508 THE MECHANICAL FUNCTIONS. 



skin. A white line is seen running longitudinally 

 the whole length of the cylinder, and another, ex- 

 actly similar to it, is met with on the opposite side : 

 the one corresponds in situation to the front, and 

 the other to the back of the stem of the future 

 feather. On laying open the matrix longitudinally, 

 as is shown in Fig. 2.30, it is found to be composed 

 of a sheath or capsule, and of a central pulpy mass, 

 termed the hulh. The capsule consists of several 

 membranous layers (c, e, s, i), which are more con- 

 solidated near the apex, and become gradually 

 softer and more delicate, as we trace them towards 

 the base of the matrix, where their formation is 

 only beginning to take place. 



The laminae and their fibrils, the assemblage of 

 which constitutes the vane of the feather, are the 

 parts which are first formed ; and their construc- 

 tion is efi^ected in the space between the outer 

 capsule (c), and the central bulb (b), in a mode 

 which is exceedingly remarkable, and different 

 from that of the formation of any other organic 

 product wdth which we are acquainted. Instead 

 of growing from a base, like hairs, and other pro- 

 ductions of the integuments, by successive deposi- 

 tions of layers, the materials which are to compose 

 the laminae are cast in moulds, where they harden, 

 and acquire the exact shape of the recipient cavi- 

 ties. The next object of our curiosity, then, is to 

 learn the way in which these moulds are con- 

 structed ; and on careful examination they appear 

 to be formed by two striated membranes, the exte- 

 rior one (e) enveloping the other (i), or interior 

 membrane. These membranes are separated by a 

 series of partitions, which commence at the edges 



