6 ZOOLOGY. 



each being separated from the actual cavity by this thin 

 peritoneum, as though they had been pushed in from 

 without, the gut having made its way in much farther. 



7. Section of Thorax. In the section across the 

 thorax (fig. 2, B) we find a very similar general arrangement, 

 but the alimentary canal (in this region called oesophagus) 

 is a straight tube, and the greater part of the ccelom is oc- 

 cupied by the large lungs and heart. On tracing the lining 

 membrane of the body-wall, here called pleura, we find 

 that its median reflected portion extends right down to the 

 ventral side, so that the right and left lungs lie in separate 

 pleural chambers. The heart is also surrounded by a similar 

 but very narrow space, the pericardial chamber, lying be- 

 tween the outer and inner folds of a similar membrane, the 

 pericardium * or perieardial membrane. 



8. Neck and Head. A transverse section through the 

 neck (fig. 2, A), shows much the same arrangement in the 

 dorsal part ; but ventrally there is no coclom oesophagus 

 and trachea are seen embedded in a solid mass of muscle and 

 other tissues. This suppression of the ccelom in all parts 

 of the body anterior to the heart is one of the most notice- 

 able characters of the ccelom of ordinary vertebrates, and 

 affords a striking contrast to the condition in many in- 

 vertebrates, such as the earthworm, where the coelom 

 extends to the front end of the head. 



A section through the head would also show an absence 

 of ccelom, but here the dorsal region no longer shows 

 vertebrae with spinal cord inside and muscles outside, but 

 the more complex skull and brain and comparatively little 

 muscle. 



9. The Diaphragm. The cavities of the thorax and 

 abdomen are separated by a partition, the diaphragm 

 (fig. 1). This is another of the structures distinctive of 

 mammals, and we shall see later that it is essentially 

 connected with the perfection of air-breathing to which 

 this group has attained. 



* The use of this term is rather ambiguous it is sometimes 

 applied to the membrane, sometimes to the cavity. 



