1)20 TEE URUAXS OF RESPIRATION 



inwards nuuieruus prolongation;? t'nnn its dct'p surface, which pass between tlie 

 lohules to surround and connect them. 



The lohuk's are irreguhir and many-sided, and consist of numerous lymph 

 follicles. They are grou{)ed around a central tubular cord of connective tissue, and 

 when unravelled the lo))e can l)e lengthened out, the lobules then apjjcaring to be 

 attached to the cord in a spiral fashion. 



The vessels. — The arteries are derived from the internal mannnary and 

 from the sui)evior and inferior thyroids. 



The veins join the left innominate and thyroid veins. 



The nerves are very minute, and ]»roceed from the. sympathetic and pneumo- 

 gastric. 



THE ORQANS OF RESPIRATIOX 



THE LUNGS AND PLEURA 



The lungs — which are two in number — are the special organs of respiration, 

 and occupy the greater portion of the chest cavity. They are separated one from 

 the other by the heart and great vessels, and the other contents of the mediastinal 

 spaces, and each is enclosed in a i)leural sac. 



THE PLEURiE 



Each lung is closely invested by an invaginated serous sac, the pleural sac. 

 The inner wall of the sac, which is closely attached to the lung substance and sur- 

 rounds its root, is the visceral pleura. The outer wall of the sac is the parietal 

 ])leura. and the two jiarts are directly continuous at the root of the lung. The 

 l)arietal pleura covers the inner surface of the thoracic wall, and it forms the lateral 

 l)oundary of the mediastinal space of the thorax. Under ordinary circumstances 

 the inner surface of the i)arietal pleura is in contact with the outer surface of the 

 visceral pleura, or at the most only a tliin layer of serous fluid intervenes, and 

 both surfaces are smooth and glistening; but if the pleural sac is opened the lung 

 shrinks, on account of its elasticity, and a space, the pleural cavity, is develo])ed 

 between the two layers. 



For convenience of description the parietal i)levu-a may Ix- divided into three 

 parts — the external or costal, the inferior or diaphragmatic, and the internal or 

 mediastinal. The costal ])ortion of the parietal pleura is in relation with the ribs, 

 the costal cartilages, the contents of the intercostal spaces, the triangularis sterni. 

 and the subcostal muscles, to which it is attached l)y the endothoracic fascia. The 

 dia])ln'agmatic portion rests upon the diaphragm, and the mediastinal part is in 

 relation with the contents of the mediastinal space — that is, on both sides, with 

 the pericardium, the phrenic nerve and its accompanying vessels, on the right side 

 in addition with the ascending aorta, the superior vena cava, the vena azygos 

 major, the right innominate vein, the innominate artery and the right vagus 

 nerve; and on the left side the additional relations are the transverse and descend- 

 ing portions of the aorta, the left common carotid and sulx-lavian arteries, the 

 thoracic duct, the left su])erior intercostal vein and the left vagus nerve. 



The upper extremity of each ])leural sac extends into the neck, reaching a ])oint 

 an inch (25 mm.) to an inch and a half (37 nun.) above the clavicle, but never 

 al)ove the neck of the fir.st ril); it is covered by a lay(T of fascia called Sibson's 

 fascia; it is in relation with the subclavian artery, which lies in a groove on its 

 antero-internal aspect, and with the .scalenus medius and anticus muscles. 



The borders of each {)leural sac are anterior, inferior and posterior. On the 



