462 RESPIRATION. 



air sacs are occasionally found to coalesce and open by a 

 single common orifice into the dilated extremity of the 

 bronchial tube. Each pulmonary lobulette receives only one 

 bronchial tube, and its air sacs and cells have no means of 

 communicating with those of other lobulettes, except through 

 this channel. Each air cell has a diameter of from -g^th to 

 rlu-th of an inch in man (Kblliker), -^iirth in the dog, -2-foth 

 in the goat, T ^-g-th in the horse, and y^th in the cow, (Eos- 

 signol.) Mandl states that in castrated animals they are 

 much smaller than in perfect males and females. The walls 

 of the air cells and sacs are similarly constructed. They are 

 composed of a single coat formed of a fibrous and an epithe- 

 lial layer. The former is chiefly composed of elastic fibres 

 which interlace in encircling the walls of the cavities, and 

 support the plexus of pulmonary capillaries. The epithelium 

 presents a single layer of cells devoid of cilia. Connecting 

 these, a very delicate basement membrane has been demon- 

 strated. 



The lobulettes vary from a quarter to a line in size, 

 and are pyriform when on the surface of the lung, but are 

 modified in shape when deeper situated. They are bound 

 together by a thin layer of elastic tissue, so as to form 

 lobules varying from a quarter to an inch in size. These 

 lobules are in turn similarly, though more loosely, connected 

 into groups, and by a still more lax union of such groups, 

 the pulmonary lobes are formed. The lobes result from the 

 breaking up of the lung by intervals, more or less deep, into 

 separate masses, each communicating with a single large 

 bronchus. The horse's lungs have no lobes, unless a slight 

 depression existing on the posterior part of the right lung, 

 and transmitting the vena cava, is to be looked upon as indi- 

 cating such a division. In the cow, sheep, and pig, the left 

 lung is divided into two lobes, and the right into four, the 



