32 PTERYLOGRAPHY. 



and both the upper and lower tail-coverts (tegmina caudce superiora et inferiora). It also 

 includes the oil-gland, to which we shall refer more particularly hereafter. It likewise receives 

 the posterior extremities of the spinal tract, the two truhcal bands of the ventral tract, and the 

 extreme portion of the lumbar tract. 



Lastly, if we are to take into consideration certain very small specially-feathered spots, we 

 might admit an anal tract (Pteryla ani) and an oil-gland-tract (Pteryla glandules uropygii olcosa;], 

 The former surrounds the anal orifice in the form of a ring of contour-feathers, and occurs most 

 distinctly in the Passerines, being particularly well shown in Parus caudatus. The latter clothes 

 the apex of the above-mentioned gland in the circumference of its orifices, and consists not 

 unfrequently of umbellated down-feathers, but usually of delicate little feathers, which occupy a 

 middle place between contour-feathers and down, but are never exposed to the light. 



CHAPTER VI. 

 OF THE SPACES. 



As the spaces and tracts mutually form each other, the form of the spaces has been already 

 pretty well indicated during the description of the tracts ; indeed, in describing the latter, some 

 notice of the former could hardly be avoided. Hence I have only to indicate briefly here what 

 was passed over in the description of the tracts, or could not well be mentioned. 



We have already seen (p. 16) that the spaces are portions of the surface of the body which 

 are not clothed with contour-feathers, but are covered by the contour-feathers or by the folded 

 wings. I think it necessary to distinguish them from the truly naked portions of the skin, 

 which are not even covered in this manner, but remain always freely visible, and have therefore 

 selected this expression. They may be investigated at the same time as the tracts, and in 

 the same manner; but for their accurate recognition an examination of the bird with the plumage 

 uninjured should never be neglected, because the more perfect the tract the more distinctly does 

 the space make its appearance. 



With regard to their structure, the spaces are sometimes quite naked, and sometimes 

 sparsely or densely clothed with down-feathers ; the latter is especially the case in water birds. 

 I have never hitherto found filoplumes upon them, but perhaps this may occur when the denser 

 plumage has more resemblance to contour-feathers, To such spaces I give the name of false 

 spaces (apteria spurid) ; and I may remark that they are usually less definitely bounded, and 

 therefore add to the difficulty of distinguishing the tracts. Moreover, there are birds in 

 which all the spaces are of the nature just described ; whilst in others they are either naked or 

 partially clothed with down. In no single bird, however, have I found the spaces perfectly 

 naked ; that is to say without any traces of down-feathers throughout their whole extent ; but 

 they are very bare, or almost naked, in a great number, as, for instance, the Passerine, 

 MacrocMres, Woodpeckers, Cuckoos, nocturnal Rapacious birds, and Gallinaceee. More fre- 

 quently the lateral neck-spaces and axillary spaces are perfectly naked, even when down-feathers 



