SKIN 



IO4I 



tissue corpuscles ' which are everywhere dispersed among them, as 

 having a most important function in the first production and sub- 

 sequent maintenance of the more definitely organised portions of 

 the fabric. In these corpuscles distinct movements, analogous r to 

 those of the sarcodic extensions of rhizopods, have been recognised 

 in transparent parts, such as the cornea of the eye and the tail of 

 the young tadpole, by observations made on these parts whilst living. 

 For the display of the characters of the fibrous tissues small and 

 thin threads may be cut with the curved 

 scissors from any part that affords 

 them ; and these must be torn astfnder 

 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 magnifying pow r er. The differ- 

 ence between the 'white' and the 

 ' yellow ' components of connective tissue 

 is at once made apparent by the effect 

 of acetic acid ; whilst the ' connec- 

 tive tissue corpuscles' are best dis- 

 tinguished by the staining process, 

 especially in the early stage of the 

 formation of these tissues (fig. 773). 



Skin ; Mucous and Serous Mem- 

 branes. The skin, which forms the ex- 

 ternal envelope of the body, is divisible FIG. 775. Vertical section of skin 

 into two principal layers : ihecutisvera of finger: A, epidermis, the 

 or l true skin,' which usually makes up 

 by far the larger part of its thickness, 

 and the 'cuticle,' 'scarfskin,' or epi- 

 dermis, which covers it. At the mouth, 

 nostrils, and the 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 pecu- 

 liar glairy secretion of mucus by which 

 its surface is protected. But those great 

 closed cavities of the body which surround the heart, lungs, intes- 

 tines, &c. are lined by membranes of a different 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 cellular membranous basis, and of a thin cuticular 

 layer, which, as it differs in many points from the epidermis, is dis- 

 tinguished as the epithelium. 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 mem- 

 branes are very copiously supplied with blood-vessels and with glan- 

 dube of various kinds ; and in the skin we also find abundance of 

 nerves arid lymphatic vessels, as well as, in some parts, of hair- 



3x 



surface of which shows depres- 

 sions a a, between the emi- 

 nences b b, on which open the 

 perspiratory ducts s ; at in is 

 seen the deeper layer of the 

 epidermis, or stratum Malpighii. 

 B, cutis vera, in which are im- 

 bedded the sweat-glands d, 

 with their ducts e, and aggre- 

 gations of fat-cells/; g, arterial 

 twig supplying the vascular 

 papillae p ; t, one of the tactile 

 papillae with its nerve. 



