286 CHAPTERS ON THE NATURAL HISTORY 



and first we will pluck them both carefully, coniuienciug at the 

 base of the mandibles in each case, and extending it down as far 

 as the root of the neck. 



It will at once be observed that in these two woodpeckers the 

 head-tract or capital pteryla (P. capitis) is very different. In 

 Harris's woodpecker there is a median naked space on top, for 

 the most part overlying the elevation caused by the epibranchials 

 of the hyoidean arches, which Mtzsch says occurs in all the 

 woodpeckers he ever examined, but this authority never in- 

 spected a specimen of our Sphyrapictts, and in this bird the head 

 is completely covered with feathers (Figs. 70 and 71), and no such 

 median naked space is to be found. Another apterium, also de- 

 scribed by Mtzsch, occurs on the sides of the head in the tem- 

 poral space. I find this in Dryobates and Colaptes, and it may be 

 present in Sphyrapicus. Moreover, as Sphyrapicus has a hyoidean 

 apparatus very much as we find it in the majority of birds where 

 the epibranchials are not curled over the top of the skull as 

 shown in Fig. 69 at c, the skin is not elevated along that region. 

 In some birds, of course, as the condors, the head is destitute of 

 feathers. The head-tract, as a rule, however, includes the head 

 and the lateral tracts of the neck, merging below into the ventral 

 and spinal tracts (Figs. 68 and 69). We may next completely fin- 

 ish the plucking of our two specimens, closely observing the posi- 

 tion of the feathers as we deliberately remove them. Now it will 

 be found that with some few birds, very few comparatively, a 

 strip of feathers of uniform width runs down the entire length of 

 the back, but as a rule the greatest amount of variation exists in 

 this particular. In the case of the two woodpeckers before us a 

 marked difference is again seen, for this spinal-tract in Harris's 

 woodpecker commences above as a narrow, median, longitudinal 

 strip, which dilates at the middle of the back as a bifurcated 

 " saddle-tract " (Fig. 69, g and h) ; then occurs an interruption 

 when a rump division of the spinal-tract commences and extends 

 down over the tufted oil-gland (j), while that part which is car- 

 ried over the caudal region (the true tail of a bird) is designated 

 as the CAUDAL-TRACT. Turning to Sphyrapicus we note that there 

 is no interruption in the spinal-tract, and that the " saddle por- 

 tion " is a lozenge-shaped area, as shown in Fig! 71. This is par- 

 ticularly interesting when taken in connection with the condi- 

 tion of the hyoid in this bird, for the arrangement is quite similar 

 to the spinal-tract as it is found in passerine birds generally. 



