136 ANIMAL PHYSIOLOGY. 



tudinal arrangement of elastic fibrous tissue, enabling them 

 to elongate when required by the stretching of the neck or 

 the expansion of the lungs; also transverse muscular fibres, 

 confined, in the trachea and bronchi, to the back part where 

 the cartilages are deficient, but circular in the other tubes, 

 and capable by their contraction of modifying the freedom 

 of entrance of air into the lungs, as is strikingly exemplified 

 in asthma, which consists of spasms of these muscles to such 

 extent as to produce difficulty of breathing. The mucous 

 membrane is furnished with mucous glands, and lined with 

 ciliated columnar epithelium. But the bronchial tubes of 

 smallest size, reduced to -^ inch in diameter, have homo- 

 geneous walls with a simple squamous lining, and each of 

 these terminates in an ultimate lobule or infundibulum, con- 

 sisting, as has been already said, of an irregular passage, 

 surrounded with air cells. These air cells or locules are cup- 

 shaped depressions, consisting of a framework of fine elastic 

 tissue, and one of the closest capillary networks in the body, 

 in which circulates the blood sent to the lungs by the pul- 

 monary artery; and in order to secure the greatest possible 

 contact of the blood with the air, the septa between the 

 locules present only one layer of capillaries exposed on both 

 sides, and protected only by a very delicate epithelium. 



Fig. 75. CAST OF INFUNDIBULA Fig. 76. AIR CELLS, with capil- 

 a T 5 ; from Henle. lary blood-vessels injected, \. 



The ultimate lobules are united in groups to form larger 

 lobules, from a quarter to half an inch in diameter, very 

 closely united one to another, but with outlines which, in 

 those of the surface of the lung, can be easily distinguished, 



