64 THE TISSUES. 



characteristic elongated or rod shaped nucleus in its centre. 

 The cells have a parallel and overlapping arrangement, held 

 together by a scanty cement-substance, and form sheets, rings, 

 or tubes of muscle. Indistinct longitudinal and transverse 

 markings are sometimes observed, the latter being probably 

 folds in the cell-walls. The cells have an envelope but no 

 separable sarcolemma. Some authorities describe involuntary 

 muscle-cells as having longitudinal ridges, or " intercellular 

 bridges," on their surface, which unite with the ridges of the 

 adjacent cells. 



Nerve -tissue. 



The elements of the nervous structures are greatly extended 

 cells, called neurons, which for practical convenience may be 

 divided into nerve-cells, nerve-fibres, and nerve-terminals. 

 Their description will be deferred to that of the nervous 

 system in general. 



Free Cells and Body-fluids. 



The tissues considered hitherto are all fixed. The present 

 class is a somewhat miscellaneous assemblage of those body- 

 elements that are not fixed, but free or fluid. 



They may be grouped as follows : 1 . Unattached cells, free 

 in the solid tissues. 2. Cellular fluids that is, fluids in 

 which free living cells are suspended, as the blood. 3. Simple 

 homogeneous fluids, mainly secretions. 4. Emulsions. 



The free cells, situated for the most part in the solid tissues, 

 but not attached to them, are represented mainly by the 

 wandering cells, the lymphoid cells of lymphoid tissue, and 

 ova. The so-called wandering cells are leukocytes or lymph- 

 corpuscles which have escaped from the bloodvessels or 

 lymphatic structures and exist free and migratory, especially 

 among the meshes of the opener or areolar forms of connec- 

 tive tissue. The small spherical lymphoid cells, with large 

 prominent nuclei, which occur crowded in the meshes of 

 retiform or lymphoid tissue, are very similar to and closely 

 related to the corpuscles of lymph. Some of the cellular 

 elements of bone-marrow might also perhaps be included in 

 this group. The ova of the lower animals are free cells ; and 



