28 BRITISH BIRDS 



in 'L'Oiseau.' Anyone who has the opportunity of dissecting a 

 Hornbill will be struck by the large and abundant air-spaces 

 between the muscles. This applies even to the Ground Hornbill 

 of Abyssinia; and yet the latter, as its name denotes, lives 

 upon the ground, while the flight of other hornbills is heavy and 

 most unsuggestive of lightness of body. These air-spaces are in 

 direct communication w r ith the windpipe. It is much easier to 

 understand their arrangement by the actual dissection of a bird. 

 We must first get a notion of the position and form of the lungs, 

 which differ very much from the lungs of other animals. In 

 a rabbit, for example, or any other mammal, the lungs lie freely on 

 each side of the heart, and are capable of being pushed here and 

 there after the body is opened, and of much expansion and diminu- 

 tion of volume during the movements of respiration. But the lungs 

 of all birds are tightly fixed to the wall of the chest cavity, being, 

 as it were, moulded on to the ribs and vertebrae ; w r hen they are 

 carefully picked away from their place, they retain the impressions 

 of the bones which they touch. There is no great possibility here 

 of independent movements on the part of the lungs. Eespiration 

 is effected in a totally different manner ; it is, in fact, bound up 

 with the mechanical filling of the air-spaces. Each of the two lungs 

 is contained within a large compartment, which is bounded exter- 

 nally by an obliquely disposed septum, often spoken of, on account 

 of its direction, as the ' oblique septum.' Others call it the dia- 

 phragm, imagining that it is the equivalent of the diaphragm in 

 the mammal, that partly fleshy, partly tendinous plate which shuts 

 off the cavity of the chest, in which lie the heart and lungs, from 

 the cavity of the abdomen, in which lie the intestines, stomach, and 

 liver. Now, this oblique septum does not by any means closely in- 

 vest the lungs ; on the contrary, a deep space is thereby shut off, at 

 the bottom of which are the lungs. This cavity is subdivided by 

 two partitions into three separate compartments. It requires a 

 very skilful manipulation to show the fact, but it can, with care, be 

 demonstrated that each of these compartments is lined by a delicate 

 membrane, which is continuous with the lung, and is actually a kind 

 of bubble, as it were, blown out of the lung ; these delicate sacs are 

 the air-sacs. There are altogether nine of them, but all these sacs 

 do not lie within the cavity bounded by the oblique septa. The 

 largest pair of all the abdominal air-sacs project into the body cavity 

 far behind the gizzard. Now these sacs are fairly easy to see in a 

 dissection ; but it is not so easy to make out that they are all of 



