CELLULAR OR FIBROUS TISSUE. 331 



a darker and stronger outline, as well as of their following 

 a more curled and tortuous course. (See Plate XXXIX. 

 fig. 7.) Acetic acid applied to a portion of mixed cellular 

 tissue, at once allows the elastic fibres to be clearly seen, 

 rendering the inelastic fibres transparent, and almost in- 

 visible. 



There are several parts of the human organisation de- 

 scribed by modern minute anatomists and physiologists as 

 being in part composed of unstriped muscular fibre ; these 

 are the skin, dartos, nipple, clitoris, penis, the ducts of the 

 larger glands, as the ductus communis choledochus, the 

 ureters, and vasa deferentia. Now, I find that all these 

 parts, which I have examined with care, owe their contrac- 

 tility, and their power of erection, to the presence of the 

 nucleated form of the elastic tissue which has been described 

 as existing in tendons and the smaller blood-vessels, and 

 not to any form of muscular fibre ; and further, that in the 

 majority of them, and especially in the dartos, penis, clitoris 

 and nipple, this elastic tissue is confined almost entirely to 

 the blood-vessels, the walls of which it constitutes r this 

 fact may be readily ascertained in the instance of the dartos 

 by taking a small fragment of that membrane when in a 

 fresh condition, and having spread it out on the surface of 

 a piece of glass without the addition of any fluid, then 

 submitting it to the microscope, when the number, size, 

 and course of the blood-vessels may be traced, and the 

 disposition of the intervening fibrous inelastic tissue recog- 

 nised : in the recent state, however, the vessels are filled with 

 blood, the presence of which prevents the satisfactory de- 

 tection of the elastic constituent of the blood-vessels : if now, 

 however, acetic acid be applied to the fragment of membrane 

 thus spread out, the inelastic fibrous tissue will become in- 

 distinct, the red blood corpuscles contained in the vessels 

 will be dissolved, and the tissue of the blood-vessels clearly 

 brought out. (Plate XLIII.^. 3.) 



The extent of contraction of which the dartos is suscep- 

 tible is very great, and the act of contraction must of course 

 exert a very powerful influence over the circulation of the 



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