444 MANUAL OF ELEMENTARY ZOOLOGY 



small intestine is beset with numerous minute processes or 

 villi, by which its surface is increased. At the junction of 

 the small and large intestine is placed a very large tube, 

 the blind gut or ccecum, marked by a spiral constriction and 

 ending blindly in a small, finger-like vermiform appendix. 

 The sacculus rotundus opens into the caecum about an 

 inch from the end opposite to the vermiform appendix; 

 the large intestine leaves it at the same end. Two regions 

 may be recognised in the large intestine. The colon is a 

 sacculated tube about a foot and a half in length; the 

 rectum is a narrower tube about two and a half feet in 

 length, in which faecal pellets can be seen. 



The spleen is a narrow, crescentic, dark-red body lying 

 close against the convex side of the stomach. 

 Glands 98 The thymus is a soft, pink mass in the 



mediastinal space at the front of the thorax. 

 The thyroid is a thin, red body consisting of two lobes, 

 one at each side of the larynx, joined by a band across the 

 ventral side of the latter. Suprarenals lie near the aorta. 

 The chest or thorax is a closed box whose side walls 

 are formed by the ribs with the muscles 

 5"san8. t0ry between them and its hinder wall by the dia- 

 phragm, which divides the main or pleuroperi- 

 toneal coelom, parting two pleural cavities in front from a 

 peritoneal cavity behind (p. 421). The windpipe comprises, 

 besides the larynx, a long tube with rings of cartilage in its 

 wall. This is the trachea or windpipe proper, which leads 

 back along the neck and in the thorax divides into two 

 bronchi which join the lungs. In these the bronchi 

 break up into numerous bronchioles, which end in minute 

 air sacs. The cavity of the thorax is enlarged from back to 

 breast by an outward movement of the ribs and from head 

 to tail by the movement of the diaphragm, which at rest is 

 convex towards the chest, but when it contracts flattens, 

 thus increasing the size of the thorax. Since the pleural 

 cavities are closed, their enlargement tends to set up a 

 vacuum within them, and thus the lungs, which are not 

 closed, expand to keep them full, drawing in air : through the 

 glottis. The air is driven out by the collapse of the chest 



1 That is, the air enters by its own pressure and expands the lungs 

 when the pressure around them in the pleural cavity is lowered. 



