounded, and projects an inch to an inch and a halt' above the first 

 rib, where it lies beneath the clavicle, the anterior scalenus muscle, and 

 the subclavian artery. 



The anterior edge or border is thin, and overlays in part the pericar- 

 dium. On the right side it lies along the middle of the sternum as low as 

 the sixth costal cartilage (fig. 97). On the left side it reaches the mid- 

 line of the chest as low as the fourth costal cartilage ; but below that spot 

 it presents a V-shaped notch (fig. 97), whose apex is opposite the outer 



