Mr W. Macgillivray on the Covering of Birds. 257 



come anterior 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 which is the true continuation of the shaft. We 

 may suppose the corky matter imposed upon its anterior surface, 

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

 the sides and face of the shaft. The posterior wall of the shaft is 

 much thicker than the others, and longitudinally grooved inter- 

 nally, or where it meets the pith ; the anterior walls are consider- 

 ably 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 with the 

 anterior coat of the shaft, but posteriorly it leaves a vacuity, 

 which extends some way up the shaft. The internal membrane 

 of the tube having reached this point, divides, a portion passing 

 upwards into the posterior vacuity, another passing to the sur- 

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

 the median groove of the shaft, over which lies a small laminar * 

 prolongation of the tube. This arrangement is what is observed 

 in quill-feathers in general ; but in most ordinary feathers there 

 is no vacuity behind, and the internal membrane makes its exit 

 undivided at the commencement of the groove. The shaft is 

 distinguished from the tube by its being opake, which is caused 

 by the internal corky substance, the external horny coat being 

 of the same nature as the tube, only attenuated, and more so, 

 as has been said, on the back, than on the face or sides of the 

 shaft. 



3. The webs (in Latin tela, in French les toiles), of which 

 there are two, one on either side of the shaft. The web is a la- 

 teral prolongation of the external layer of the coat of the shaft, 

 into a series of filamentous substances, ordinarily placed in ap- 

 position, and by their association in this manner forming a stifle 

 ish elastic expansion. The filaments of which the web consists 

 are named barbs. 



The barb, barba, barbe, is a very thin linear membrane, being 

 an attenuated continuation of the outer pellicle of the shaft, and 

 arising from it at the angle formed by the meeting of the dorsal 



JULY SEPTEMBER 18S7. R 



