PIGMENT. Ixiii 



being probably a manifestation of that property on which the more conspicuous 

 motions of animals are known to depend, namely, vital contractility ; and this view- 

 has at least the advantage of referring the phenomenon to the operation of a vital 

 property already recognised as a source of moving power in the animal body. But, 

 assuming this view to be sound, so far as regards the nature of the motile property 

 brought into play, it affords no explanation of the cause by which the contractility 

 is excited and the cilia maintained in constant action. 



It is true that nothing resembling a muscular apparatus in the ordinary sense of 

 the term, has been shown to be connected with the cilia, nor is it necessary to 

 suppose the existence of any such ; for it must be remembered that, while the organic 

 substance on which vital contractility depends is probably uniformly the same in 

 composition, it does not everywhere assume the same form and texture. The 

 anatomical characters of human voluntary muscle differ widely from those of most 

 involuntary muscular structures, and still more from the contractile tissues of 

 some of the lowest invertebrate animals, although the movements must in all these 

 cases be referred to the same principle. The heart of the embryo beats while yet 

 but a mass of cells, united, to all appearance, by amorphous matter, in which no 

 fibres are seen ; yet no one would doubt that its motions depend then on the same 

 property as at a later period, when its structure is fully developed. 



In its persistence after systemic death and in parts separated from the rest of the 

 body, the ciliary motion agrees with the motion of certain muscular organs, as the 

 heart, for example ; and the agreement extends even to the regular or rhythmic cha- 

 racter of the motion in these circumstances. It is true, the one endures much longer 

 than the other ; but the difference appears to be one only of degree, for differences 

 of the same kind are known to prevail among muscles themselves. No one, for 

 instance, doubts that the auricle of the heart is muscular, because it beats longer after 

 death than the ventricle ; nor, because a frog's heart continues to act a much longer 

 time than a quadruped's, is it inferred that its motion depends on a power of a dif- 

 ferent nature. And the view here taken of the nature of the ciliary motion derives 

 strength from the consideration that the phenomenon lasts longest in cold-blooded 

 animals, in which vital contractility also is of longest endurance. In the effects of 

 heat and cold, as far as observed, there is also an agreement between the movement 

 of cilia and that of muscular parts ; while, on the other hand, it must be allowed 

 that electricity does not appear to excite their activity. The effects of narcotics 

 afford little room for inference, seeing that our knowledge of their local action on 

 muscular irritability is by no means exact ; but in one instance, at least, an agent, 

 chloroform vapour, which stops the action of the freshly excised heart of a frog, 

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

 difficulty with which the tissues permit the narcotic fluid to penetrate, which cir- 

 cumstance must needs affect 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 discovery of vibrating cilia on the spores and other parts of certain crypto - 

 gamic vegetables may perhaps be deemed a strong 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. 



The cells of the cuticle, and of other textures which more or less resemble 

 it in structure, sometimes contain a black or brown matter, which gives a 

 dark colour to the parts over which, the cells are spread. A well-marked 

 example of such pigment-cells in the human body is afforded by the black 

 coating which lines the choroid membrane of the eye and covers the pos- 

 terior surface of the iris. They are found in the epidermis of the Negro 

 and other dark races of mankind, and in the more dusky parts of the 



