46 GENERAL ANATOMY OF THE TISSUES. 



cells, occur among animals is not yet determined, but the 

 homogeneous chitin-ihvestments of the intestine, and of the 

 external surface in the Articulata, appear to be of this nature.] 



§ 17. 

 Contractility of the Cells. — Among the vital phenomena of 

 cells must be enumerated those contractions which are mani- 

 fested by cell-membranes and also by cell-contents. Contractile 

 cell-membranes are possessed by many if not all Protozoa ; and 

 among subordinate cells, by the yelk-cells of the Planarise, the 

 heart-cells of many embryos (Alytes, Sepia, Limax), the cells of 

 the tail of embryo Botrylli. The cilia also, as processes of the 

 cell-membrane, may be mentioned here. Contractile cell-con- 

 tents are found in the fibre-cells of the smooth muscles, in the 

 stellate cells of the skin of the embryo of Limax, and in the 

 animal muscular fibres ; which last, as they consist of a number 

 of united cells, may be here enumerated. Here also I place 

 the contractile phenomena exhibited by the contents of the 

 Protozoa (contractile vesicles) and by the Rhizopoda. 1 



[Donders has recently promulgated the view that it is only 

 the cell-contents which are contractile, not the cell-membranes. 

 Although it must be granted, that it is difficult, in the cases in 

 question, to decide what part of the cell contracts, yet it seems 

 very hazardous to endeavour to refer the movements of the 

 cilia in plants and animals, in free and combined cells, to 

 indemonstrable contents in these cilia communicating with the 

 cell. In the cells of the Planarise and in the Protozoa, any 

 one who has actually seen the movements will hardly refer 

 them to anything but the cell-membranes. In the transversely 



1 [To this list of contractile cells must be added the colourless corpuscle of the 

 blood of man, the Frog and the Skate, and probably that of other Vertebrata 

 (Wharton Jones, I. c), the cells which lie in the meshes of the areolar tissue of 

 the disc of theMedusce (Cyantea), and the young epithelium cells (mucus-corpuscles) 

 of the mucous membranes, in which most distinct protean movements, like those 

 of the colourless corpuscle, may be observed. It is certainly the membrane which 

 contracts in these cases, for it pushes out processes which are only subsequently 

 filled by the granular contents. 



In the lower plants {Alga) the occurrence of contractile processes in the shape of 

 cilia is universal, and the contractility of the cell substance in the zoospores of 

 Volvox is evinced by the occurrence of a rhythmically contracting space in them. 

 (See Busk on Volvox globator, 'Quarterly Journ. of Micr. Sc.,' No. 2, 1853.)— Eds.] 



