383 



CHAPTER XV. 

 DISSECTION OF THE PIGEON. Columba livia. 



BIRDS are in many respects the most highly differentiated 

 vertebrates. While agreeing in their essential morphological 

 characters with the other air-breathing vertebrates, and more 

 especially with reptiles, they present a number of very special 

 characters, chiefly connected with the power of flight. 



The modifications in the skeleton have been noticed in the 

 preceding chapter : in the other systems the most noteworthy 

 features are (1) the development of feathers, which are very 

 highly specialised products of the epidermis ; (2) the great size 

 of the pectoral muscles, by which flight is effected ; (3) the pro- 

 longation of the bronchi through the lungs into spacious thin- 

 walled air- sacs, lying between the viscera, and prolonged into 

 several of the bones ; (4) the persistence of the fourth right 

 aortic arch, instead of as in mammals the fourth left, to form 

 the arch of the aorta ; (5) the lateral, instead of dorsal, posi- 

 tion of the optic lobes of the brain ; and (6) in the female, the 

 disappearance, during development, of the ovary and oviduct 

 of the right side. 



Physiologically, the most interesting points are : the high 

 temperature of the blood, which exceeds that of mammals, 

 varying between 100 and 110 F. in different birds ; and the 

 modifications in the mechanism of respiration, expiration 

 being the active movement, and inspiration almost purely 

 passive. 



Birds are oviparous, and the eggs, which are of large size, 

 are protected by a calcareous shell. The eggs are fertilised in 

 the upper part of the oviduct before the shell is formed, and 



