Chap. V.] GENERAL HISTOLOGY. 81 



to resist inflation, and to expel the contained air during 

 expiration. 



In birds the minute lung structure is different. There are no 

 infundibula and no air-cells, though the finest tubuli are 

 minutely sacculated. These minute tubuli which form a close 

 plexus in the lung are placed in communication with each 

 other by minute perforations in their walls. It will be remem- 

 bered (p. 49) that the lungs are in connection with a system of 

 large air-sacs. 



In the frog the lung is a hollow structure, pitted on its 

 inner surface with sacculations, formed by the infolding of the 

 walls. These sacculations are more numerous in the anterior 

 part of the lung. In their walls is the capillary plexus. Their 

 edges are often covered with ciliated cells, but the cavities of 

 the sacculations are lined with flattened epithelium. 



14. The Kidney. The kidney of the rabbit is hollowed out 

 on one side, its inner edge forming a depression or hilus. Here 

 the ureter enters and enlarges into a funnel-like dilatation, the 

 pelvis, which ends in a number of smaller dilatations, the calyces. 

 Into the pelvis there projects a conical mass of kidney sub- 

 stance, the urinary pyramid. The substance of the kidney is 

 divisible into an outer portion, the cortex, which has a dotted 

 appearance, and an inner portion, the medulla, which is radially 

 striated. 



In a section of a kidney in which the blood-vessels have been 

 injected, arterioles are seen running horizontally in the inter- 

 mediate region between cortex and medulla. They send up verti- 

 cal branches into the cortical layer, which in turn branch, each 

 branch ending in a convoluted tuft of blood-vessels called a 

 glomerulus (Fig. 29, v.) Each glomerulus is (as shown diagram- 

 matically to the right of the figure) enclosed within a capsule 

 (lined with flattened epithelial plates), which forms the dilated 

 end of a urinary tubule. The blood passes into the glomerulus 

 by an afferent vessel, and out by an efferent vessel. It then 

 breaks up into a delicate capillary plexus within the cortex 



