THE DOMESTIC PIGEON 373 



The Excretory System. The kidneys (Fig. 195) are dark, 

 three-lobed organs fitting closely into cavities beneath the 

 backbone. The ureters open into the cloaca. 



The Skeletal System. The skull (Fig. 196) is large, with 

 a comparatively large, rounded cranium. The articulation 

 of the cranium with the first vertebra is made by a single 

 condyle, as in the reptiles. The vertebrae of the neck, or 

 cervical region, are free ; those of the thoracic region, pelvic 

 region, and caudal region are more or less united. There 

 are several pairs of ribs. The caudal vertebras are termi- 

 nated by a peculiar bone called, from its shape, the plow- 

 share bone. The shoulder girdle is composed of a pair of 

 narrow scapulas, to which are attached two coracoids, con- 

 necting the scapulas and the sternum, and a V-shaped bone, 

 the wishbone, formed from the union of the two clavicles. 

 A greatly enlarged sternum with a prominent ridge, or keel 

 (Fig. 196), serves as a surface for attachment of the mus- 

 cles of flight. The hip girdle is united to the vertebras of 

 the pelvic region. A study of Fig. 196 and comparison with 

 the skeleton of the frog will make clear in what respects the 

 appendages are different. 



The Nervous and Muscular Systems. The brain is larger 

 in the pigeon, in proportion to the size of the animal, than 

 in the amphibians and reptiles. The cerebrum (Fig. 195) 

 and the cerebellum are especially large. One of the principal 

 functions of the cerebellum is to control the muscles which 

 bring about the balancing of the body. It is apparent that 

 this is of greater importance in the birds and fishes than in 

 the broader-bodied amphibians and reptiles. The optic lobes, 

 pressed to one side by the large cerebrum, are also well 

 marked, in correlation with the unusually large eyes and the 

 dependence of the pigeon on the sense of sight. The olfactory 

 lobes are relatively small, and the sense of smell is not at all 

 keen. The medulla is bent downwards, as in the reptiles. 



