AVES. 231 



narrow barbs on each side, flattened at right angles to the axis 

 of the feather. Each barb possesses a proximal and a distal row 

 of minute barbules. These overlap and hold the barbs together, 

 and, for this purpose, the distal barbules are provided with 

 diminutive booklets. A minute aperture, the inferior umbilicus, 

 leads into the proximal end of the quill, and a similar superior 

 umbilicus into its distal end on the ventral side of the feather. 



Ventral side = below in quill -feathers, next body in the others. The shaft 

 is longitudinally grooved on this side. 



In the fowl, but not in the pigeon, a small after-shaft which 

 resembles the vane in structure is attached near the superior 

 umbilicus. 



The Jiloplumes possess a minute thread-like stem, but the barbs 

 and barbules are only represented by a tuft of disconnected 

 processes. 



The oil-gland belongs to the skin. It is made up of numerous 

 branched tubules, epidermic in origin, and lined by glandular-cells. 



The most important histological point is the way of develop- 

 ment of the feathers. A small papilla is formed on the surface, 

 the base of which sinks down into the dermis and thus becomes 

 enclosed in a pit, the feather-follicle, from which its pointed end 

 grows out. The papilla is known as the feather-germ. It con- 

 tains a vascular core of dermis, and the epidermis covering its 

 surface is gradually moulded into the feather. The quill is 

 formed around the base of the germ, the vane around the free 

 part, from which it splits off and expands. The inferior umbilicus 

 is the point where the vascular core entered the feather, the 

 superior umbilicus shows where it left the unsplit quill-part to 

 form the centre round which the vane was once folded. 



This description refers to the first-formed feathers. The papillse of new 

 feathers are formed in connection with the old follicles, and never project 

 on the free surface. 



The feathers are renewed periodically during life, the old ones 

 being cast off at moulting. 



The dermis contains a network of muscle-fibres attached to the 

 feather-follicles. By the contraction of this the feathers can be 

 erected. Blood-vessels, lymphatics, and nerves are present, and 

 a large number of ovoid touch-corpuscles. 



3. The endoskeleton (Figs. 68 and 69). This, as in the frog, 



