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 
1s 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. 
Fic. 361.— Parts of a feather.—After Nitzsch. 
Z., Four barbs (8.) bearing anterior harbules (4.B8.) and posterior barbules 
(P.BB.); Lf., six barbs (B.) in section, showing interlocking of barbules ; 
fff, 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 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 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 
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