DISSECTION OF THE BACK AND THORAX. 123 



there is reached a tube of comparatively small calibre which belongs 

 exclusively to one lobule, and is therefore termed a lobular or terminal 

 bronchus. The left bronchus has a length of three or four inches before 

 dividing, but the right immediately gives off from its outer side a con- 

 siderable branch (Plate 26). Within each lobule the terminal bronchus 

 ramifies, forming smaller tubes or hronchioles,th.e last and smallest of which 

 lead into recesses or dilatations. Each such dilatation is termed an 

 alveolar passage, and it is bounded by delicate sacculated walls, 

 each sacculation being an infundibulum. The infundibula are them- 

 selves sacculated, the minute recesses of their walls being termed 

 air-cells. The air-cell is thus the ultimate part of the air passages within 

 the lung, and a group of air-cells forms an infundibulum. The wall 

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

 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 segmented 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 lymphatics. 



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 fibrous capsule for the lung. Lymphatic vessels 

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

 pleural, perivascular (around the pulmonary vessels), and peribronchial. 



DISSECTION OP 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 fuiTOw, it divides, one branch descending in that furrow, while the 

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



