APPENDICES OF THE SKIN. 95 
hard. At this time, the sheath bursts open at the point, 
and the withered medulla appears encircled by the shaft 
and barbs. As the growth takes place from below, new 
portions of the feather are pushed forwards with the me- 
dulla, in the same position. Between the barbs may like- 
wise be observed numercus threads and scales, which are 
the dried remains of the fluid in which they were formed. 
These scales, and the withered medulla, either fall off na- 
turally, or are removed by the animal in the act of preen- 
mg. 
When that part of the feather containing the web is 
formed, the remaining part of the medulla becomes enve- 
loped with the matter of the quill, which is a continuation 
of the shaft. It is pervious at both extremities, for the en- 
trance and egress of the medulla. Perhaps a distinct idea 
may be formed of the structure of a feather, by consider- 
~ ing the whole as at first like an elongated hollow cone, in 
which a portion of the base retains its form, and constitutes 
the quill; while the remainder, towards the apex, has 
opened on one side, to form the shaft and the web; 
the medulla becoming exposed in the last part, while it 
continues covered in the former. ; 
Where more shafts than one arise from the same quill, 
they appear to be formed on opposite sides of the medulla. 
This part always appears in the centre; and the grooved 
sides of the shaft face each other. 
The quill does not attain its full growth at the first, like 
the shaft and the web. It continues to increase in length 
for a considerable period, during which the corresponding 
part of the medulla continues soft and vascular. As the 
animal cannot reach the medulla which is contained in the 
quill, it remains, and forms that tubular chambered mem- 
brane, with which all are familiar, as it appears in common 
writing quills, 
