ORGANIC FORM AND ARCHITECTURE 697 



cated. A residue of unmodified cytoplasm is often to be observed 

 on the side of the fibre, and a slight sheath or sarcolemma rcprescnU 

 the cell-membrane. In the superficial cytoplasm, as in man, or more 

 deeply, as in the frog, there are several nuclei. On the surface of 

 the fibre there are also the branched endings of motor and sensory 

 nerves. Numerous muscle fibres wrapped up in a sheath of connective 

 tissue or fascia compose a muscle, which in Vertebrate animak is 

 usually attached to pieces of skeleton by means of sinews or tendons 

 of living connective tissue. In Arthropods the highly developed 

 muscles are attached to the skeletal parts by non-Hving strips of 

 chitin, so that there is a marked structural difference betwelh 

 Vertebrate and Arthropod tendons. 



CONNECTIVE TISSUE.— While nervous and muscular tissues 



are well defined, those included as connective form a heterogeneous 

 group. Their chief functions are to enswathe, to bind, and to 

 support. In some cases, like the fascia that covers a muscle, the 

 component cells are bound together without any intercellular matrix. 

 Not infrequently they become laden with fat, or sometimes with 

 pigment. Very different, however, are those forms of connective 

 tissue in which there is aji intercellular matrix, which the cells 

 make, as in gristle or cartilage. Connective cells are often very 

 irregular in shape, giving off fine processes in the matrix, well seen 

 in bone cells. Another modification is seen when the connective 

 cell forms a long fibre, as is well illustrated by tendons. Besides 

 the firm forms of connective tissue, we are logically bound to include 

 fluid tissues — the blood and the lymph; and in Invertebrate animals 

 the coelomic or perivisceral fluid may be very important, as in the 

 sea-urchin. 



General. — The concept of a tissue implies aggregation of cells, 

 and sometimes integration as well, as in nervous tissue, where the 

 cells work into one another's hands; but we re-emphasise that in 

 most animals with differentiated fixed tissues there is a common 

 medium of fluid tissue, usually the blood. And besides this fluid 

 tissue there are more or less independent cells, the amoeboid 

 wanderers, notably those leucocytes that are able to migrate out of 

 the blood and the larger phagocytes (macrophages) which move 

 about in the tissues. If a tissue be compared to a street of similar 

 houses, there are also some houses that move from one street to 

 another, as we may see them doing in America. 



We cannot leave the animal tissues without again referring to 

 the method of tissue culture especially associated with the work 

 of Ross Harrison and Carrel. It has been found possible to keep 

 small fragments of tissue, especially young tissue, alive in isolation 

 in suitable media, and even to induce them to exhibit cell-division 

 and growth. It has been possible to discover certain media which 



