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. 
Fic. 363.—Disarticulation of bird’s skull.—After Gadow. 
Membrane bones shaded. 
B.Oc., basioccipital; £.Oc., exoccipital; S.Oc., supraoccipital ; 
Pa., parietal; F., frontal; Wa., nasal; f., premaxilla; JZ, 
maxilla; /z., jugal; Q7., quadrato-jugal; Qu., quadrate ; Ze., 
periotic; Sg., squamosal; AS., alisphenoid; B.S., basi- 
sphenoid; O.S., orbito-sphenoid; Px.5f%., presphenoid ; zo., 
vomer; zOS., interorbital septum; £., ethmoid; Se., nasal 
septum; De., dentary; S%., splenial; Az., angular; Sa., sur- 
angular; A~., articular; 47K., 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- 
