THE HORSE. 



149 



but it is paler than it, and not near so sensi- 

 tive. Its arterial ramifications, also less 

 abundant than upon the glottis, exhale a 

 vapor from its surface ; independently of 

 which, it is kept continually lubricated by 

 mucus, furnished from its numerous lacunce, 

 to defend it from anything acriminous that 

 may be contained in the breath. 



Bronchial Tubes. — The trachea having 

 entered the thorax, bifurcates into the two 

 hroncliial tubes. Of them, the right is the 

 more capacious canal, on account of having 

 communication with the larger division of 

 the lungs ; the left the longer one, in conse- 

 quence 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-lilce or angular pro- 

 jection extending down between the bron- 

 chial tubes, filling up that space which 

 would otherwise be left open from the di- 

 vergent manner in which they branch off: 

 it is quite loosely attached, in order that 

 the branches may accommodate themselves 

 to the motions of the neighboring 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 segments 

 of the circle, overlapping one another, and 

 united together and invested by an elastic 

 cellular substance : they also differ in hav- 

 ing no muscular band, another fact connected 

 with the physiology of that part. The bron- 

 chial tubes, in penetrating the substance of 

 the lungs, subdivide — the right into tln-ee 

 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. Li the larger 

 branches, we may dissect out five and even 

 six segments of cartilage, held together by 

 a thin but dense and elastic cellular sub- 

 stance ; 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 may be 



traced upon the continuation of the lining 

 membrane, which in these intimate parts 

 compose the entire parietes of the tube. 

 In the larger branches this membrane 

 (which is continuous throughout the bron- 

 chial system) assumes a plicated disposi- 

 sition — ^apparently, to admit the more 

 readily of expansion. 



THYROID GLANDS. 



Two egg-shaped, apparently glandular 

 bodies, attached just below the larynx to 

 the sides of the trachea, and united in fi-ont 

 of that tube by an intervening portion of 

 the same substance, which, by way of dis- 

 tinction, 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 tex- 

 ture, when cut into, which I am at present 

 ignorant of the precise nature of. They 

 are well supplied with blood-vessels, and 

 have many small nerves going to them. 

 Their physiology still remains obscm^e. 



OF THE LUNGS AND PLEURA. 



The lungs are the essential organs of res- 

 piration : the pleura is but the membrane 

 by which they are invested. 



PLEURA. 



The pleura is a fine, semi-transparent 

 membrane, lining the cavity of the chest, 

 and giving a covering to the lungs. By 

 that portion of it which is called the medi- 

 astimun, the cavity is divided into the right 

 and left sides of the thorax. 



General Conformation. — If the lungs be 

 exposed, by breaking off one or two of the 

 ribs, we shall perceive that their surface, as 

 well as that of the cavity itself, is every- 

 where smooth, polished, and humid. This 

 is owing to the extensive investment of the 

 pleura, the surface of which is now pre- 

 sented ; so that, in reality, without break- 

 ing the surface, nothing but pleura can be 

 touched ; although, from its extreme tenuity 

 and pellucidity, the viscera appear, on a 

 superficial view, to present their own bare 



