STRUCTURE OF BIRDS. 7I 



tic substance, of a white colour, having much of the mechanical 

 nature of cork, and which may be named the internal suberose 

 substance of the shaft. It is separated longitudinally by a line 

 proceeding from the groove of the face of the shaft, and this 

 division can be traced along its whole extent, even to the back, 

 on the external surface of which there is sometimes a corres- 

 ponding sunk line ; but the two pieces of the corky matter are 

 in close contact along this dividing line, and do not even se- 

 parate distinctly by tearing them asunder. The external part, 

 or horny envelope or case of the shaft, is much thinner than 

 the tube, the latter of which is prolonged farther along the back 

 of the shaft than along its face, although there is no line of 

 distinction between them. Some further explanations, however, 

 are necessary here, before the structure of the shaft can be 

 rightly understood. Where the tube terminates on the face of 

 the feather, and where the groove of the shaft commences, the 

 line of union of the dorsal and lateral surface of the shaft meets 

 its fellow of the other side, having gradually left the posterior 

 margin of the shaft, crossed its side obliquely, and become an- 

 terior at this point ; so that, at the commencement of the shaft, 

 what is naturally considered as the back of the shaft forms the 

 whole circumference of it, and does not become the real or 

 geometrical back, until it has reached a certain height. It is 

 this back only w^hich is the true continuation of the shaft. We 

 may suppose the corky matter imposed upon its anterior sur- 

 face, and covered over by a prolongation of it, forming the 

 coating of the sides and face of the shaft. The posterior w^all 

 of the shaft is much thicker than the others, and marked with 

 a number of longitudinal grooves internally, or where it meets 

 the pith. The anterior walls are considerably thinner, and 

 the lateral comparatively very thin. About the point of union 

 of the two lines mentioned, on the face of the feather, the 

 corky matter commences, and is in contact w^th the anterior 

 coat of the shaft, but posteriorly it leaves a vacuity, which ex- 

 tends some way up the shaft. The internal membrane of the 

 tube having reached this point, divides, a portion passing up- 

 wards into the posterior vacuity, another passing to the surface 

 of the feather, by a small aperture at the commencement, of 



