102 
Elementary Manual of Zoology. 
before any of the other organs are interfered with, as they 
collapse when cut into and are then difficult to recognise. 
The air-sacs are numerous and lie in different parts of the 
body cavity and neck ; they are difficult, however, to disseet 
out completely. Fill a, as they are, with warm air, they 
serve to increase the buoyancy of the body in the atmos- 
phere. When uninjured, they can be inflated by means of 
a, blow-pipe inserted in the trachea. 
(2) In the middle line in front you will be able to see the base of 
the trachea where it divides into two branches (bronchi} 
which communicate with the lungs. You can at once re- 
cognise the trachea by the close-set cartilaginous rings by 
which it is supported. Just above the insertion of the 
bronchi is the organ of voice (syrinx). This is clothed 
with muscles which appear like a swelling in the - trachea 
and should be examined carefully, as the manner of their 
insertion into the’ ends of the bronchial semi-rings is a 
character used in classifying the group (Passeres) to 
which the sparrow belongs. 
(8) Immediately below the syrinx, in the middle line, is the large 
heart with its numerous vessels, The heart in birds is 
divided into four chambers, »7z.,a right and left auricle 
and corresponding ventricles. The ventricles occupy the 
thick muscular apex of the heart and the partition be- 
tween them cannot be recognised without further dissee- 
tion. The right and left auricles can be easily dis- 
tinguished as thin-walled vessels filled with blood, situat- 
ed on either side in front of the thick-walled ventricles. 
Opening into the right auricle you will be alle to see the 
base of the large thin-walled vein (right anterior vena 
cava) which returns the blood from the jugular, pectoral 
and brachial veins. Inthe middle line, arising between 
the auricles, you will see the two white thick-walled 
innominate arteries, which the aorta gives off almost 
immediately after leaving the right ventricle. By mov- 
ing the tissues a little, but without further dissection, 
you will be able to trace where the innominate artery on 
either sides breaks up into the carotid artery, which sup- 
plies the side of the head, and ‘the sub-clavian artery, 
which supplies the wing and breast. 
(4) Behind the heart lie the large, dark-coloured lobes of the 
liver. 
At this stage in the dissection the bird should be pinned out 
under water, so as to float the organs that are exposed. Dissect off the 
