CHAP, viii.] ORGANS OF RESPIRATION AND SECRETION. 223 



category, or the essential respiratory organs, are those which place 

 the blood in intimate relation with the inspired air. To the first 

 category helong the windpipe, and the grosser structure of the 

 lungs, with the muscles and other parts which aid in drawing air 

 inwards and in again expelling it. To the second category belong 

 the minute cells which form the ultimate and essential parts of the 

 lungs the " air-cells " or " alveoli." 



In the cat there is but one real set of respiratory organs ; a 

 certain respiratory action can indeed be also effected by the skin, 

 but its amount is so minute it may be practically disregarded. 

 Water ordinarily contains atmospheric air mixed up with it, but 

 the cat has no organs by which the air thus contained can be 

 respired, either by the external skin or by the lungs : for if water be 

 introduced into the latter its introduction causes death, as also does 

 the continued immersion of the whole skin in water. Nevertheless, 

 as we shall see, the actual respiratory surface the inner surface of 

 the lungs is always moist, and bathed with a thin watery film. 



3. The tube by which air is introduced into the cat's body is 

 the windpipe, or trachea, which is relatively very capacious, and 

 opens anteriorly in the back of the floor of the mouth, while pos- 

 teriorly it divides into two branches, each of which is called a 

 bronchus, and penetrates into one of the two lungs. At its front end 

 the trachea expands into a membranous and cartilaginous box-like 

 structure called the larynx, and it is the larynx which opens into 

 the mouth behind the root of the tongue. The trachea passes 

 downwards and backwards down the neck and along the thorax on 

 the ventral side of the oesophagus. It remains permanently hollow, 

 like an artery, its cavity being kept open by means of a series of 

 incomplete cartilaginous rings which, surround the windpipe in 

 front and at the sides, but do not extend into its dorsal wall (ad- 

 joining the oesophagus), which is soft. The ventral surface of its 

 hinder half is in contact with this thymus gland. 



Its lower end lies above the sternum and the arch of the aorta. 

 Above the pulmonary artery, the trachea divides into the two 

 bronchi. The right bronchus is short, and passes horizontally into 

 the root of the right lung. The left bronchus is somewhat the 

 longer, and passes to the left lung behind the arch of the aorta. 

 Within the lung the bronchi divide and subdivide, like the branches, 

 branchlets, and twigs of a tree. 



The cartilages of the trachea are forty -five in number, and are 

 held in juxtaposition by fibrous membrane which embeds them. 

 The highest cartilage is connected with and underlies the larynx. 

 The soft layer which completes the trachea above, where the carti- 

 laginous rings are incomplete, contains organic muscular fibres, 

 internal to which is a stratum of elastic fibres which extend thence 

 all round within the cartilages. More internally the tube is lined 

 with mucous membrane, covered on its surface by a columnar and 

 ciliated epithelium. 



The bronchi have the same structure as the trachea, except that 



