CHAPTER V. 



THE SKIN. 



THE skin everywhere consists of two layers very different from one another in structure. 

 The lower layer, which is much the thicker, consists of fibrous connective tissue, 

 forming a dense tough membrane. This layer is about -^-th of an inch thick (1 mm.). 

 It is clothed externally by a thin layer of cells, the longer axis of which is perpen- 

 dicular to the surface of the body : this is the epidermis. The lower layer or derma 

 contains the scales, the ends of which project backwards through the epidermis, and 

 the pigment cells or chromatophores. On the lower side of the head anteriorly the 

 skin forms a number of flexible papilke, or filaments, which are delicate tactile organs. 

 The skin is continued over the eyes, but the part which covers the eye is thin and 

 perfectly transparent. Scales are present in the skin everywhere, except over the eyes 

 and in the region of the tactile filaments. The scales can be detached from the skin 

 without difficulty, and when separated and examined with a low power of the 

 microscope present the appearance shown in Fig. 1, PI. XIV. The largest scales are 

 in the central region of the surface of either side. The scales consist of plates of 

 fibrous tissue hardened by the deposition of lime-salts. Each has the form of a 

 rectangle at the posterior end of which is a portion bounded internally by two straight 

 lines meeting at an obtuse angle, outwardly by a semicircular curve, and covered with 

 sixteen rows of spines. In each row the outermost spine is the largest, the others 

 becoming gradually shorter : there are five spines in each row, and the rows radiate 

 from the angle at which the internal bounding lines of the spinous area meet. This 

 spinous area is the only portion of the scale which is exposed at the surface of the skin, 

 the remainder being imbedded in a socket and overlapped by adjacent scales ; they 

 are placed with their longer axes directed backwards, and each overlaps the one 

 behind it. They are arranged quincuncially, that is, each scale in any transverse row 

 lies over the line where the edges of two adjacent scales of the row behind meet one 

 another. In consequence of this quincuncial arrangement the transverse rows, which 

 are perpendicular to the longitudinal axis of the fish, are not easily recognised ; but two 

 series of oblique rows are visible, one series running downwards from right to left, the 

 other downwards from left to right. In the same way in the pattern of a wall paper 

 when there is a figure repeated at equal distances in lines crossing one another at right 



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