Rev. P. Keith on the Susceptibilities of Living Structures. 167 



nute slice taken from the skin or fixt of an animal recently 

 slaughtered, or from the pulpy part of any plant or fruit. It is 

 composed, according to Bichat*, of an assemblage of whitish 

 filaments traversing a fine and filmy membrane, or of thin and 

 transparent films, which with the filaments form little cells. 

 The film is as fine as a soap-bubble, and resembles, by the 

 crossing of the filaments, a sort of irregular net-work. This 

 description is equally correct whether we regard it as applying 

 to plants or to animals. In the living animal they contain a 

 serum and fat; in the living vegetable, a limpid juice; and in 

 the healthy condition of either, they are to be seen in what 

 may be called their natural state, — that is, in a state occupy- 

 ing a space that is comparatively very small. But the tissue 

 which they form is capable of a considerable degree of exten- 

 sion, as is evident from the increase of fat which enlarges the 

 bulk of the body ; and from the occurrence of tumours and 

 swellings, whether of the animal or vegetable subject, as well 

 as from the movements of every limb or branch. Thus in 

 elevating the arm to a perpendicular position, the tissue of 

 the arm-pit is extended to more than double its former length ; 

 and in the forcible bending of the bough of a tree there oc- 

 curs an extension of a similar sort. If the extension is pushed 

 too far the tissue will break, but in the natural and ordinary 

 movements of the fabric it is not pushed on to rupture. 



As this tissue is capable of a considerable degree of exten- 

 sion, so it is capable of a correspondent degree of contraction. 

 For as soon as the distending cause has ceased to act, the 

 tissue shrinks again, spontaneously, to its original dimensions ; 

 as in the case of the loss of emhoniioint^ or of the resolution 

 of tumours or swellings, or of the shrivelling up of the re- 

 laxed scrotum, or of the contraction of the abdomen and litems 

 when the period of gestation is past. This is what Bichat 

 calls contractility par faut d'extensioiif. It is always greatest 

 in young subjects. Thus though a young person may become 

 thin, still the skin lies close to the interior organs, because the 

 cellular tissue possesses yet the power of contracting itself 

 completely ; but if an old man becomes thin, the skin hangs 

 loose and in wrinkles, because the cellular tissue has lost its 

 power of recoil. 



In the ordinary state of the living fabric there is a certain 

 degree of tension to which every organ is subjected : the loco- 

 motive muscles by their antagonists; the hollow muscles by 

 thu substances which they include; the veins and arteries by 

 the circulating blood; and the alveolar processes by the teeth. 

 Divide a locomotive muscle by means of a transverse section, 



• liechcrchci P/i^s. p. 66. f Ibid. p. 77. 



and 



