THE TRACHEA. 137 



in the human subject, in the interval behind; where a third of 

 the ring is defective, its place being supplied by a condensed cel- 

 lular substance; but in the bullock it is there also. In the small- 

 er ramifications of the bronchia, where a complete circle- is 

 formed by the juxta-position of the several little cartilaginous 

 segments, it is probable that this tissue contributes to the whole 

 periphery of the ramification. 



This ligamentous structure possesses great elasticity, which 

 is manifested by the rapid shortening of the trachea, when its 

 two extremities are stretched apart and then suddenly let loose. 

 And it is the continuance of this quality of elasticity, in the mi- 

 nute ramifications of the bronchia, which proves the existence of 

 this tissue there, even when it cannot be very distinctly seen. 



The Muscular Structure of the trachea exists at the cartilagi- 

 lious deficiency in its posterior third, and consists in a thin mus- 

 cular plane whose fibres pass transversely between the interrupt- 

 ed extremities of its cartilaginous rings. These transverse 

 fibres begin at the first ring, and exist all the way down: they 

 arise from the internal faces of the rings, and the intermediate 

 elastic ligamentous tissue; about a line beyond their extremities. 

 Anteriorly, they are covered by the lining membrane of the 

 trachea, and posteriorly, by the dense cellular tissue just spoken 

 of. 



A similar arrangement of muscular structure occurs in the 

 bronchia and for some distance into the lungs. Where the car- 

 tilages become scattered and irregular, the muscular fibres are 

 said, by J. F. Meckel, and by M. Reisseissen,* to perform the 

 whole circuit of the bronchial ramification, and to be visible 

 even beyond the existence of the cartilaginous pieces. Scem- 

 mering expresses a doubt of this arrangement.! It is very dif- 



De Fabrica Pulmonis. Berlin, 1822. M. Laennecsays (Traite, de 1'Ausculta- 

 tion, Paris, 1826, vol. ii. p. 18Q,) that he has sought in vain lo verify these obser- 

 vations of Reisseissen, but that the manifest existence of circular fibres upon 

 branches of a middling size and the phenomena of many kinds of asthma, induce 

 him to view, as a thing well established, the temporary occlusion of the small 

 bronchial ramifications, by a spasmodic contraction of their parietes. 



t Extimse autem vel poslicre ejus fibrse per long-itudinem, a carlilagine cricoi- 

 dea ad pulmones usque descendant ac, vel in ipsis tracheae ramis haud parum 

 conspicui sunt. De Corp. Hum. Fabrica. 



13* 



