REPARATION OF THE TISSUES. 997 



Secondly, certain tissues of comparatively simple structure and 

 chemical composition, and of low vital endowments, appear to be capa- 

 ble of regeneration. Such are the areolar and fibrous tissues, elastic 

 tissue, and bone, which fulfil mechanical uses in the body, serving to 

 connect and support its various parts. 



Lastly, bloodvessels, lymphatics, and nerves, tissues which pene- 

 trate other parts or organs, are likewise endowed with this power. 



Other tissues and organs of a special kind, which have a complex 

 structure, higher chemical constitution, or peculiar properties or func- 

 tions such as true cartilage, muscle, the gray substance of the ner- 

 vous centres, the essential parts of the organs of special sensation, the 

 cutis and its glands, the secreting and excreting glands, and the duct- 

 less glands are not regenerated after injury or destruction. 



The regeneration of particular tissues is accomplished by the multi- 

 plication and evolution of previously formed cell-elements, whether 

 these be gyrnnoplasts, nuclei, or nucleoli; and by the modification of 

 the intercellular or internuclear elements, or matrix, within the sphere 

 of action of those nutritive centres. In this way, the epidermis and 

 epithelium are speedily reproduced. The mode of formation of new 

 lymph-corpuscles and blood-corpuscles, already described (p. 992), is to 

 be explained in a similar way. The connective, membranous, fibrous, 

 or tendinous areolar tissues, and the elastic tissues, are regenerated 

 in the same manner as that in which they are developed. Connective 

 tissue is the chief medium of restoration or repair in wounds, or ulcers 

 of tissues or parts, which, like muscles, glands, and the cutis, are not 

 reproduced. In its growth, it becomes penetrated by new capillaries 

 and lymphatics, which are developed after the manner already described 

 as their original mode of formation. The development of new vessels, 

 in the meshes of effused lymph or blood, in the restoration of the lost 

 tail or limbs of the Amphibia, and also in tumors, is accomplished in 

 the same way. Cartilage, if removed by accident, or softened and 

 absorbed in disease, is not regenerated, but cup-shaped cavities are left, 

 which may wear smooth ; if it be rent or broken across, it does not 

 unite, but the separated parts become connected by strong fibrous or 

 osseous belts. New cartilage is produced in certain tumors. Bony 

 tissue is regenerated with remarkable facility; the process always 

 takes place by the intra-membranous form of ossification. The intra- 

 cartilaginous form, however, occurs in tumors. Injury to a muscle, 

 such as division of its fibres, provided that the cut ends have not re- 

 tracted too far from each other, is repaired by a uniting band of dense 

 connective tissue, which re-establishes the continuity and office of the 

 muscle ; but when a whole muscle is torn across, it may retract, and 

 form altogether new connections, or it may cease to be used, and then 

 undergo fatty degeneration. A divided nerve is quickly united by con- 

 nective tissue ; in the cicatrix, nerve-fibres are afterwards formed, 

 which join the divided fibres, and completely restore their functions, 

 whether these be reflex, sensory, or motor. The nerve-fibres beyond 

 the line of section usually lose their medullary substance or sheath, 

 which previously undergoes a granular and fatty degeneration ; but 

 the tubular sheath, the axis-fibre, and the nuclei remain. When the 



