138 ORGANS OF RESPIRATION. 



ficult in such minute structure to arrive at a satisfactory con- 

 clusion; careful observations, latterly made, have, however, in- 

 duced me to adopt the same conviction as Reisseissen. Longi- 

 tudinal muscular fibres are said by Portal to exist between the 

 contiguous margins of the cartilaginous rings, but the fact is far 

 from being ascertained.* 



The use of the muscular tissue has been pointed out, by the late 

 Dr. Physic, as follows: "In expectoration, it diminishes the cali- 

 ber of the air tubes, so that the air having to pass out with increased 

 rapidity through them, its momentum will bring up the inspissated 

 fluid which may be in its way." This very ingenious theory has 

 subsequently been advanced by M. Cruveilheir, of Paris, possibly 

 without a knowledge of his having been anticipated; but cer- 

 tainly not without the claims of the eminent individual to whom 

 we owe it, having been established by its publication,! 



The Mucous Membrane of the trachea lines its whole interior 

 periphery, from the larynx to the bronchia, and is continued, un- 

 der the same circumstances, through the latter to their minute 

 divisions. It adheres very closely to the contiguous structure, and 

 is continued, in the substance of the lungs, beyond the traces of 

 any of the other tissues which compose the trachea; it indeed 

 terminates in the air cells. It is very vascular, like other mu- 

 cous membranes; and also, like them, the venous appears to pre- 

 vail over the arterial vascularity. A successful minute injection 

 makes it look as if it consisted of a tissue of blood vessels; thin 

 and red, it presents an abundance of slightly elevated longitudi- 

 nal folds: one of the latter, conspicuous for its greater size, ex- 

 ists at the commencement of the left bronchus, and is yet more 

 developed in the still-born 'infant. 



The exterior circumference of the mucous membrane is stud- 

 ded with Muciparous Glands, about the size of millet seed. These 

 glands are particularly conspicuous and abundant on the pos- 

 terior part of the trachea and of the bronchia, where the defi- 

 ciency of the cartilaginous rings is supplied by the membranous 

 structure only; and more of them exist at the lower part of the 



* Anat. Med. f Wistar's Anatomy, 3d edition, vol. ii. p. 64. Phil. 182L 



