INTRODUCTION. xvii 



It is not my intention here to go much into the 

 internal structure of birds, but a few characteristic points 

 may be noted. It will have been noticed by most 

 observant people that a large part of the body is made 

 up by the " breast," the great pectoral muscles which 

 move the wings. The keel on the centre of the breast- 

 bone affords increased space for the attachment of these. 

 In most non-flying birds, the pectoral muscles having 

 dwindled, this keel is wanting. The merrythought, or 

 wish-bone, corresponds to the collar-bone in man, and 

 varies a great deal in development in birds. When 

 well developed, it indicates good powers of flight. 



With regard to the internal organs, it will be noticed 

 that the lungs and kidneys are closely pressed against 

 the body wall, so much so that they are indented by the 

 adjacent bones. The reproductive organs are also 

 closely attached to the wall of the body, and outside 

 the breeding season are much reduced in size, so that 

 the sex is often not easy to distinguish by dissection. 



The bones of birds are generally hollow to a greater 

 or less extent, and contain air which passes into them 

 from air- sacs situated in different parts of the bodyj 

 which in their turn are connected with the lungs. This 

 circulation of air in the body of birds may perhaps 

 account for their sensitiveness to changes of tempera- 

 ture, which has for ages given them the reputation of 

 weather prophets. The vocal organ of birds is the 

 syrinx, situated at the base of the windpipe ; the 

 tongue does not influence the voice. 



The bodily temperature in birds is very high, equalling 

 fever heat in man. It is doubtful if they ever become 

 torpid like beasts and reptiles, but Humming-birds, 

 when exposed to a temperature too low for them, will 

 become insensible, and will revive when warmed, as 

 also will some young nestling birds, like young Canaries 

 and Pigeons. 



The digestion of birds is very powerful, the seed- 



b 



