306 STRUOTUEE AND CLASSIFICATION OF BIRDS 



middle line posteriorly the dorsal tract is again strongish. 

 The lumbar region is also strongly feathered, and the thigh is 

 covered by a strong tract which ends very abruptly below. 



The ventral tract is much weaker, and on the neck 

 gradually merges into the lateral space which extends from 

 a short way below the head to the shoulder. Below this the 

 ventral tract is still weaker, and does not bifurcate until the 

 upper end of the carina sterni. The median apterion is 

 oblong and narrow, and reaches the cloacal aperture. In 

 the abdominal region the two tracts get stronger. The 

 pterylosis of Columba livia as figured by Nitzsch hardly 



i 1 III II I I M ^1 JI -. J 



Gizzards op, a, PHlopus jainhu, 

 b, Treron calva (after Garkod). 



Fio. 155. — a, Gizzard of Garpophaga 

 latrans. b, One of Hoent Tuber- 

 cles IN Section. (After Gakkod.) 



differs, and is typical of the pigeons in general ; there is no 

 down. 



All pigeons have a well-developed crop, the presence of 

 which organ is presumably related to their fruit- and grain- 

 eating mode of life. 



In Garpophaga, a fruit-eating pigeon, the gizzard is very 

 weak ; but in other pigeons this organ is very strong, its 

 walls being even ossified in Caloenas nicoharica} In Ptilopus 

 a cross section of the gizzard has the peculiar form shown 

 in the accompanying drawing, where it is compared with a 



' Vekeeaux and Des Mubs describe an exaggeration like this in Phosnm'Mna 

 goUath, where also the tubercles are ossified (Bev. Mag. Zodl. 1862, p. 138). 

 See also Flower, ' On the Structure of the Gizzard of the Nicobar Pigeon,' 

 P. Z. 8. 1860, p. 330. 



