22G RESPIRATORY SYSTEM. 



longer one, in consequence of having- to cross under the pos- 

 terior aorta, in its course to the left division of the lungs. The 

 last cartilage of the main pipe has a spear-like or angular pro- 

 jection extending down between the bronchial tubes, filling up 

 that space which would otherwise be left open from the diver- 

 gent manner in which they branch oft': it is quite loosely at- 

 tached, in order that the branches may accommodate themselves 

 to the motions of the neighbouring parts. The bronchial tubes 

 vary in structure from the trunk that gives origin to them : instead 

 of their rings being formed of entire pieces of cartilage, they are 

 constituted of several separate pieces, making up so many seg- 

 ments of the circle, overlapping one another, and united together 

 and invested by an elastic cellular substance : they also differ in 

 having no muscular band, another fact connected with the physio- 

 logy of that part. The bronchial tubes, in penetrating the sub- 

 stance of the lungs, subdivide — the right into three principal 

 branches, the left into two ; from which spring innumerable others 

 that grow smaller and smaller, until the ramifications become so 

 reduced that they are no longer traceable by the naked eye. In 

 the larger branches we may dissect out five and even six segments 

 of cartilage, held together by a thin, but dense and elastic cellular 

 substance : in the smaller divisions, only two are found, and they 

 are diminished in size ; and in the smallest visible ramifications 

 of all, cartilage is altogether wanting, though, in many places, 

 marks of the rings n)ay be traced upon the continuation of the 

 lining membrane, which in these intimate parts composes the en- 

 tire parietes of the tube. In the larger branches this membrane 

 (whicli is continuous throughout the bronchial system) assumes a 

 plicated disposition — apparently, to admit the more readily of ex- 

 pansion. 



Thyroid Glands, 

 Two egg-shaped, apparently glandular bodies, attached just 

 below the larynx to the sides of the trachea, and united in front 

 of that tube by an intervening poition of the same substance, which, 

 by way of distinction, is by some called the isthmus. They are 

 enveloped and attached in their situation by cellular membrane ; 

 are larger and more vascular in the young than in the old subject ; 

 and exhibit a spongy texture when cut into, which I am at pre- 

 sent ignorant of the precise nature of. They are well supplied 

 with bloodvessels, and have many small nerves going to them. 

 Their physiology still remains obscure. 



OF THE LUNGS AND PLEURA. 



The lungs are the essential organs of respiration : tlie pleura is 

 but the metnbrane by which they are invested. 



