166 ELEMENTS OF ORNITHOLOGY, 
kinds of birds, sending out branches, bifurcating or suffering 
interruption, as the case may be. Fig. 147 (1). 
The ventral pteryla extends from the throat to the root 
of the tail, and may vary in disposition, as already stated, with 
respect to the dorsal pteryla. Fig. 146 (8). 
The cranial pteryla, or tract of the head, more or less 
extensively invests the head and becomes continuous with the 
dorsal pteryla. (4). 
The caudal pteryla, or tail-tract, comprises both the dorsal 
and ventral surface of the tail, wherein the rectrices and upper 
and lower coverts are inserted. (6). 
The humeral pteryle, or shoulder-tracts, are constant but 
small. One crosses obliquely backwards and inwards over the 
shoulder from the upper part of the upper arm. Fig. 147 (2). 
The alar pteryle, or wing-tracts, run each along the solid 
part of one of the wings from the upper arm to the end of the 
pinion. (5). 
The femoral pteryle, or thigh-tracts, form two oblique bands, 
one outside either side. They are very variable. Fig. 147 (3). 
The crural pteryle, or leg-tracts, cover those portions of 
the pelvic limb below the knee which have feathers inserted 
into them. (7). 
Downy feathers generally clothe the body (partially or en- 
tirely) independently of the pteryle, and beneath the other 
feathers distinguished as contour feathers. But down may be 
absent altogether, as in the Woodpeckers. 
