THE BIRD ITSELF 
In tlie matter of the senses it is probable that birds do not 
possess the sense of smell in a high degree : their vision is 
acute (the coloured part of the eye is known as the iris; pi. 
irtdcs) as is also their sense of hearing^. 
The blood of birds is hotter than that of any other class 
of animals. The respiratory system also offers peculiar fea- 
tures — the organ of voice or syrinx is situated well down in 
the body cavity at the junction of the trachea and bronchial 
tubes. In breathing the air is drawn quite through the Inngrs 
into a series of membranous air-sacs distributed about the 
body. A bird in fact can be inflated by pumping^ air into these 
sacs, and the air spaces extend in many cases into the bones. 
These air sacs no doubt act as a reservoir of air on which the 
bird can draw when flying and it is also supposed that they 
regulate the temperature of the body. 
Considerable misunderstanding usually prevails with re- 
gard to the various parts of the bird's dig'estive tract and words 
like ^^crop" are indiscriminately used. The crop is a thin- 
walled bag at the lower end of the gullet or cesophagus in 
which the food is temporarily stored and softened. From this 
it passes into the stomach proper which consists of two parts, 
the first being- soft and glandular (provcHtncnlus) and the 
second muscular and thick walled, (gizzard). In the gizjsard 
the food is ground up, a work assisted by the small pieces of 
stones and grit which the bird deliberately swallows. Many 
modifications of the digestive tract are found in birds and it 
has been demonstrated that the actual manner in which the 
intestine is coiled provides an important clue to the classifica- 
tion of birds which in many respects is a vexed question. 
No modern bird has teeth. Unless there is a sexual 
difference in the plumage of a species it is usual ly impossible 
to tdl the sex of a bird 7ifithout dissection. Normally there 
are no external organs of generation and even in the 
case of male and female having distinct plumages care 
must be exercised as instances are known in which old 
females assume the male plumage and furthermore in many 
of these cases all the young males wear the adult female 
plumage. 
[21] 
