1180 



THE ORGANS OF VOICE AND RESPIRATION 



tube is completed by fibrous membrane. The cartilages are placed horizontally above each 

 other, separated by narrow intervals bridged by fibroelastic tissue. They measure about \ of 

 an inch in depth, and ^ of an inch in thickness, tapering at their posterior ends. Their outer 

 surfaces are flattened, but internally they are convex, from being thicker in the middle than at 

 the margins. Two or more of the cartilages often unite, partially or completely, and are some- 

 times bifurcated at their extremities. They are highly elastic, but sometimes become calcified 

 in advanced life. In the right bronchus the cartilages vary in number from six to eight; in the 

 left, from nine to twelve. They are shorter and narrower than those of the trachea. The 

 peculiar cartilages are the first and the last. 



The first cartilage is broader than the rest, and sometimes divided at one end; it is con- 

 nected by fibrous membrane with the lower border of the cricoid cartilage, with which or with 

 the succeeding cartilage it is sometimes blended. 



The last cartilage is thick and broad in the middle, in consequence of its lower border being 

 prolonged into a triangular hook-shaped process which curves downward and backward between 

 the two bronchi. It terminates on each side in an imperfect ring which encloses the com- 

 mencement of the bronchi. The cartilage above the last is often somewhat broader than the 

 rest at its centre. 



Dorsal view. Ventral view. 



FIGS. 894 and 895. Radiographs of trachea and bronchi filled with fusible metal. (After ,T. A. Blake.) 



The Fibrous Membrane. The cartilages are enclosed in a fibroelastic membrane which 

 forms a double layer, one layer, the thicker of the two, passing over the outer surface of the 

 ring, the other over the inner surface; at the upper and lower margins of the cartilages these two 

 layers blend together to form a single membrane, which intervenes between the rings. They 

 are thus, as it were, embedded in the membrane. In the space behind, between the extremities 

 of the rings, the membrane forms a single distinct layer. 



The muscular fibres are disposed in two layers, longitudinal and transverse. 



The longitudinal fibres are external, and consist merely of a few scattered longitudinal bundles 

 of fibres. 



The transverse fibres, the Trachealis muscle of Todd and Bowman, form a thin internal 

 layer which extends transversely between the ends of the cartilages and the intervals between 

 them, thus extending the entire length of the posterior part of the trachea. The muscular fibres 

 are of the unstriped variety. 



The mucous membrane is continuous above with that of the larynx, and below with that of 

 the bronchi. Microscopically, it consists of stratified ciliated epithelial cells, among which a 

 number of goblet cells are seen; the basal cells are often branched and rest upon the basement 

 membrane, beneath which is a layer of fibroelastic tissue containing diffuse lymphoid tissue. 

 The tunica propria blends with the next coat, the submucosa, and here are found the cartilage 

 rings and a number of mucous glands, the tracheal glands. 



The tracheal glands (ylandulae tracheales) are found in great abundance at the posterior part 

 of the trachea. They are racemose glands, and consist of a basement membrane lined by col- 

 umnar mucus-secreting cells. They are situated at the back of the trachea, outside the 'layer 

 of muscular tissue, between it and the outer fibrous layer. Their excretory ducts pierce 'the . 



