200 VERTEBRATE ZOOLOGY 



principal muscles of flight have their origin in the sternum, the 

 size of these muscles and consequently the powers of flight are 

 directly correlated with the size of the sternum. Birds that do 

 not fly, as the ostrich, have a very small sternum, on which there 

 is often no keel. Projecting from the lateral edge of the sternum 

 on each side, near its middle, is the prominent middle xiphoid 

 process. Behind this is a deep notch, and behind that the small 

 external xiphoid process, which fuses posteriorly with the body of 

 the sternum, inclosing a small opening. In front of the middle 

 xiphoid processes are the articular surfaces of the ribs, and at the 

 anterior end of the sternum those of the coracoid bones. 



Exercise 43. Draw a lateral view of the thorax, showing the thoracic 



vertebrae, the ribs, and the sternum. 

 Exercise 44. Draw a ventral view of the sternum. 



The Skull. The skull is made up of two portions, the cranium 

 and the visceral skeleton. The former protects the brain and the 

 organs of special sense; the latter forms the framework of the 

 face, the beak, and the tongue. 



The cranium, which forms the entire top and back part of 

 the skull, is characterized by the great lightness of the bones 

 composing it and their tendency to become ankylosed. They are 

 not compact but spongy ; the air cells in them can be seen through 

 the outer wall by holding the cranium up to the light. In an old 

 bird ankylosis has gone on so far that the sutures marking the 

 boundaries of the bones have largely disappeared; in a young 

 bird, however, they are still present. 



The visceral skeleton, which forms the ventral part of the skull, 

 is also characterized by the lightness and delicacy of its bones 

 and the absence of teeth. It is much less firmly joined with the 

 cranium than in mammals. Care must be taken to preserve the 

 lower jaw and the hyoid apparatus. 



Study the posterior surface of the skull. The large circular 

 foramen magnum is seen here, through which the spinal cord joins 

 the brain. Beneath it is the occipital condyle, by which the skull 

 articulates with the atlas; note carefully its shape. The hinder 

 end of the skull is formed in the young bird by four bones : the 



