THE MICROSCOPE IN ANIMAL HISTOLOGY. 213 



lined by mucous membrane, with vascular walls full of 

 folds, and containing, near the bladder, a number of mu- 

 cous glands. 



3. Respiratory Organs. The lungs receive air by the 

 trachea and venous blood from the rio-ht side of the heart 



O 



to transmit to the left side. They may be compared, as 

 to form and development, to racemose glands. The ex- 

 cretory ducts are represented by the bronchial ramifica- 

 tions, and the acini by the air-vesicles. 



The ciliated mucous membrane of the bronchial twigs 

 gradually loses its laminated structure until only a single 

 layer remains. Their muscular layer also ceases before 

 arriving at the air-cells. At the end of the last bronchial 

 tubules we find thin-walled canals called alveolar passages. 

 These are again subdivided and end in peculiar dilatations 

 called primary pulmonary lobules, or infundibula (Plate 

 XXIII, Fig. 172). The air-cells, vesicles, or alveoli, are 

 saccular dilatations in the walls of the primary lobules, 

 opening directly into a common cavity. Their walls con- 

 sist of delicate membrane of connective tissue, often con- 

 taining black pigment, probably from inhalation of car- 

 bonaceous matter, or a deposit of melanin. 



The pulmonary artery subdivides, and follows the rami- 

 fications of the bronchi to the pulmonary vesicles. Here 

 a multitude of capillary tubes form a network over the 

 alveoli, only separated from the air by the most delicate 

 membrane (Plate XXII, Fig. 168). In the frog we find 

 the whole respiratory portion lined with a continuous 

 layer of flattened epithelia. A similar lining is found in 

 the mammalian foetus, but in the adult the number and 

 character of the epithelial scales is greatly changed. Large 

 non-nucleated plates are seen with occasional traces of the 

 original bioplasm. In inflammatory affections, however, 

 these may multiply, giving rise to catarrhal desquamation. 



4. G-enerative Organs. The histology of the organs of 

 reproduction is quite elaborate, and the plan of this work 



