331 
character. It is to the structure of the denser and thicker parts 
that the following remarks chiefly apply. The free surface is hard, 
generally rough, and always stained of a deep yellow colour. It is 
marked by sulci, usually deep, parallel, and disposed longitudinally 
on the disks, and finer, intersecting, or irregular in the other por- 
tions ; these correspond with depressions in the membrane below, 
but,there are also linear grooves, not extending through the entire 
thickness, variously disposed in different birds, which add to the 
roughness of the surface. On carefully raising this layer, very nu- 
merous minute, delicate, white threads, attached to its under sur- 
face, are seen to be drawn out of the membrane below: these give 
to this surface, when detached, a soft, villous appearance. 
The membrane upon which this epithelial stratum rests is thick, 
tough, and white, and is in contact externally with the muscular sub- 
stance of the gizzard. When examined microscopically, it is found 
to consist of two portions—the deeper one composed of fibrous tissue, 
to the outer surface of which the muscular fibres are attached; the 
other superficial, comprising about three-fourths of the thickness of 
the entire membrane, composed of numerous tubular glands, or fol- 
licles, placed side by side, simple (in all the birds that I have exa- 
mined *), and terminating below in rounded closed extremities ; their 
general shape much resembling that of a chemist’s test tube 
(Pl. CLXV. fig. 3). A fibrous stroma, continuous with the deeper 
layer of tissue, extends between, and supports the tubes. A hori- 
zontal section through this stratum shows very well the mode in which 
the follicles are arranged. In some birds they appear closely packed 
together without any definite plan. In others they are disposed in 
lines or linear groups; this is the prevailing arrangement in the 
passerine birds. In a third series, comprising the fowls, duck, &c., 
they are collected into definite groups, oval, or polygonal in outline, 
and each containing from twelve to twenty-four tubes (Pl. CLXV. 
fig. 1). In these cases a fine fibrous stroma passes between the 
individual tubes, while a coarser intermediate substance separates 
the groups. Each tube is composed of an outer wall, lined by a single 
layer of nucleated cells, and contains within it a cylindrical, nearly 
transparent, solid body, which, being attached by its upper end to 
the cuticular layer, is readily drawn out of the tube, and is, in fact, 
one of the fine filaments mentioned above as visible to the naked 
eye. A closer examination of these cylinders, when pulled out from 
their follicles, shows that they have a fibrous structure, with a cen- 
tral axis of a different nature, rather darker, and sometimes distinetly 
granular ; and that their surface is covered by numerous polygonal 
scales, giving it an imbricated appearance, like that of a young hair. 
On the addition of caustic potash they swell out, become more trans- 
parent, and their structure is rendered distinct (Pl. CLXV. fig. 4). 
The horny layer itself presents in all granivorous birds that I have 
examined this common character : it is composed of numerous pa- 
rallel, rod-like, solid bodies, placed side by side, extending from the 
* Molin describes them as compound in the Parrot. 
