HEALTH AND DISEASE 



is highly vascular. The upper part is supplied with the olfactory nerve, 

 which is the sj^ecial nerve of smell, whilst the lower part is supplied with 

 branches of the fifth pair of nerves, which confer upon it a high degree of 

 common sensation. Several objects are gained by the inhaled air passing- 

 over this surface. The impressions received by the sense of smell, and 

 conducted to the brain, serve to place the animal on guard against enemies, 

 whilst they inform it of the proximity of food, water, or other objects of 

 desire. The acuteness of their perception in these respects is well known. 



But it does more. It 

 warms and moistens the 

 air as it is inspired, and 

 thus prevents cold, raw, 

 and dry air from acting 

 directly on the delicate 

 tissue of the lungs. 

 Having reached the back 

 of the nose, the current 

 of air crosses the pharynx 

 and enters the larynx, 

 shown in side view in 

 fig. 198, and looked at 

 from above in fig. 199. 

 The larynx is a cartila- 

 ginous chest or box con- 

 taining the organ of voice. 

 Continuing its course 

 the inspired air travels 

 through the trachea or 

 wind-pipe and the bron- 

 chial tubes, the divisions of which penetrate, and indeed form, a large 

 proportion of the substance of the lungs. In the act of expiration the 

 air passes through the several passages just named in the opposite direc- 

 tion. 



The Lungs. — When full of air the lungs are two voluminous soft 

 elastic organs of pink colour, which occupy with the heart nearly the whole 

 of the thoracic cavity. In old animals they are more or less mottled 

 with black. The right lung is rather larger than the left, and each is 

 covered with a thin sheet of a serous membrane, named the pleiira, which 

 also lines the inner surface of the ribs and walls of the thorax. The 

 opposed surfaces of this membrane are polished, and at all times lubricated 

 with a watery secretion, so that in the acts of respiration the gliding 



Fig 198 -The Larynx 



A, Long Cornu of Os Hyoides. B, Short Comu. c, Thyroid or 



Heel-like Process. D, Spur Process. E, Epiglottis. F, Glottis. 



G, Cricoid Cartilage. H, Thyroid Cartilage. I, First Ring of Trachea. 

 J, Arytsenoid Cartilage. 



