THE SPACES. 35 



cervical nor pectoral portion, and the ventral portion consists of three narrow, parallel, insular 

 bands, I have already spoken in describing the ventral tract of that bird (see p. 28). 



The purpose of this space must apparently be sought for in the fact that the neck, when it 

 is retracted, and, in consequence, forms a large projection at its lower extremity, meeting the 

 equally prominent ridge of the breast, would disproportionately increase the amount of the 

 feathery covering if it were present on this spot, and thus, of course, deform the body of the 

 bird, instead of making this projecting part as inconspicuous as possible, by concealing it under 

 the neighbouring plumage. But for this purpose the region of the body in which the above- 

 mentioned projections occur must itself be destitute of feathers. The inferior space may, like- 

 wise, perform an important part during incubation, as I have already indicated. 



IV. THE SPINAL SPACE (Apt. spinale). 



This is produced by a longitudinal division of the spinal tract, which it renders double at 

 those places where it occurs. This space is very often wanting, and when it is present it is 

 short and even insular. It is usually seen only on the back, at those spots where the spinal tract 

 is dilated. Only in Ardea and Cancroma (Plate VIII) it makes its appearance in the place of 

 the otherwise simple cervical portion of the spinal tract, and is bounded by the lateral neck- 

 tracts, which are continued as spinal tracts upon the back. In HUampliastus (Plate V, fig. 13), 

 on the other hand, it occurs only on the trunk, and not upon the neck and nape. The other 

 variations of this space may be easily ascertained from the differences of the spinal tract which 

 have already been described. 



V. THE UPPER WING-SPACE (Apt. alee superius). 



This space extends over a larger or smaller portion of the upper arm and the great wing- 

 membrane, separates the axillary tract from the other feathered parts, and is covered by the 

 axillary feathers which form the tract just mentioned. It is probably never wanting, except in 

 some birds with continuous plumage ; but is sometimes very small and almost abortive, as in many 

 water-birds. It is large in the aerial birds, especially in the Passerines (Plate III) and Picince 

 (Plate V). 



VI. THE LOWER WING-SPACE (Apt. dice inferius). 



This is generally much larger than the preceding space, and extends over the lower part of 

 the upper arm, the great wing-membrane, the axillary wing-membrane, and even the forearm, but 

 is covered anteriorly by the contour-feathers inserted in this margin of the great wing-membrane, 

 which are broadened and lengthened for this very purpose. It is usually a continuation of the lateral 

 space of the trunk, and is directly connected therewith, when not restricted to the great wing-mem- 

 brane alone. In some birds, as, for example, the Accipitrina; (Plate II, fig. 1), and the water-birds, 

 it contains in the middle of its surface a row of contour-feathers, and is more or less concealed by 

 them ; but it also disappears when the plumage of the anterior margin of the great wing-mern- 



