ß38 ^'^- HANS GADOW, 



the rings. In any case the tracheal rings will give way at their 

 weakest point. 



The first of these two assumptions seems to be the less acceptable, 

 because of the following reasons. In birds the cartilaginous rings begin, 

 according to Wunderlich (Sparrow and Duck) on the lateral sides 

 of the trachea, according to Rathke on the ventral side, and grow 

 towards the median line; they always meet latest on the dorsal side, 

 and it is here that some of them, when at all, remain open; they 

 agreeing in this respect with Mammalian couditions. The ossification 

 of the rings begins always on the ventral side and proceeds dorsal- 

 wards. 



The males of certain Ducks e. g. lletopiana, possess a globular 

 but flattened swelling at the beginuing of the last third of the trachea ; 

 this swellings is composed of about 15 broadened and widened rings 

 which ossify and almost completely fuse with eachother into an 

 osseous bulla, but on the dorsal side a longitudinal space of from 

 2 — 3 mm in width remains, where the cartilaginous rings remain un- 

 ossified. 



In various Passerine birds, which like many Cotinginae, e. g. Chas- 

 morhynchus^ and Tracheophonae, e. g. Grallaria and Hylactes, have a 

 very loud voice, and which cause their tracheae to swell up con- 

 siderably, many of the rings in the vibrating membranes are extremely 

 thin and often deficient. They have all the appearance of having 

 been stretched out and ultimately rent asunder. 



Dromaeus affords the only instance amongst all birds known 

 of a ventral deficiency in otherwise well developed cartilaginous 

 rings. The bulging out could not well proceed towards the dorsal 

 side, because there the trachea lies upon the Oesophagus and the 

 muscles of the neck, whilst on the ventral side only the skin offers 

 any additional resistance. 



The tracheal pouch may therefore be looked upon as a ventral 

 hernia of the trachea, acquired by the birds themselves, and inherited 

 by their offspring to at least a preliminary extent. It has obvious, 

 although distant analogies in the laryngeal and pharyngeal sacs of 

 various Mammals, and how these can shape, and exert a lasting in- 

 fluence upon neighbouring skeletal structures, we shall be able to see 

 in the next chapter. 



1) I have found recently that J. Bland Sütton has also described 

 and figured the „tracheal cyst" of Dromaeus, and regards it as a hernia. 

 On the origin of certain cysts, in: J. Anat. andPhys., vol. 20 (1886), p. 432 f. 



