68 



THE BRITISH NATURALIST. 



[March 



lungs, as above stated, are retrograde from the Lacertilian type, but 

 in the chelonians and crocodiles each bronchus instead of opening 

 directly into its lung traverses its length, and by lateral apertures 

 communicates with pouches each of which resembles the lung of an 

 ordinary lizard. 



In a few existing reptiles (e.g. Chameleons and some tortoises) we 

 find traces of additional respiratory apparatus in the air-sacs which in 

 some extinct reptiles and in the majority of birds, fossil and recent, attain 

 a much greater development. In most birds these air-sacs lie not only 

 amongst the viscera, but send prolongations into the bones, and even 

 give off sub-cutaneous air-sacs ; the result is not only a greatly increased 

 aerating surface, but a diminution of the specific gravity of the animal, 

 a peculiarity not indeed essential to flight but of great subsidiary 

 importance. The anatomy of the lungs and diaphragm, which latter 

 is present in a more or less complete state in many reptiles, supports 

 and confirms the definition of a bird as a specialization of the reptilian 

 type for flight. 



In the highest great division of the back-boned animals, the milk-givers, 

 who suckle their young and are scientifically designated Mammalia, the 

 lungs attain their highest development ; widely as the members of this 

 great division differ in structure and habits, all breathe by lungs, and if 

 deprived for more than a very short time of oxygen, die. In all, there 

 are two lungs, communicating with the wind pipe by bronchi which divide 

 and sub-divide (diminishing in diameter at every division) into minute 

 tubes or bronchioles. Each bronchiole is a cylindrical tube which in turn 

 divides dichotomously and rapidly several times, terminating in irregular 

 alveolar passages. The air-cells lie in great abundance round these 

 terminal passages and may be regarded as primarily hemispherical in 

 form, though made polygonal by mutual compression. The connective 

 tissue surrounding these air-cells is so disposed that all the cells derived 

 from one bronchiole are more closely knit together than they are bound 

 to those of a neighbouring bronchiole. It will be evident that each 

 bronchiole, with the assemblage of its members, forms a miniature lung, 

 usually called a lobule ; the lobules are compacted together by connective 

 tissue to form lobes, several of which go to make up each lung. The 

 number of the lobes and bronchi varies in different groups of mammals, 

 and is of some value in classification ; occasional variations from the 

 generic type occur, and the number of the lobes of the right and left 

 lungs are usually different. For instance, in man there are two lobes in 

 the left lung and three in the right ; in the baboons, the left lung is two- 

 lobed, the right four-lobed ; in the group to which the hedgehog belongs 

 the right lung is usually four-lobed, the left may possess from one to 

 three lobes. As an instance of the classificatory value of bronchial 

 peculiarities it will suffice to refer to the walrus, in which the right 



