NO. 2 LUNGS OF THE ALLIGATOR — REESE 9 



mass of cells, and doubtless represent the anlagen of the carti- 

 laginous rings, as noted above. The lung cavities, I 1 — f , have about 

 the same appearance as described in connection with figure 22, 

 though the epithelium is, perhaps, somewhat thinner, and the 

 crescents are not quite so marked. No attempt has been made to 

 show these details with the low power used in this figure. 



Figure 27 represents a ventral view of the body of an alligator 

 of about 15 cm. length, dissected to show the respiratory organs 

 and the neighboring structures. 



The trachea, t, is seen lying against the ventral wall of the large 

 cesophagus, cc; its numerous cartilaginous rings are easily seen. At 

 the anterior end of the trunchus, tr, the trachea disappears beneath 

 (dorsad to) the thyroid gland, tg, and its division into the two 

 bronchi cannot be seen in this figure. The lungs, I, are elongated 

 bodies, lying on each side of and mostly anterior to the heart. Their 

 medial borders are covered by the auricles, au, and the thymus 

 glands, ty, while the posterior end of each lies beneath (dorsad to) 

 the corresponding lobe of the liver, li. The alveolar appearance of 

 the lungs is easily seen with the naked eye. A thymus gland, ty, 

 is seen on either side of the posterior region of the trachea ; it con- 

 sists of a lobulated mass posteriorly and of an anteriorly directed 

 cylindrical portion extending forwards into the neck. The heart 

 and liver need not be described here. In the cut surface of the neck 

 may be seen the spinal cord, sp, and the notochord, n, surrounded 

 by the now cartilaginous vertebral column. The yolk or umbilical 

 stalk, u, is shown, somewhat diagrammatically, just caudad to the 

 opened body cavity. 



Figure 28 represents, in outline, the respiratory organs of an 

 alligator of about 75 cm. length ; this animal was probably two years 

 old and its lungs should have reached approximately their adult 

 condition. 



Extending from the glottis, gl, at the base of the tongue, to, is 

 the fairly wide trachea, t; between the lungs it' divides symmetrically 

 into the two bronchi, b, which enter their respective lungs a little 

 cephalad to the middle region of these organs. The pulmonary 

 veins, pv, are shown at the posterior edge of the bronchi ; the 

 corresponding arteries are not shown in this figure. 



The internal structure of the adult lung has been described by 

 Miller (3) and others and need not be noted here. 



The thyroid gland, tg, is shown against the trachea just cephalad 

 to the origin of the bronchi. 



