892 ORGANS OF RESPIRATION. 



ticularly strong and numerous opposite the bifurcation of the trachea. The 

 elastic longitudinal fibres serve to restore the windpipe to its ordinary size 

 after it has been stretched in the direction of its length. 



The glands. The trachea is provided with very numerous mucous gland?, 

 the constant secretion from which serves to lubricate its internal surface. 

 The largest of these glands are small roundish lenticular bodies, situated at 

 the back part of the tube, lying close upon the outer surface of the fibrous 

 layer, or occupying little recesses formed between its meshes : these are 

 compound glands ; their excretory ducts pass forwards between the muscular 

 fibres and open on the mucous membrane, where multitudes of minute 

 orifices are perceptible. Other similar but smaller glands are found upon 

 and within the fibrous membrane between the cartilaginous rings. Lastly, 

 there appear to be still smaller glands lying close beneath the mucous 

 coat. 



The mucous membrane. This is smooth and of a pale pinkish white 

 colour in health, though when congested or inflamed, it becomes in- 

 tensely purple or crimson. It is covered with a ciliated columnar epithelium, 

 the vibratile movements of which, as may be best seen at the back of the 

 trachea of an animal, tend to drive the mucous secretion upwards towards 

 the larynx. The epithelium is stratified, oval nucleated cells being disposed 

 several rows deep, beneath the columnar cells which bear the cilia. 



Vessels and Nerves. The arteries of the trachea are principally derived 

 from the inferior thyroid. The larger branches run for some distance longi- 

 tudinally, and then form a superficial plexus with rounded meshes. The 

 veins enter the adjacent plexuses of the thyroid veins. The nerves come 

 from the trunk and recurrent branches of the pneumo-gastric, and from the 

 sympathetic system. 



Structure of the Bronchi. 



The general structure of the bronchi corresponds with that of the trachea 

 in every particular. Their cartilaginous rings, which resemble those of the 

 trachea in being imperfect behind, are, however, shorter and narrower. The 

 number of rings in the right bronchus varies from six to eight, whilst in the 

 left the number is from nine to twelve. 



The bronchi are supplied by the bronchial arteries and veins, and the 

 nerves are from the same source as those of the trachea. 



THE LUNGS AND PLEURA. 



The lungs, placed one on the right and the other on the left of the heart 

 and large vessels, occupy by far the larger part of the cavity of the chest, 

 and during life are always in accurate contact with the internal surface of 

 its wall. Each lung is attached at a comparatively small part of its inner 

 or median surface by a part named the root, and by a thin membranous fold 

 which is continued downwards from it. In other directions the lung is free 

 and its surface is closely covered by a serous membrane, belonging to itself 

 and to the corresponding side of the thorax, and named accordingly, the 

 right or left pleura. 



THE PLEUBJE. 



The pleura are two independent serous membranes forming two shut sacs, 

 quite distinct from each other, which line the right and left sides of the 

 thoracic cavity, form by their approximation in the middle line the medias- 



