OF THE UNITED STATES 289 



Birds also exhibit a great variation in their spinal tracts, and J 

 trust some of the readers of this chapter will demonstrate this 

 to their own satisfaction. 



Still referring to Figure 69, we note that the humeral region of 

 each arm is obliquely crossed by a narrow feather strip (e), 

 which is known as the humeral-tract (the single long bone of the 

 arm is the humerus), while in our woodpeckers we see a character 

 quite peculiar to them, being present in both of our specimens, 

 and shown at /, a little separate tract, called the inner humeral- 

 tract. Nitzsch found only one woodpecker that lacked this char- 

 acteristic feature, a Sumatran type (P. luridus). Such feather- 

 ing as a Ming may show, aside from the flight feathers, is col- 

 lectively spoken of as the alar tract (Fig. 69, q). 



Over the region of the thigh there is a tract known as the 

 femoral tract (Fig. 69, o), which is but very faintly indicated in 

 woodpeckers, not noticeable at all in most specimens and spe- 

 cies. 



Then across the leg there is a well defined strip which we call 

 the crural tract, shown in Fig. (i!) at i, and is even more con- 

 spicuously seen in Sphyrapicus (Fig. 71). 



We may also see from these figures that the apteria, or naked 

 spaces, are sparsely covered over in some places by small, downy 

 feathers of an elementary character. 



Turning next to Figures 69 B and 70, we are enabled to study 

 the feather tracts as they are found to occur upon the ventral 

 aspect of the body — and here again we note that no little differ- 

 ence obtains between our two specimens of woodpeckers. 



Most often the ventral tract is a single broad one, extending 

 from the region of the shoulder in front down to the vent, being 

 situated about half way between the middle line of the body and 

 the side, and not so well marked for its lower moiety as it is 

 above (Fig. 69 B, m and n). In these woodpeckers, however, the 

 upper part of this ventral-tract bifurcates, giving rise to an ex- 

 ternal tract seen at m and of quite a different form in Dryobates 

 and Sphyrapicus. 



A circlet of feathers usually surrounds the vent, and in Har- 

 ris's Woodpecker there extends down over the anterior aspect of 

 the coccygeal region, from this point, on either side, a narrow 

 feather tract, which, taken in connection with the median line 

 over the same part, we might collectively name the postventral 

 tract, in the absence of any other designation known to me for 

 it. 



