39S THE MECHANICAL FUNCTIONS. 



that of the formation of any other organic product with 

 which we are acquainted. Instead of growing from a base, 

 like hairs, and other productions of the integuments, by suc- 

 cessive depositions 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 ca^aties. The 

 next object of our curiosity, then, is to learn the way in 

 which these moulds are constructed; and on careful exami- 

 nation they appear to be formed by two striated membranes, 

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

 membrane. These membranes are separated by a series of 

 partitions, which commence at the edges of the longitudinal 

 white band, seen in Fig. 229, and wind obliquely upwards 

 till they reach the opposite longitudinal band already de- 

 scribed, where they join a longitudinal partition which oc- 

 cupies a line answering to that posterior band. Thus they 

 leave between them narrow spaces, which constitute so 

 many compartments for the deposition, as in a mould, of the 

 material of each lamina. The course of these channels, and 

 their junction at the back of the matrix is seen at s, Fig. 

 230. It is exceedingly probable, though from the minute- 

 ness of the parts it is scarcely possible to obtain ocular de- 

 monstration of the fact, that the fibrils of the laminae are 

 formed in a similar manner, by being moulded in still more 

 minute compartments, formed by transverse membranous 

 partitions. 



The proper office of the bulb, after it has supplied the 

 materials for the formation of the laminae, is to construct 

 the stem of the feather, and unite the laminae to its sides. 

 For this purpose the anterior portion of the bulb deposites 

 on its surface a plate of horny substance, while another plate 

 is formed by the posterior part in the interior of the bulb. 

 Thus the bulb becomes divided into two portions, one ante- 

 rior and the other posterior. The former of these, after 

 having finished the external plate, proceeds to form the 

 spongy substance, which is to connect the two plates, and 

 the posterior portion of the bulb embraces the inner plate, 



