OF THE SECRETIONS. 



243 



families, as will hereafter be shown. In the vertebrata it is 

 composed of three very distinct layers of unequal thickness 

 (fig. 250) ; the lower and the thickest layer is the corium, 

 (c, c), or true skin, and is the part which is tanned into 

 leather. Its surface presents numerous papillae, in which the 

 nerves of general sensation terminate ; they also contain a fine 

 net-work of blood-vessels, usually termed the vascular layer. 

 The superficial layer is the epidermis, or cuticle ; the cells of 

 which it is composed are distinct at its inner portion, but 

 become dried and flattened as they are pushed outwards. It 

 is destitute of vessels and nerves, and, consequently, is in- 

 sensible. Between these two layers, and more especially 

 connected with the cuticle, is the rete mucosum, a very thin 

 layer of cells, some of which contain the pigment which gives 

 the complexion to the different races of men and animals. 

 The scales of reptiles, the nails and claws of mammals, and the 

 solid covering of the Crustacea are merely modifications of the 

 epidermis; on the other hand, the feathers of birds, and the 

 scales of fishes, are derived from the vascular layer. 



[ 413*. Dutrochet investigated the phenomena called endos- 

 mose and exosmose more carefully than had yet been done, 



Fig. 247. 



and designated them by these names.* 

 Berzelius has given an excellent con- 

 densed view of the subject : " The phe- 

 nomena exhibited by bodies in solu- 

 tion," he observes, " in traversing 

 solid living parts, do not depend solely 

 on the properties which bodies in solu- 

 tion have of diffusing themselves evenly 

 through the fluids which are their men- 

 strua ; the animal membranes and the 

 water contribute their share, inasmuch 

 as the water passes with the dissolved 

 substance, and from this results a phe- 

 nomenon, which in its effects resembles 

 in every respect an absorption. For the 

 sake of illustration, let a, a, fig. 247, be a tube open at both 

 ends, but having a piece of moist bladder tied around its lower 

 extremity ; let a solution of any salt be now poured into the 



* Memoires pour servir a 1'Histoire Anatomique et Physiologique des 

 Vegetaux et Anitnaux, Paris, 1837. 



E2 



