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 right side of the heart 

 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. Tbeir 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 infandibula (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 desquarnation. 



4. Generative Organs. The histology of the organs of 

 reproduction is quite elaborate, and the plan of this work 



