434 ZOOLOGY FOR MEDICAL STUDENTS chap. 



Archaeopteryx shows us that birds originally possessed long tails like 

 those of lizards. 



In the skull a conspicuous feature is the large rounded cranium, 

 enclosing the brain and continued forwards into an elongated face region, 

 its individual bones completely fused together in the adult. The face 

 region is connected with the cranium dorsally by a thin flexible layer of 

 bone, while on each side it is continued back as a chain of slender bones 

 (maxilla, jugal, quadrato-jugal) to the ventral end of the movable 

 quadrate. The bony palate or roof of the mouth is prolonged backwards 

 into the elongated palatine and pterygoid bones — the latter at their hind 

 ends also being attached to the ventral end of the quadrate. The special 

 functional meaning of these arrangements is that they allow the upper 

 jaw to be tilted upwards when the mouth is opened, the quadrate swinging 

 forwards, and the palatine and pterygoid sliding forwards along the 

 smooth presphenoid bone of the base of the cranium. 



As might be anticipated from the peculiarly modified limbs, the 

 limb-girdles show characteristic features. The pectoral girdle is based 

 upon the very large sternum which projects ventrally as a prominent 

 flat keel that serves for the origin of the large depressor muscles of the 

 wing. In the running birds (Ostrich, Kiwi), where the wings have 

 become reduced, the keel of the sternum also has disappeared. The 

 girdle itself consists of a strong coracoid attached at its ventral end to 

 the sternum, and a slender scapula or shoulder-blade. What correspond 

 to clavicles meet ventrally and fuse to form the characteristic furcula or 

 " merry-thought " bone. Scapula, coracoid and furcula at their meeting- 

 point bound a rounded foramen through which, as over a pulley, there 

 passes the tendon of the supra-coracoid muscle. This muscle, originating 

 from the sternum and having the end of its tendon attached to the dorsal 

 side of the humerus, plays an important part in raising the wing during 

 .flight. 



In correlation with the very high grade of evolution reached by the 

 birds both in their general structure and in their habits we find that they 

 show a high stage of development of their nervous system. The eyes 

 are of relatively enormous size and of the greatest complexity of structure. 

 The otocysts are also highly developed and the lagena forms a curved 

 cochlea devoted to the very acute sense of hearing. The cerebellum, 

 in correlation with the complexity of movement, is large and complicated. 

 The optic lobes are of great size — in association with the large size of 

 the eyes. The hemispheres are large but the pallium — the portion 

 which reaches its highest development in the mammals — is thin and 

 comparatively inconspicuous. 



