SKELETON. 



659 



toes (perforati muscles) are stretched automatically when the ankle is 

 bent in perching. In some birds, an ambiens muscle, inserted on the 

 front of the pubis, is continued down the anterior side of the femur, 

 and its tendon, bending round the knee to the opposite side of the tibia, 

 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 

 bending the first toe, and its importance has been exaggerated. The 

 bending of the toes is mainly due to the perforati muscles. 



In connection with the muscular system, it may also be noted that 

 the walls- of the gizzard consist of thick muscles radiating around 

 tendinous discs. Two small sterno-tracheal muscles ascend from 

 sternum to trachea, and are apt to be confused in dissection with the 

 carotid arteries. Complex muscles are associated with the song-box. 



Fr. 



} .0c. 



FIG. 363. Disarticulation of bird's skull. After Gadow. 

 Membrane bones shaded. 



B.Oc., basioccipital ; .Oc. t exoccipital ; S.Oc., supraoccipital ; 

 Pa., parietal ; Fr. t frontal; JVa., nasal ; /;., premaxilla ; Af. t 

 maxilla ; /., jugal ; Qj. t quadrato-jugal ; Qu. t quadrate ; pe.> 

 periotic ; S#., squamosal ; AS., alisphenoid ; .S. t basi- 

 sphenoid ; O.S., orbito -sphenoid ; Pr.Sph., presphenoid ; v0., 

 vomer ; tOS., interorbital septum; A'., ethmoid; Se., nasal 

 septum; Z>e. t dentary ; Sp., splenial ; An., angular; Sa., sur- 

 angular ; Ar., articular ; MK. t Meckel's cartilage. 



Skeleton. The skeleton of birds is lightly built, with 

 much strength and surface for its weight, on the hollow 

 girder principle. The texture of the bone is often very 

 spongy, and air-sacs from the lungs may be continued into 

 many of the bones, which are then more or less completely 

 destitute of marrow in adult life. In the pigeon, most of 

 the bones, except those of the tail, forearm, hand, and 

 hind-limb, contain air-spaces. Another general character is 

 the marked tendency to fusion of bones, as seen in the skull, 

 dorsal vertebrae, sacral vertebrae, ploughshare bone, carpo- 

 metacarpus, and tarso-metatarsus. 



The vertebral column is divided into five regions cer- 



