604 VERTBBEATED ANIMALS. 



obvious to the eye, and these into the entire muscle ; whilst it 

 also forms the membranous divisions between distinct muscles. 

 In like manner it unites the elements of nerves, glands, &c., 

 binds together the fat-cells into minute masses, these into larger 

 ones, and so on ; and in this way it penetrates and forme part 

 of all the softer organs of the body. For the display of the 

 characters of these tissues, small and thin shreds may be cut 

 with the curved scissors from any part that affords them ; and 

 these must be torn asunder with needles under the simple micro- 

 scope, until the fibres are separated to a degree sufficient to 

 enable them to be examined to advantage under a higher magnify- 

 ing power. 



418. Skin, Mucous and Serous Membranes. — The Skin which 

 forms the external envelope of the body, is divisible into two 

 principal layers ; the "true skin," which usually makes up by 

 far the larger part of its thickness, and the "cuticle," "scarf 

 skin," or Epidermis, which covers it. At the mouth, nostrils, 

 and other orifices of the open cavities and canals of the body, 

 the skin passes into the membrane that lines these, which is 

 distinguished as the Mucous membrane, from the peculiar glairy 

 secretion of mucus by which its surface is protected. But the 

 great closed cavities of the body, which surround the heart, 

 lungs, intestines, &c., are lined by membranes of a diflerent 

 kind; which, as they secrete only a thin serous fluid from their 

 surfaces, are known as Serous membranes. Both Mucous and 

 Serous membranes consist, like the skin, of a proper membra- 

 nous basis, and of a thin cuticular layer, which, as it difiers in 

 many points from the epidermis, is distin|;uished as the Epithe- 

 lium (§421). The substance of the "true skin" and of the 

 "mucous" and "serous" membranes, is principally composed of 

 the fibrous tissues last described ; but the skin and the mucous 

 membranes are very copiously supplied with bloodvessels and 

 with glandulse of various kinds ; and in the skin we also find 

 abundance of nerves and lymphatic vessels, as well as, in some 

 parts, of hair-follicles. The distribution of the vessels in the 

 skin and mucous membranes, which is one of the most interest- 

 ing features in their structure, and which will come under our 

 notice hereafter (Figs. 328, 329), is intimately connected with 

 their several functions. In serous membranes, on the other 

 hand, the supply of bloodvessels is more scanty, their function 

 being simply protective. 



419. Epidermic and Epithelial Cell-layers. — The Epidermis or 

 "cuticle" covers the exterior surfaces of the body, as a thin semi- 

 transparent pellicle, which is shown by microscopic examination 

 to consist of a series of layers of cells, which are continually 

 wearing off at the external surface, and are being renewed at 

 the surface of the true skin ; so that the newest and deepest 

 layers gradually become the oldest and most supei-ficial, and are 

 at last thrown ofii'by slow desquamation. In their progress from 



