ANIMAL ORGANIZATION. ( 85 



pelled in a direction contrary to its proper course, it raises 

 the loose edge of the valve, which, being applied to the op- 

 posite side of the canal, effectually closes the passage. On 

 the contrary, it presents no obstacle to the natural flow of 

 the contents of the vessel, both edges being then closely ap- 

 plied to the same side. Frequently two, or even three 

 valves are used at the same part, their edges being made to 

 meet in the middle of the passage, like the flood-gates, or 

 locks of a canal.* Among the numberless instances of ex- 

 press contrivance which are met with in the examination of 

 the fabric of animals, there is perhaps none more striking 

 and more palpable, than this admirable mechanism of the 

 valves. 



As we ascend from the simpler to the more complicated 

 systems of organization, adapted to a greater range of facul- 

 ties, we find greater diversity in the mechanical means em- 

 ployed for carrying on the functions of life. Textures of 

 greater strength than can be constructed by membranes 

 alone become necessary for the security, the support, and 

 the defence of important organs; and more especially for the 

 execution of extensive movements. For obtaining: these 

 advantages a peculiar species of fibres is provided, formed 

 of a much denser substance than even the most consolidated 

 forms of cellular texture. The animal product termed albu- 

 men possesses a much stronger cohesive power than gelatin^ 

 which is the basis of membrane. The addition of albumen, 

 therefore, procures the quality required: and the fibres that 

 are produced by its combination with gelatin are opaque, 

 and of a glistening white colour. By interlacing fibres thus 

 composed, a close texture is formed, which is exceedingly 

 tough and unyielding. Th^sQ Jibrous textures, as they are 

 termed, while they retain the flexibility of membranes, 

 greatly surpass them in strength; but, being at the same time 

 incapable of extension, they are necessarily devoid of elasti- 



* Fig. 27, representing" the section of a vessel, is Intended to show the po- 

 sition of the valves when applied lo the sides of the vessel, by the stream 

 nnoving- onwards in the direction pointed out by the arrow. In Fig". 28, they 

 are seen closing the passage by the retrograde pressure of tlie current. 



