34 PTERYLOGRAPHY. 



II. THE LATERAL SPACE of THE TRUNK (Aptcrium triinci luteralc}. 



This is of constant occurrence, and chiefly assists in producing the incontinuous plumage. 



Although it is the largest of all the spaces, and is only covered by the folded wings, it is not 



visible during flight, because the contour-feathers of the lateral branch of the inferior tract lie 



over it, or, if this is but small, the lower accessory wings assist in concealing it. It is smallest in 



Palamedea, the plumage of which has no vacant space except in the axillary region ; usually, 



however, it is very large. Occupying the whole side of the trunk, it becomes broader as the 



spinal tract is narrower; it surrounds the greater part of the wing and thigh, and is continued 



from the shoulders between the spinal and inferior tracts, which bound it, to a greater or less 



distance backwards, often even as far as the tail. Its anterior and dorsal part, situated between 



the spinal and inferior tracts, passes into the lateral neck-space ; the portion placed beneath the 



axilla, on the contrary, passes into the inferior wing-space. It also extends itself over the 



thigh, the spaces of which I am inclined to regard as parts of the lateral space of the trunk, 



because they stand off very little from the trunk. Thus, this space includes the femoral space. 



When the external lateral branch of the inferior tract occurs this space surrounds it, as the sea 



flows round a peninsula, and in fact separates it from the main stem of the inferior space. The 



purpose of this space seems not to be only the facilitation of the movements of the wing, but 



likewise to serve for the reception of the folded wing, in such a manner that it may be supported 



upon the feathers of the branch of the ventral tract. Hence, whenever the wings do not fit 



themselves very closely to the body in repose, or do not rest upon the feathers of this branch, the 



lateral space is very small in proportion to the size of the wings. The opening between the outer 



branch and the main body of the inferior tract appears, however, to favour the movement of the 



leg, and also, as we have already seen, to assist in incubation. 



III. THE INFERIOR SPACE (Apt. mcsoyastrai) . 



This runs as a simple median space in the middle line of the ventral surface, between 

 the two stems of the inferior tract, and commences at various points in the gular region. In a 

 few birds only, e.g. the Herons (Plate VIII), it originates near the chin ; in general it makes its 

 first appearance upon the anterior half of the neck, and runs along the breast and belly to the 

 anus. Hence, it may be divided into three parts. The gular portion is wanting almost 

 entirely in most Palmipedes and in Alcedo ispida, as, indeed, generally in all birds with a con- 

 tinuous neck- plumage. The pectoral portion, like the ventral, differs chiefly in breadth, and in 

 this respect stands in an inverse ratio to the inferior tract the broader the latter the narrower the 

 inferior space, and vice versa. I find it broadest in many aerial birds, such as the Actipitrirue 

 (Plate II), Passerines (Plate III), and MacrocMres (Plate III) ; in most Cuculinee (Plate IV), 

 the Picince (Plate V), and in Vpupa (Plate VI, fig. 3). It is particularly narrow in the G alii nee 

 (Plate VII) and many aquatic birds, as, for example, the Fulicaria (Plate VIII), Limicola; 

 (Plate IX), Longipennes (Plate IX), and Nasulce (Plate X) ; but in all these the ventral part 

 gradually becomes broader. I found it to be narrowest in the Stcganopodes, TJnyuirostrcs, and 

 Pygopodes (Plate X). Of its peculiar form in Otis (Plate VIII, fig, 1), in which there is neither 



