SKELETON. 633 



parallel with one another, yet slightly divergent. The central pair 

 predominate, and fuse to form the rachis ; their neighbours gradually 

 become the lateral barbs. The external sheath falls off ; the core of 

 dermis is wholly nutritive, and disappears as the feather ceases to grow. 

 On the four toes and on the base of the legs there are horny epidermic 

 scales, the presence of which reminds us of the affinities between Birds 

 and Reptiles. The toes are clawed, the thumb is often clawed ; the 

 second digit very rarely. Only in the embryos of the hoatzin 

 (Opisthocoimis) and of the ostriches (Stnithio and Rhea] is the third digit 

 clawed. The beak is covered by a horny sheath, which is annually 

 moulted in the puffin. The dermis is very thin and vascular, and 

 is rich in tactile nerve-endings or Pacinian corpuscles, especially 

 abundant in the cere. The only skin gland the preen gland secretes 

 an oily fluid, which some birds use in preening their feathers. It is 

 absent in the ostrich, emu, cassowary, and in a few Carinate birds. 



Muscular system.- -The largest breast muscle (pectoralis 

 major) arises from the sternum and its keel, and from the 

 clavicle, is inserted on the ventral surface of the humerus, 

 and depresses the wing. The smaller but longer pectoralis 

 minor or subclavian, exposed when the large one is reflected, 

 raises the wing. It arises from the keel and sides of the 

 sternum, and is continued over the shoulder (through the 

 foramen triosseum, which serves as a pulley) to its insertion 

 on the dorsal surface of the humerus. Arising chiefly from 

 the coracoid, but in part from the sternum, and inserted on 

 the humerus, is a small coraco-brachialis, which helps a little 

 in raising the wing. There are several yet smaller muscles. 



Interesting also is the mechanism of perching. When the bird sits 

 on its perch, the toes clasp this tightly. The flexor tendons of the 

 toes are stretched automatically when the leg is bent in perching. 

 Furthermore, an ambiens muscle, inserted on the front of the pubis, 

 is continued down the anterior side of the femur, and its tendon, bend- 

 ing round the knee to the opposite side of the tibia, is inferiorly con- 

 nected with the flexors of two digits. When the leg is bent in sitting, 

 the ambiens tendon is stretched, and the digits clasp the branch. Thus 

 the bird, when asleep, does not fall off its perch. It is only in some 

 birds, however, that the ambiens muscle is present. 



In connection with the muscular system, it may also be noted that 

 the walls of the gizzard consist of thick muscles radiating around 

 tendinous discs. Two small sterno-tracheal muscles ascend from 

 sternum to trachea, and are apt to be confused in dissection with the 

 carotid arteries. Complex muscles are associated with the vocal chords 

 in the song- box. 



Skeleton. The skeleton of birds is lightly built, with 

 much strength and surface for its weight, on the hollow 



