ADAPTATIONS OF THE SKELETON 69 



the attachment of the rectrices or tail feathers. It is 

 interesting to remember that no Uving bird has more than 

 a very short tail as far as vertebrje go, the apparent long 

 tails of pheasants and the like being, of course, made up of 

 feathers only. On the other hand, the extinct Archae- 

 opteryx, the earhest bird known, had a long tail like a 

 lizard's, with about twenty vertebrae. 



§ 6. Muscular System 



Birds show a high degree of muscular development. 

 In some pigeons the pectoral muscles weigh half the whole 

 bird. Every one who has picked the neck of a boiled fowl 

 knows how many small muscles there are working this part 

 of the body, a contrast to the scanty musculature of the stiff 

 back-region. In birds that eat hard food the posterior 

 part of the stomach has become a very muscular gizzard. 

 Some of the adaptations of the muscles used in flight will 

 be referred to later, but something may be said here in 

 regard to perching. The toes are bent by perforati muscles 

 which are inserted above the ankle-joint, but while these 

 are contracted voluntarily, as may be watched in a bird of 

 prey, there is also an automatic clinching. As the bird 

 settles down on the tree the leg is bent at an angle ; this 

 stretches the tendons, and thus makes the toes grip the 

 perch. In some birds there is a special perching muscle, 

 the ambiens, inserted on the front of the pubis and con- 

 tinued down the anterior side of the femur. Its tendon, 

 bending round the knee-joint to the opposite side of the 

 tibio-tarsus, is inferiorly connected with the tendon of the 

 flexor of the second or third toe, or with the third and 

 fourth. It has nothing to do with the flexing of the first 

 toe, and its importance has been exaggerated. The bending 

 of the toes is mainly due to the active contraction of the 

 perforati muscles already alluded to, and to the automatic 

 stretching of the tendons as the legs are bent down. 



When we consider the muscles of flight, of running, and 

 of perching, the muscles working the rib-system and raising 



