26 LABORATORY MANUAL FOR VERTEBRATE ANATOMY 



such an extent that the tympanic membrane is no longer visible. The skin 

 around the entrance to the meatus tends to elevate as a fold; this fold together 

 with the meatus constitutes the external ear. 



3. Trunk. The trunk is very firm and inflexible, owing to a fusion of the 

 bones of the back. Pass your fingers along the back of the plucked bird and 

 feel the skeleton. Feel also in the median ventral line of the trunk the pro- 

 jecting keel of the breastbone, to which are attached the great wing muscles. 

 It is the presence of these muscles, the " breast" of the bird, which produces the 

 plump contour of the trunk. The trunk bears the two pairs of limbs, of which 

 the anterior pair is remarkably modified into wings or organs of flight. The 

 hind limbs have also undergone considerable modification as a result of the 

 biped mode of walking used by birds. 



The parts of the wing, which are homologous to those of the fore limb of 

 other vertebrates, should be studied on the plucked bird. In the folded condi- 

 tion the sections of the wing make angles with each other like the letter Z. 

 The upper arm is short, directed posteriorly, and slightly twisted on its axis so 

 as to bring the preaxial margin on the dorsal side. The forearm is longer and 

 directed forward. The wrist and hand are fused together, and the whole is con- 

 siderably elongated and directed caudad. There are but three digits, which 

 are regarded as the second, third, and fourth. The second digit is the projection 

 found just below the joint between forearm and wrist; the third digit forms the 

 terminal point of the wing; the fourth cannot be seen externally. When the 

 wing is extended, its parts have nearly the primitive position described under 

 Necturus. The great quills of the wings are known as remiges: those of the 

 hand are called primaries; of the forearm, secondaries; and of the upper arm, 

 humerals. The primaries differ from the others in that the soft part of the feather 

 is wider on the posterior side of the central axis than on the anterior side. The 

 remiges are borne on the postaxial margin of the wing, and the deep large feather 

 follicles exposed by their removal should be noted on the plucked bird. 



The hind limb is partially clothed with feathers and partially with horny 

 scales, identical with those found in reptiles. The digits, of which there are but 

 four the fifth being absent terminate in claws. The position of the hind 

 limb with reference to the body is greatly altered. The whole limb, instead of 

 extending straight out laterally from the body, is directed ventrally, thus raising 

 the animal high above the ground. In order to achieve this result it is obvious 

 that the limb must have been rotated 90 from the primitive position so that the 

 original dorsal surface now faces anteriorly that is, has become preaxial. The 

 toes are consequently directed forward, with the exception of the first which, 

 through a secondary modification connected with the perching habit, points 

 posteriorly. 



4. Tail. The tail stump bears a half-circle of large quills, known as rectrices. 

 Under the base of the tail is the anus or cloacal aperture, a transverse opening 



