52 THE RESPIRATORY SYSTEM. 
THE RESPIRATORY SYSTEM. 
TRACHEA. 
What is the structure of the trachea? 
It is a tube, the outer portion of which is composed of fibrous tissue. 
At intervals in its course this is rendered firm and stiff by incomplete 
rings of hyaline cartilage, joined at their free ends by smooth 
muscle fibres. The inner coat of the trachea is mucous membrane 
and consists of a layer of cylindrical, ciliated cells resting upon a 
basement membrane, under which is the corium, made up of loose 
connective tissue, containing cells of various shapes. The outer por- 
tion of the mucous membrane is made up of a layer of elastic tissue. 
The sub-mucous coat, which is between the outer fibrous coat and the 
mucous membrane, is composed of loose areolar tissue in which 
numerous mucous glands are imbedded. 
BRONCHI. 
In what respect does the structure of the bronchi differ from that 
of the trachea ? 
The larger bronchi are about the same in structure with the excep- 
tion that between the mucous membrane and the sub-mucous coat 
there is a layer of smooth muscle cells, 
How do the bronchi change in structure as they are followed to 
their ultimate ramifications ? 
The outer connective-tissue coat becomes thinner, the cartilage 
plates become smaller, more infrequent and finally disappear, as also 
do the mucous glands. The other layers grow gradually thinner, and 
finally there is left a single row of cylindrical, ciliated cells upon the 
basement membrane, surrounded by a few muscle cells in the connec- 
tive-tissue layer. In the smallest tubes the wall is made up of a few 
connective-tissue fibres and muscle cells, and lined with cuboidal, cili- 
ated, and lastly, with flattened epithelial, cells. 
LUNG. 
Describe a pulmonary lobule. 
It is pyramidal in shape, and is better seen upon the surface of the 
lung. This surface appears to be divided by narrow lines into irreg- 
ular, polygonal-shaped spaces, which are the bases of these pyramidal- 
shaped pulmonary lobules. Within the lung these lobules are not so 
regular in shape. They are all separated one from another by narrow 
connective-tissue septa, and each lobule is made up of a collection of 
air vesicles and air passages grouped around a terminal bronchus. 
