PIGMENT. 51 



the ciliary motion. Something, moreover, may depend on the facility or difficulty 

 with which the tissues permit the narcotic fluid to penetrate, which circumstance 

 must needs influence the rapidity and extent of its operation. Again, we see 

 differences in the mode in which the cilia themselves are affected by the same 

 agent : thus, fresh water instantly arrests their motion in certain cases, while it 

 has no such effect in others. 



The existence of \'ibrating cilia on the spores and other parts of certain crypto- 

 gamic vegetables may perhaps be considered to afford an argument on the 

 opposite side ; but it is by no means proved that the sensible motions of plants 

 (such, at least, as are not purely physical), and those of animals, do not depend 

 on one common vital property. 



PIGMENT. 



Fig. 26. 



The cells of the cuticle, and of other epithelial structures, sometimes 

 contain a black or bro^\^l matter, which gives a dark colour to the parts 

 over Avhich the cells are spread. A well-marked example of such 

 j)igmented cells in the human body is atforded by the black coating 

 which lines the choroid membrane of the eye, and covers the posterior 

 surface of the iris. Pigment is also met with in certain cells of the 

 investing membrane (pia mater) of the spinal cord, in the membranous 

 labyrinth of the ear, and (with brownish yellow pigment) on the olfactory 

 region of the nose. 



The pigment, strictly so called, which is contained within the cells, 

 consists of black or brown granules 

 or molecules of a round or oblong 

 shape, and almost too small for exact 

 measurement. These molecules are 

 densely packed together in some 

 cells ; in others they are more scat- 

 tered, and then it may be seen that 

 there is a certain amount of colour- 

 less matter included along with 

 them. When they escape from the 

 ruptured cells, they exhibit very 

 strikingly the " Bro^vnian " mole- 

 cular movement ; and in conse- 

 quence of this movement the ap- 

 parent figure of the particles is sub- 

 ject to change. It is worthy of 

 remark, that when viewed singly 

 with a very high magnifying power 

 they look transparent and almost 

 colourless, and it is only when they 

 are heaped together that their blackness distinctly appears. The 

 nucleus is colourless, but is very generally hidden from view by the 

 black particles. 



The dark colour of the negro is known to have its seat in the cuticle, 

 and chiefly in the deeper and softer part named the rete mucosum. It 

 is caused by dark-brown colouring matter within the cells, either diffused 

 through their substance or in form of granules — usually more densely 

 aggregated round the nucleus. The dark parts of the European skin 

 owe their colour and its different shades to the presence of pigment 

 granules in the cells in diflferent proportions. Lastly, it cannot be 



K 2 



Fig. 26. — Pigmented Ei'iTHELiu-M Cklls 

 FROM THE Choroid ; magnified 370 

 DIAMETERS (Henle). 



A, cells still cohering, seen on their 

 surf.ace ; a, nucleus indistinctly seen. 

 In the other cells the nucleus is con- 

 cealed by the pigment granules. 



B, two cells seen in profile ; a, the 

 outer or posterior part containing scarcely 

 any pigment. 



