DISSECTION OP THE BACK AND THORAX. 125 



ivall of an air-cell consists of a delicate membrane supporting the 

 japillary plexus of the pulmonary vessels, and lined towards the air 

 passage by a single layer of squamous cells. The bronchial tubes 

 comprise in their walls : (1) an outer fibro-elastic coat sustaining seg- 

 mented rings of cartilage ; (2) within the preceding, a complete coat 

 of non-striped muscular fibres, circularly arranged ; (3) an inner fibro- 

 elastic coat ; (4) a mucous membrane with a ciliated epithelium on its 

 free surface. Numerous mucous glands lie in the outer fibrous coat, 

 and discharge their secretion into the bronchus. The bronchi in their 

 ramifications are accompanied by divisions of the pulmonary artery 

 and veins, these two sets of vessels being connected by the capillary 

 plexus on the air-cells. Along the bronchi run also the much 

 smaller branches of the bronchial vessels, as well as nerves and lym- 

 phatics. 



Connective-tissue forms a framework for the lung. It surrounds 

 and connects the bronchi and vessels as they run together in the lung 

 substance; it connects and isolates the adjacent lobules; and beneath 

 the pleura it forms a thin fibrous capsule for the lung. Lymphatic vessels 

 are abundantly distributed in it, and form three principal sets, viz., 

 subpleural, perivascular (around the pulmonary vessels), and peri- 

 bronchial. 



DISSECTION OF THE HEART. 



The Vessels of the Heart. 



The Coronary Arteries (Plates 23 and 24) carry arterial blood to 

 nourish the heart-wall. They are two in number, distinguished as right 

 and left. Each arises from the common aorta, and has its mouth in 

 one of the sinuses of Valsalva. 



The Right Coronary Artery passes forwards to the right of the pul- 

 monary artery at its root ; and, encircling the right auricular appendix, 

 it places itself in the auriculo-ventricular furrow, in which it passes to 

 the right side of the heart. On reaching the origin of the right ventri- 

 cular furrow it divides, one branch descending in that furrow, while the 

 other continues the course of the main trunk in the auriculo-ventricular 

 o-roove. The terminal twigs of the vertical branch enter the heart 

 a little above the apex; the horizontal branch reaches as far as the 

 posterior border of the heart. 



The Left Coronary Artery passes outwards and to the left, between 

 the pulmonary artery and the left auricular appendix. Reaching the 

 auriculo-ventricular furrow at this point, it divides into a vertical and a 

 horizontal branch. The former descends in the left ventricular furrow ; 

 the latter turns backwards along the auriculo-ventricular furrow. The 

 terminal portion of the vertical branch turns round the anterior border 

 of the heart, and ends in twigs that enter the ventricular wall on the 



