50G THE MKCHANICAL FUNCTIONS. 



another ; and while the outer covering of the stem 

 is growing from one membrane, the interior spongy 

 tissue is forming in other places, in various stages 

 of softness or consolidation : so that the whole com- 

 poses a system of operations, which may be said 

 to resemble in its complication at least, although 

 on a microscopic scale, an extensive manufactory. 

 Hence will be readily understood how great must 

 be the difficulty of tracing all the steps of these 

 multifarious processes, which are carried on in so 

 small a space : and this difficulty is much increased 

 from the circumstance that the organ in which 

 they take place is itself only developed as the work 

 proceeds ; its different parts being produced suc- 

 cessively in proportion as they are wanted, and 

 their form and structure undergoing frequent varia- 

 tion in the course of their developement. 



The most elaborate, and apparently accurate re- 

 searches on this intricate subject, are those lately 

 undertaken by M. Frederick Cuvier, from whose 

 memoir* I have selected the following abridged 

 statement of the principal results of his observa- 

 tions. It will be necessary in order to obtain a 

 clear idea of the several steps of the process to be 

 described, to advert to the structure of a feather in 

 its finished state. For this purpose we need only 

 examine a common feather, such as that repre- 

 sented in Fig. 228, where s is the posterior surface 

 of the solid stem, which, it will be perceived, is 

 divided into two parts by a longitudinal groove, 

 and from either side of which proceed a series of 

 laminae, composing, with their fibrils, what is 



* Memoires du Museum, xiii. 327 ; and Annales des Sciences 

 Naturelles, ix. 113. 



