MUSCULAR SYSTEM. 507 



comes cornified, and forms the whole feather. The pro- 

 cess by which this cylinder of cells becomes horny is re- 

 markable ; in the upper part ridges are formed which separate 

 from one another as a set of barbs, the lower part re- 

 mains intact as the quill. When we pull the horny sheath 

 off a young feather, we disclose a set of barbs lying almost 

 parallel with one another, but slightly divergent. The 

 central one predominates as the rachis, and its neighbours 

 gradually become its lateral barbs. As for the external 

 sheath, it falls off, while the core of dermis is wholly nutritive, 

 and disappears as the feather ceases to grow. 



On the toes and on the base of the legs little epidermic 

 scales occur. The toes are clawed, and in some birds the 

 same is true of the thumb and first finger. Only in the 

 embryos of the hoatzin (Opisthocomus) and of the ostriches 

 {Struthio and Rhea) is the second finger clawed. The beak 

 is covered by a horny sheath, which is annually moulted in 

 the pufKn. The dermis is very thin and vascular, and is 

 rich in tactile nerve endings or Paccinian corpuscles, which 

 are especially abundant in the cere. The only skin-gland 

 — the preen-gland — secretes an oily fluid, with which the 

 bird anoints its feathers. It is absent in the ostrich, emu, 

 cassowary, and kiwi, and in a few Carinate birds. 



Muscular System. — The largest pectoral muscle arises 

 from the sternum and its keel, and from the clavicle ; it is 

 inserted on the humerus and depresses the wing. It is 

 called the pectoralis major. The smaller but longer muscle 

 — the pectoralis minor — exposed when the large one is 

 reflected, elevates the wing. It arises from the keel and 

 sides of the sternum, and is continued over the shoulder 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 continued upwards in flexor muscles 

 over the metatarsal joint to the tibia, and are flexed auto- 

 matically when the leg is bent during perching. Further- 

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



