26 LABORATORY MANUAL FOR VERTEBRATE ANATOMY 



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

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

 with the meatus constitutes the external ear. 



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

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

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

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

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

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

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

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

 biped mode of walking used by birds. 



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

 other vertebrates, should be studied on the plucked bird. In the folded con< 

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

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

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

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

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

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

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

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

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

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

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

 liumcrils. The primaries differ from the others in that the soft part of the feath 

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

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

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



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

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

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

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

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

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

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

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

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

 through a secondary modification connected with the perching habit, poin 

 posteriorly. 



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

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



