512 



SEROUS AND SYNOVIAL MEMBRANES. 



such an inference. Observation shows that 

 in the human body a variety of structures 

 exist, which are united by the characteristics 

 not only of considerable analogy of office, but 

 also of similarity of structure, almost complete 

 identity of chemical composition, and intimacy 

 of pathological relations. 



Adopting the possession of these common 

 properties as a natural and safe basis of 

 classification, we form a group in which are 

 included all those tissues which serve to limit, 

 define, or facilitate movement. The class of 

 structures thus constituted was formerly 

 termed " the Cellular System ; " but the cel- 

 lularity which the name connotes, as it was 

 never supposed to be predicable of all its 

 members, so it is now known to be erro- 

 neously used of that part of them to which it 

 was originally applied ; and they have there- 

 fore been preferably arranged under the head 

 of " Passive Organs of Locomotion." And 

 if any should consider this term open to the 

 lesser objection of specifying a general, but 

 not essential fact, that of " Passive Organs of 

 Movement " might be again substituted. 



On this view we may regard serous mem- 

 branes as forming one of a group of tissues. 

 A further analysis of this group shows it to 

 be composed of several members, separated 

 from each other by differences, in which we 

 may recognise a progressive, though some- 

 what interrupted, series of gradations. These 

 differences we shall now proceed rapidly to 

 trace. 



Two important microscopical elements 

 pervade all these structures, and will there- 

 fore demand some attention. These are the 

 white and yellow fibrous tissues. 



The white fibrous tissue (fig. 395. a) consists 

 of bands or bundles of a very variable width, 

 which, unless artificially stretched, take a 

 sinuous or wavy course ; and, at distant in- 

 tervals, include cell-nuclei in their substance. 

 They are marked with striae, which take the 

 direction of their length, and, by their mutual 

 proximity, give a fibrous or fibrillated ap- 



Fig. 395. 



a, White fibrous tissue ; b, Yellow fibrous tissue. 



After Todd and Bowman. Magnified 320 diame- 

 ers.) 



pearance to the whole mass. But these 

 markings are not exactly parallel to the 

 borders of the band; and since the tissue, 

 though easily divided longitudinally to almost 

 any degree of minuteness, cannot be split up 

 into uniform and definite fibrils of a diameter 

 corresponding with the transverse width which 

 intervenes between one of these striae and 

 another; and since it is also swelled up into 

 one shapeless and semitransparent mass by 

 the action of acetic acid ; it seems highly 

 probable that they are limited to the surface 

 of the bundle, or its immediate neighbourhood. 

 At any rate, they do not sufficiently divide 

 the mass to give it a filamentous constitution, 

 or to render it " fibrous " in the true sense ol 

 the word. 



The yellow fibrous tissue (fig- 395. 6) is 

 contrasted with the preceding form, not only 

 by its colour, but equally by its minute struc- 

 ture and properties. It consists of separate 

 fibres, the size of which varies considerably 

 in different parts, and, in a lesser degree, in 

 any one specimen. They are exceedingly 

 disposed to curl up, often assuming almost a 

 spiral form ; and are rendered very distinctly 

 visible by the dark margin which their great 

 refracting power gives them. Their branchings 

 are generally dichotomous, and the processes 

 thus given off* are of a size which nearly equals 

 that of the original stem ; and they may often 

 be traced to their union with neighbouring 

 ones, so as to form a kind of trellis- work. 



The first form exists alone in tendons, 

 ligaments, and the stronger fasciae latae ; its 

 inextensibility and strength admirably adapting 

 it to the use of mere passive resistance to an 

 external force. The second is highly elastic, 

 whence it is often termed " the elastic tissue :" 

 it is chiefly found where, along with a certain 

 amount of yielding, is also required a complete 

 restoration of the previous state without any 

 further expenditure of muscular force, the 

 long duration of an action often rendering it 

 advantageous to avoid the fatigue which the 

 constant exercise of volition and muscle would 

 imply. And as these conditions are much 

 rarer than the simply mechanical wants which 

 the preceding form is destined to supply, so 

 also is the tissue which fulfils them, being 

 found separately in but a few parts of the 

 body; viz. in the ligamenta subfiava, and in 

 certain portions of the vocal and respiratory 

 apparatus. Here it is in sparing quantity ; 

 but in the vast ligamentum nuchae, which 

 suspends the ponderous heads of the horned 

 graminivora, the uses of the tissue are ex- 

 emplified in a very striking manner. 



Chemically, they are distinguished by the 

 white fibrous tissue containing much gelatine, 

 or rather yielding it by boiling ; while, from 

 the yellow, none can be obtained. They are 

 both little disposed to putrefaction, and retain 

 their peculiar physical properties almost un- 

 impaired by time.* 



* In an Egyptian mummy, I lately found these 

 tissues (after moistening) displaying as perfect a 

 structure as a specimen of yesterday could have 

 done. 



