36 PTERYLOGRAPHY. 



brane is disseminated over the surface of the latter. In many other birds, on the contrary, this 

 space is either entirely naked or sparsely or densely clothed with down. I find it to be broadest 

 in the Passerine (Plate III), in many of the Cuculina (Plate IV), in Vpupa (Plate VI, fig. 3), 

 Alcedo (Plate VI, fig. 5), the Picina (Plate V), Fulicaria> (Plate VIII), Erodii (Plate VIII), 

 and some other birds. The accessory wing of the forearm, however, when it occurs together 

 with this space, is attached to it and partly covers it. 



Both the upper and lower wing-spaces are necessary for facilitating the movements, and espe- 

 cially the folding, of the wing. 



VII. THE CRURAL SPACE (Apt. crurale). 



This usually separates the femoral tract from the rest of the plumage of the leg, and appears 

 as a naked ring at the upper end of the tibial region, on the inner surface of which it spreads 

 out, and generally covers it. It is sometimes entirely or nearly naked, sometimes densely 

 clothed with down -feathers. In the water birds it is generally very indistinct, and by no means 

 definitely bounded ; indeed, it is always of subordinate importance in the pterylographic cha- 

 racters of birds. Its purpose is, doubtless, to assist the free movement of the leg, and especially 

 to allow of its being bent backward. 



VIII. THE HEAD-SPACES (Apteria capitis). 



Besides the perfectly naked spots which may be detected upon the heads of many birds , 

 there are in this region several truly featherless spaces, which, however, are concealed by the 

 neighbouring plumage. A featherless space of this kind occurs, for example, in the Wood- 

 peckers in the middle of the vertex, following its longitudinal direction (Plate V, fig. 15), and a 

 second smaller circular one is seen on each side close to this. Beneath the former lie the cornua 

 of the hyoid bone, which are rolled up when in repose. In Vpupa cpops (Plate VI, figs. 3 

 and 4) I have likewise met with both these spaces ; and in the Cockatoos a large round vertical 

 space is present behind the transverse crest. In Coracias, Oriolus, and most, but not all, 

 of the Passerine (Plate III, figs. 2, 3, 7, 9, and 13), I have also remarked a small naked spot 

 near the eye, which might be named the temporal space (apterium temporale] ; but in these the 

 median space is wanting. A proportionately larger space appears at the nape in TrocMlus 

 moschitus (Plate III, fig. 19), and may be indicated as the nuclial space (apt. nuchale). I have 

 also met with this, although in a less perfect form, in Colitis capcnsis (Plate VI, fig. 11). 



To the general considerations which properly close with the description of these various 

 spaces, I shall add a few words upon the remarkable powder-down-tracts and upon the oil- 

 aland of the rump, as I cannot find any better place in which to give a general account of 

 them. Indeed, the former, as tracts, belong more properly to the end of the fifth chapter; 

 but as these tracts do not consist of contour-feathers, but of down, and, moreover, do not occur 

 in all or many birds, I thought it better to exclude them from the description of the feather- 

 tracts. Still less does the description of the anal gland or oil-gland of the rump appear to belong 



