THE TRACHEA. 967 



from being thicker in the middle than at the margins. Two or more of the carti- 

 lages often unite, partially or completely, and are sometimes bifurcated at their 

 extremities. They are highly elastic, but 

 sometimes become calcified in advanced 

 life. In the right bronchus the carti- 

 lages 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 connected by fibrous mem- 

 brane with the lower border of the cricoid cartilage, with which or with the suc- 

 ceeding 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 down- 

 ward and backward between the two bronchi. It terminates on each side in an 

 imperfect ring which encloses the commencement of the bronchi. The cartilage 

 above the last is somewhat broader than the rest at its centre. 



The Fibrous Membrane. The cartilages are enclosed in an elastic fibrous 

 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 connects the rings one with another. They are thus, as it were, 

 imbedded 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 the most external, and consist merely of a few scattered 

 Longitudinal bundles of fibres. 



The transverse fibres (Trachealis muscle, Todd and Bowman), the most internal, 

 form a thin layer, which extends transversely between the ends of the cartilages 

 and the intervals between them at 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 areolar and lymphoid 

 tissue, and presents a well-marked basement-membrane, supporting a layer of 

 columnar, ciliated epithelium, between the deeper ends of which are smaller tri- 

 angular cells, the bases of which, often branched, are attached to the basement- 

 membrane. These triangular cells are mucus-secreting, and may be seen as goblet- 

 or chalice-cells when their contents have been discharged. In the deepest part of 

 the mucous membrane, and especially between the mucous and submucous layers, 

 longitudinally arranged fibres are very abundant and form a distinct layer. 



* / 



The Tracheal glands are found in great abundance at the posterior part of the 

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

 columnar 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 muscular and inner fibrous layers, and pass through the 

 submucous and mucous layers to open on the surface of the mucous membrane. 

 Some glands of smaller size are also found at the sides of the trachea, between the 

 layers of fibrous tissue connecting the rings, and others immediately beneath the 

 mucous coat. The secretion from these glands serves to lubricate the inner surface 

 of the trachea. 



Vessels and Nerves. The trachea is supplied with blood by the inferior 

 thyroid arteries. The veins terminate in the thyroid venous plexus. The nerves 

 are derived from the pneurnogastric and its recurrent branches and from the 

 sympathetic. 



