70 INTRODUCTIOX. 



the appearance of a very thin transparent membranous tube, 

 divided internally by transverse dissepiments. At each of 

 these dissepiments the tube separates on pulling it gently, and 

 each portion so obtained presents the appearance of an inverted 

 funnel, the prolonged extremity of which, being continued into 

 that of the next above it, an internal tube is produced, which 

 occupies the centre of the membrane. This membrane is, in 

 ordinary language, teniied the pith^ from its resemblance, if not 

 in nature, at least in position, to the pith of a plant. It might, 

 wdth more propriety, be named the internal membrane of the 

 tube. The tube is invested externally with a sort of close 

 sheath, consisting of several layers of condensed cellular mem- 

 brane. With regard to the texture of the tube itself, it would 

 seem to be composed internally, and in its greatest thickness, 

 of a uniform horny substance, which, in many species, however, 

 shews longitudinal fibres, while the outer part, though not to a 

 great depth from the surface, is composed of transverse or an- 

 nular fibres. Hence the reason w^hy, in making a pen, the slit 

 is always cleanest when the outer layer has been scraped oiF. 

 The longitudinal fibres are distinctly seen in the quills of the 

 domestic cock, and of Gallinaceous birds in general. The tube 

 terminates above, that is, distally with respect to the body of 

 the bird in the shaft. 



2. The shaft or stem^ Fig. 17, ^, is a continuation of the 

 tube, but considerably altered in form. It is generally as fol- 

 lows : From being of equal diameter with the tube, it gradu- 

 ally diminishes, so as to terminate in a point. Considered in 

 respect to its length, it is more or less curved, the outer, upper, 

 or anterior part, or back, as it may be called, being convex, the 

 inner, under, or posterior part, or face, concave. The back is 

 more or less convex, but generally in a small degree, considered 

 in its transverse section. The face is formed of two convex 

 surfaces, separated by a groove which runs along its whole 

 length, or of two inclined planes meeting at an obtuse angle. 

 The two sides are more or less plane, and gradually approxi- 

 mating, as is equally the case with the back and face, from the 

 base toward the tip, where all four meet, and so terminate in 

 a point. Internally the shaft consists of a soft, compact, elas- 



