FORMATION OF TISSUES. xxiii 



determined by the same circumstances. Thus, uric acid, known to be present in 

 minute quantity in the blood, is gathered up by the cells of the kidneys ; and other 

 chemical compounds existing in the circulating fluid may be segregated in Hke 

 manner by cells, to be discharged by excretion. Also, in the converse operation of 

 absorption of aliment, the cells of the intestinal epithelium become charged with 

 particles of fat. But, Avhile an alteration in the contents of a cell may be thus 

 brought about by the imbibition of one kind of matter in preference to another, the 

 contained substance may be also changed in its qualities by a process of conversion 

 taking place within the cell. 



4. Cells ultimately present differences in their relations to each other. 



a. They may remain isolated, as in the instance of the corpuscles of blood, 

 chyle, and lymph, and those formed in certain secretions. 



b. They may be united into a continuous tissue, by means of a cementing 

 intercellular substance ; the epithelium and cuticle, the nails and hairs, 

 afford instances of this. In cartilage, where this occurs, the capsules of the 

 cells may become more or less blended with the intercellular substance. 



c. The parietes of neighbouring cells meet at particular points, and, 

 absorption taking place, their cavities become united. It is supposed that 

 ramified cells may thus open into one another ; and Schwann conceives that 

 the networks of capillary vessels originate in this way. 



Intercellular Substance. Of the matter which lies between cells the 

 intercellular substance and its relation to them, it may be observed that 

 sometimes it is in very small quantity, and seems merely to cement the 

 cells together, as in epidermis and epithelium ; at other times it is more 

 abundant, and forms a sort of matrix in which the cells are imbedded, as in 

 cartilage. It is homogeneous, translucent, and firm in most cartilages, and 

 pervaded by fibres in yellow cartilage. In connective tissue it consists of 

 fibres, with soft interstitial matter, which is scanty in the denser varieties, 

 but abundant in the lax tissue of the umbilical cord ; in bone the inter- 

 cellular substance is calcified and mostly fibrous. As to the production of 

 the intercellular substance, there can be little doubt that in cartilage it is 

 derived from the cells. Formed as capsules round the cells by excretion 

 from their surface, or by conversion of their proper substance, and being 

 blended into a uniform mass, it accumulates while the cells multiply, and 

 while fresh material is supplied to them from the blood, which they convert 

 into chondrinous substance. Kolliker supposes that, in like manner, 

 simple membrane, such as the membranes propriw of gland-ducts, may be 

 produced by excretion from a series of cells ; in which case such membranes 

 would come under the same description as intercellular substance. The 

 source of the intercellular substance is in other cases not so apparent, but 

 it may be presumed that the cells have some influence in its nutrition and 

 maintenance. 



From what has been said it will be obvious that cells and nuclei play an important 

 part in the growth of textures, and probably in nutrition. The former process is 

 usually accompanied by a great multiplication of cells or nuclei, the peculiar consti- 

 tuent of which the protoplasm seems to be specially endowed with the faculty of 

 propagation by division, and of increase by appropriating and converting new matter. 

 It is conceivable that in this way it may serve for the extension of growing tissue 

 and the development of structural elements from the crude materials of growth. 

 Again, in the nutrition of a mass of tissue the crude material may undergo preparation 

 by the cells or nuclei that lie in the insterstices of the structure. 



The existence of this protoplasmic germinative substance is very general, perhaps 

 indeed universal, in the animal and vegetable kingdoms. But, M'hilst in the great 



