432 ZOOLOGY FOR MEDICAL STUDENTS chap. 



explains how it is that a wounded bird with a broken bone projecting 

 from its body can sometimes go on breathing when an attempt is made 

 to suffocate it by compressing the mouth and nostrils. 



The birds possess a very characteristic type of stomach. The 

 oesophagus is highly glandular and somewhat dilated at its posterior 

 end to form the proventriculus. This opens into the rounded stomach, 

 which is also glandular but in which the secretion is of a quite peculiar 

 kind, in that it spreads over the inner surface and hardens into a tough 

 horny protective coat. The wall of the stomach is very thick and muscular 

 and the organ acts as a gizzard for grinding up the food, the process being 

 aided by stones which the bird swallows, while the inner surface is pro- 

 tected from injury by the horny secretion already mentioned. 



The metanephros of the bird is elongated, closely fitted in between 

 the transverse processes of the sacral vertebrae, and its secretion is 

 noteworthy for the small proportion of water it contains. The excess 

 of water appears in the bird to pass to the exterior mainly as vapour in 

 the expired air from the lungs . 



In the reproductive system the main feature is the asymmetry, the 

 right ovary and oviduct being reduced and non-functional. This is 

 probably correlated with the very great size of the bird's egg and its 

 rigid shell, and the dangers that would arise from two such eggs passing 

 to the exterior synchronously. 



The heart shows complete division of the ventricle, the right ventricle 

 being wrapped as it were round the left as may be seen in a cross-section. 

 The conus undergoes the same division by a primary septum as in the 

 reptile but there is no further subdivision of the systemic cavity. This 

 latter feature is correlated with the fact that in the bird (Fig. 185) the 

 left systemic aortic arch has disappeared, and no doubt this disappearance 

 we may in turn correlate with the increasing perfection of the lung 

 entailing a large and larger proportion of the available blood in the 

 right ventricle being drawn off into the pulmonary artery (cf. Fig. 183). 



The ventral or external carotids (Fig. i85,z'.c)are throughout the greater 

 part of their length reduced to insignificant vestiges, the entire blood- 

 supply to the head passing by way of the dorsal or internal carotids {d.c). 

 These arteries become approximated together immediately ventral to the 

 vertebral column of the neck and in many birds they become completely 

 fused into a single vessel. When this happens a further development 

 may take place through the disappearance of the hinder, unfused portion 

 of the artery either on the right (Grebe, Quails, Woodpeckers, ordinary 

 perching birds) or on the left side (African Bustard) so that the blood 

 for the head region is derived entirely from the left or the right side. 



