68 GENERAL ANATOMY OF THE TISSUES. 



cells, but is deposited from the blood-plasma or is even formed out of 

 cells, does not come under this head. It may be said to be present in 

 a smaller quantity and more fluid, however, not only in the cellular 

 tissues, but also in the higher structures, among which there everywhere 

 exists a small quantity of connecting substance. Intercellular spaces 

 developed by the excretions of cells between one another, have not 

 been demonstrated with certainty in animals, yet it is probable that 

 most glandular cavities and those of the heart and of the great vessels 

 are of this nature, since they appear to rise by the excretion of fluid in 

 the interior of originally compact masses of cells. 



My view, that the genuine, membrance prop^nce and the vitreous 

 membranes are formed as excretions, is founded particularly upon the 

 examination of the chorda dorsalis and of the renal canals, in which it 

 may be readily shown that the structureless membranes are secondary 

 formations, arise in intimate union with the cells of this part, and from 

 the very first appear perfectly homogeneous. The supposition of many 

 authors, especially of Reichert, that these membranes belong to the 

 homogeneous connective tissue, is readily refuted by chemical examina- 

 tion, since they yield no gelatine, but consist of a substance which most 

 closely approximates to the sarcolemma and elastic tissue (comp. Men- 

 sonides in 'Nederl. Lancet.' d. iv. GOi, and Bonders, ibid. August, 1851, 

 p. 73). To what extent homogeneous membranes formed by excretions 

 from cells, occur among animals is not yet determined, but the homoge- 

 neous chitin-investments 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 manifested 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 Pladarise, 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-contents 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.* 



* [To this list of contractile cells must be added tlie colorless corpuscle of the blood of 

 man, the Frog, and the Skate, and probably that of other Vertebrata (Wharton Jones, /. c), 

 the cells which lie in the meshes of the areolar tissue of the disc of the Medusce {Cyaticpa) 

 and the young epithelium cells (mucus-corpuscles) of the mucous membranes, in which 

 most distinct protean movements, like those of the colorless corpuscle, may be observed. 



