STRUCTURE AND ARRANGEMENT OF MUSCLES. 



453 



.^% 



Fig. 301. 



of melanin. The pigment-granules may be diffused in the cell, or aggregated around the nucleus ; 

 in the former Case, the skin of the frog appears dark in colour, in the latter, it is but slightly 

 pigmented (fig. 301). The question has been raised whether they are actual cells or merely 

 spaces, branched, and containing a 

 fluid with granules in suspension. In 

 any case, they undergo marked changes 

 of shape under various influences. If 

 the motor-nerve to one leg of a frog be 

 divided, the skin of the leg on that 

 side becomes gradually darker in colour 

 than the intact leg. A similar result 

 is seen in the curara experiment, when 

 all parts are ligatured except the nerve. 

 Local applications affect the state of 

 diffusion of the pigment, as v. "Wittich 

 found that turpentine or electricity 

 caused the cells of the tree-frog to con- 

 tract, and the same effect is produced 

 by light. In Rana temporaria local 

 irritation has little effect, but light, on 

 the contrary, has, although the effect 

 of light seems to be brought about Pigment-cells from the web of frog s foot ; a, cell with 

 through the eye, probably by a reflex pigment-granules diffused ; b, granules more concen- 

 mechanism (Lister). A pale-coloured t ra ted ; c, more concentrated still ; d, cells with guanin- 

 frog, put in a dark place, assumes, after granules (Stirling). 



a time, a different colour, as the pigment is diffused in the dark ; but if it be exposed to a 

 bright light it soon becomes pale again. The same phenomenon may be seen on studying the 

 web of a frog's leg under the microscope. The marked variations of colour within a certain 

 range in the chameleon is due to the condition of the pigment-cells -in its skin, covered as 

 they are by epidermis, containing a thin stratum of air (Brilcke). When it is poisoned with 

 strychnin, its whole body turns pale ; if it be ill, its body becomes spotted in a dendritic 

 fashion, and if its cutaneous nerves be divided, the area supplied by the nerve changes to black. 

 The condition of its skin, therefore, is readily affected by the condition of its nervous system, 

 for psychical excitement also alters its colour. If the sympathetic nerve in the neck of a turbot 

 be divided, the skin on the dorsal part of the head becomes black. It is notorious that the 

 colour of fishes is adapted to the colour of their environment. If the nerve proceeding from 

 the stellate ganglion in the mantle of a cuttle-fish be divided, the skin on one half of the body 

 becomes pale.] 



[Guanin in Cells. Besides the pigment-cells in the web of a frog's foot (especi- 

 ally in Rana temporaria) there are other cells which contain granules of guanin 

 (fig. 301, d). If the web of a frog's foot be mounted in Canada balsam and 

 examined microscopically between crossed Nicol's prisms, each guanin-cell is seen 

 to contain numerous very strongly doubly refractive granules of guanin ( 283).] 



292. STRUCTURE AND ARRANGEMENT OF MUSCLES. [Muscular 

 Tissue is endowed with contractility, so that when it is acted upon by certain forms 

 of energy or stimuli, it contracts. There are two varieties of this tissue 



(1) Striped, striated (or voluntary) ; 



(2) Non-striped, smooth, organic (or involuntary). 



Some muscles are completely under the control of the will, and are hence called 

 " voluntary," and others are not directly subject to the control of the will, and are 

 hence called " involuntary j" the former are for the most part striped, and the 

 latter non-striped ; but the heart-muscle, although striped, is an involuntary 

 muscle.] 



1. Striped Muscles. The surface of a muscle is covered with a connective-tissue envelope or 

 perimysium externum, from which septa, carrying blood-vessels and nerves, the perimysium 

 internum, pass into the substance of the muscle, so as to divide it into bundles of fibres or 

 fasciculi, which are fine in the eye-muscles and coarse in the glutei. In each such com- 

 partment or mesh, there lie a number of rmtscular fibres arranged more or less parallel to each 

 other. [The fibres are held together by delicate connective-tissue or endomysium, which sur- 

 rounds groups of the fibres ; each fibre being, as it were, separated from its neighbour by 

 delicate fibrillar connective-tissue.] Each muscular fibre is surrounded with a rich plexus of 

 capillaries [which form an elongated meshwork, lying between adjacent fibres, but never pene- 



