58 Vertebrata. 



birds of most powerful flight, but the bones of very 

 young birds contain marrow. 



32. Muscles and Viscera. The muscle or flesh 

 of birds consists of very close fibres, and the sinews 

 or tendons are often converted into bone. There is 

 an enormous muscle on the front of the breast, the 

 great pectoral, whose action is to depress the wing ; 

 beneath this is a smaller, or second pectoral muscle 

 with oblique fibres, arranged like the barbs of a feather, 

 and converging to a tendon which, winding round a 

 pulley at the top of the coracoid, is inserted into the 

 top of the humerus and raises the wing ; this is the 

 second pectoral. In the legs of many birds there is 

 to be found superficially on the front of the thigh a 

 slender little muscle, which, starting from the front of 

 the pelvis, passes down the upper or front surface of 

 the thigh, winds round to the back of the knee and 

 runs by a tendon into the superficial flexor (or bender) 

 muscle, for the longest toe (plantaris) ; a second 

 muscle (the peroneus), from the outside of the leg can 

 generally be traced into the same toe-muscle. These 

 muscles are supposed to be of importance in the 

 action of perching, and as their tendons pass over 

 several joints they probably have a complex action. 



The digestive system of birds consists of the fol- 

 lowing parts : first, the bill or prehensile organ, vary- 

 ing in shape and texture according to the nature of, 

 and mode of obtaining, the food upon which the bird 

 subsists ; secondly, the tongue, rarely soft, usually 

 hard and horny, often barbed ; thirdly, the long food- 

 passage, or oesophagus, which, above the furculum, 

 usually dilates into a crop (fig. 24, #), below which is 



