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 numerous 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- 

 ing. 



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. 



