8o ANIMAL BIOLOGY. [Part I. 



In the case both of the salivary glands and of the pancreas the 

 alveoli are surrounded by delicate bands of connective tissue, 

 and are grouped into lobules, surrounded by stronger bands, 

 while the lobules are further grouped into lobes separated from 

 each other by yet stronger connective tissue continuous with 

 that of the capsule which encloses the whole gland. 



The structure of the liver is very different from that of the 

 salivary glands or pancreas. The substance of the organ is more 

 or less distinctly divided by connective tissue partitions into a 

 a vast number of lobules about -^ inch in diameter. In a 

 section of a liver in which the blood-vessels have been injected 

 (29, iv.), there are seen round the lobule several inter-lobular 

 venules, ultimate branches of the portal system. They send a 

 fine network of capillaries inwards towards the centre of the 

 lobule, where the blood is collected by an intra-lobular venule, one 

 of the ultimate factors of the hepatic vein. In 29, iii. we have a 

 section in which not the blood-vessels but the bile canaliculi 

 have been injected. They form a network between the liver 

 cells, which are thus clearly marked out, and seen to be poly- 

 gonal and nucleated. 



13. The Lung. The trachea of the rabbit is lined with 

 ciliated epithelium. It branches into two bronchi, each of which 

 again branches, rebranches, divides, and subdivides into a vast 

 number of ciliated tubes, the finer and more delicate of which 

 are the bronchioles. Each bronchiole finally branches into wider 

 non-ciliated infundibula, which, with their small branches, form 

 the ultimate termination of this extensive system of tubes. 

 Finally, the walls of each infundibulum are closely beset with 

 minute bags, the air-cells or alveoli, which open into the infun- 

 dibulum. The air-cells are lined with flattened non-ciliated epi- 

 thelial scales, beneath which lies a dense network of pulmonary 

 capillaries. Here it is that the plasma of the blood gives up its 

 carbonic acid gas, and the exhausted red corpuscles receive 

 a fresh supply of oxygen. Each lobule is separated from its 

 neighbours by delicate bands of fibrous connective tissue, by the 

 action of which the infundibula and air-cells have a tendency 



