ANIMAL TISSUES. 369 



The white fibrous tissue is (when perfectly cleared of the areolar) 

 of a silvery lustre, and is composed of bundles of fibres running for 

 the most part in a parallel direction ; but if there be more than one 

 plane of fibres, they often cross or interlace with each other : in some 

 specimens it is difficult to make out the fibres distinctly, except in cer- 

 tain lights ; and in these cases it appears that this tissue may be com- 

 posed of a longitudinally striated membrane, which may be now and 

 then split up into fibres. The white fibrous tissue is principally em- 

 ployed in the formation of ligaments and tendons, a purpose for 

 which it is admirably fitted on account of its inelasticity ; it also is 

 concerned in the formation of fibrous membranes, viz. the pericardium, 

 dura mater, periosteum, perichondrium, the sclerotic coat of the eye, 

 and all the different fasciae. It is sparingly supplied with blood-vessels 

 and nerves : the former always run in the areolar tissue, connecting 

 the bundles of fibres together ; but in the generality of the fibrous 

 tissues the blood-vessels are not well seen, except in the dura mater 

 and in the periosteum. 



The yellow fibrous tissue is highly elastic ; it consists of bundles 

 of fibres covered with, and connected together by, areolar tissue : the 

 fibres are of a yellow colour, in some cases round, in others flattened ; 

 they are not always parallel, but frequently bifurcate and anastomose 

 with other neighbouring fibres. It is always rather difficult to separate 

 the fibres from each other ; and when they are separated, the elasticity 

 of each individual fibre is shown by its tendency to curl up at the end. 

 The fibres in the human subject vary in diameter from the 1 -5000th 

 to 1-1 0,000th of an inch. The acetic acid of ordinary strength does 

 not act on the yellow fibrous tissue ; nor after maceration in water or 

 spirit for a very long time does its elasticity diminish. Very long 

 boiling is said to extract from it a minute quantity of a substance 

 allied to gelatine ; neither nuclei nor a trace of cells can be seen in it 

 after the addition of acetic acid : these are readily seen when white 

 fibrous element is treated with this acid. 



Muscular Fibre. There are three different kinds of muscular fibre 

 found in the animal body: 1st, muscle of the skeleton; 2d, muscle of 

 the heart ; and 3d, muscle of the stomach, intestines, &c. The func- 

 tions of muscular substance may be referred to two kinds voluntary 

 and involuntary. The muscles endowed with voluntary power are those 

 of the skeleton; the involuntary are those of the heart, stomach, &c. 



Muscular fibre is held together by a very delicate tubular sheath, 

 nearly resembling simple structureless membrane. It cannot always 

 be discerned ; but when the two ends are drawn asunder it will be 



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