MUSCULAR SYSTEM. 



657 



a horny sheath, which is annually moulted in the puffin. A moulting 

 of claws occurs in the grouse. The dermis is 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. 



ABB 



FIG. 361. Parts of a feather. After Nitzsch. 



/., Four barbs (B.) bearing anterior barbules (A.BB.) and posterior barbules 

 (P.BB.) ; //., six barbs (It.) in section, showing interlocking of barbules ; 

 ///., anterior barbule with barbicels (.//.). 



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 triosscum, which serves as a pulley) to its insertion 

 on the dorsal surface of the humerus. Arising chiefly from 

 the cpracoid, but in part from the sternum, and inserted 

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

 a little in depressing 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 



