PLEURA—KIDNEY. 53 
How do the bronchi enter these lobules ? 
They may enter from any side and after entering they break up into 
irregular tubular cavities (the air passages). The walls of these 
cavities pouch out and form the air vesicles or alveoli. 
Describe the structure of the wall of an air vesicle. 
In foetal life each vesicle or alveolus is lined with cuboidal or 
roundish-shaped cells which, when breathing is begun, become flattened 
so as to cover the wall of the expanded air vesicle. In the adult lung 
the walls of the alveoli are composed of a wide-meshed net of elastic 
fibres, the meshes of which are filled with a homogeneous structure 
containing a few oval nuclei. Upon this layer are seen cells of two 
kinds, viz.: some that are round and granular, and larger, irregular- 
shaped nucleated cells that are very thin and transparent. 
Describe the blood supply of the lung. 
Blood is carried to the lungs by the pulmonary and bronchial 
arteries. That from the bronchial artery being for the nourishment of 
the lung tissue. The pulmonary artery divides and subdivides until it 
forms a delicate capillary network which lies between the walls of the 
air vesicles. From this network the blood is returned by the pulmo- 
nary veins. 
PLEURA. 
What is the structure of the pulmonary pleura ? 
It is composed of a layer of dense fibrous tissue well supplied with 
blood and lymphatic vessels. Here and there are seen in it little 
nodules of lymphoid tissue. This is the “deep” layer. The super- 
ficial layer is “endothelial” in character. 
THE URINARY ORGANS. 
KIDNEY. 
What is the general structure of the kidney ? 
In the cut section of a rabbit’s kidney a low magnifying power will 
show that the outer part is covered with a dense connective-tissue coat 
called the capsule. Within this is a broad zone of kidney tissue called 
the cortex. The remaining or inner portion of the kidney is called the 
medullary portion or the Malpighian pyramid, as it terminates in a 
blunt projection in the pelvis of the kidney. This projection is called 
the papilla and forms the apex of the Malpighian pyramid, the base of 
