93- THE RESPIRATORY SYSTEM. 
colour is a light slate-blue, but scattered over this there are numerous dark patches 
of various sizes, and also dark intersecting lines. The coloration of the lung 
differs considerably at different periods of life. In early childhood the lung is — 
rosy-pink, and the darker colour and the imottling of the surface which appear 
later are due to the pulmonary substance, and chiefly the interstitial areolar tissue 
becoming impregnated more or less completely with atmospheric dust and minute 
particles of soot. 
At every breath foreign matter of this kind is inhaled, but only a small proportion of it 
reaches the lung tissue. The greater part of it becomes entangled in the slimy mucus which 
coats the mucous membrane of the larger air-passages, and is gradually got rid of along with 
the mucus through the activity of the cilia attached to the lining epithelium. By the constant 
upward sweep of these a current towards the pharynx is established. The fine dust and soot 
particles which reach the finer recesses of the lungs, and ultimately the interstitial issue, is 
partly conveyed away by the lymphatic vessels to the bronchial glands, which in consequence 
become in many cases absolutely black. The colour of the lung, therefore, depends to some 
extent upon the purity of the atmosphere which is inhaled, and it thus happens that in coal- 
miners the surface of the ling may be very nearly uniformly black. 
The feetal lung differs in a marked degree from the lung in an individual who 
has breathed. After respiration is fully established, the lung soon comes to occupy 
almost the whole space allotted to it in the pleural cavity; in the foetus, on the 
other hand, the lung is packed away at the back, and occupies a relatively much 
smaller amount of space in the thoracic cavity. Further, it is firm to the touch, 
and sinks in water. It is only when air and an increased supply of blood are 
introduced into the lung that it assumes the soft spongy and buoyant qualities 
which are characteristic of the adult lung. 
Form of the Lungs.—The lungs are accurately adapted to the walls of the 
pleural chambers in which they are placed, and in the natural state they bear on 
6 7 14 
15 
Pulmonary veins 
Lig. latum 
A B 
Fic. 632.— MEDIASTINAL SURFACES OF THE TWO LUNGS OF A SUBJECT HARDENED BY FORMALIN-INJECTION. 
A, Right lung. B, Left lung. 
1. Base. 7. Groove for vena azygos major. 13. Pulmonary artery (left). 
2. Fissure. 8. [parterial bronchus. 14. Apex pulmonis. 
3. Cardiac depression. 9. Pulmonary artery (right). 15. Groove for left subclavian artery. 
4. Groove for innominate vein. 10. Fissure. 16. Groove for left innominate vein. 
5. Groove for innominate artery. 11. Groove for aorta. 17. Cardiac depression 
6. Apex pulmonis. 12. Bronchus. 18. Fissure. 19. Base. 
the surface impressions and elevations which are an exact counterpart of the 
irregularities on the walls of the cavity in which they lie. 
When care has been taken to harden it im situ, each lung presents an apex 
