STRUCTURE OF THE ELEMENTARY TISSUES. 35 



either by part of the cells becoming fibrils, the others remaining as con- 

 nective-tissue corpuscles, or by the fibrils being developed from the out- 

 side layers of the protoplasm of the cells, which grow up again to their 

 original size and remain imbedded among the fibres. This process gives 

 rise to fibres arranged in the one case in interlacing networks (areolar 

 tissue), in the other in parallel bundles (white fibrous tissue). In the 

 mature forms of purely fibrous tissue not only the remnants of the cell- 

 substance, but even the nuclei may disappear. The embryonic tissue, 

 from which elastic fibres are developed, is composed of fusiform cells, and 

 a structureless intercellular substance by the gradual fibrillation of which 

 elastic fibres are formed. The fusiform cells dwindle in size and event- 

 ually disappear so completely that in mature elastic tissue hardly a trace 

 of them is to be found: meanwhile the elastic fibres steadily increase in 

 size. 



Another theory of the development of the connective-tissue fibrils 

 supposes that they arise from deposits in the intercellular substance and 

 not from the cells themselves; these deposits, in the case of elastic fibres, 

 appearing first of all in the form of rows of granules, which, joining 

 together, form long fibrils. It seems probable that even if this view be 

 correct, the cells themselves have a considerable influence in the produc- 

 tion of the deposits outside them. 



Functions of Areolar and Fibrous Tissue. The main function 

 of connective tissue is mechanical rather than vital: it fulfils the subsidi- 

 ary but important use of supporting and connecting the various tissues 

 and organs of the body. 



In glands the trabeculae of connective tissue form an interstitial frame- 

 work in which the parenchyma or secreting gland-tissue is lodged: in 

 muscles and nerves the septa of connective tissue support the bundles of 

 fibres, which form the essential part of the structure. 



Elastic tissue, by virtue of its elasticity, has other important uses: 

 these, again, are mechanical rather than vital. ^ Thus the ligamentum 

 nuchas of the horse or ox acts very much as an India-rubber band in the 

 same position would. It maintains the head in a proper position without 

 any muscular exertion; and when the head has been lowered by the action 

 of the flexor muscles of the neck, and the ligamentum nuchae thus 

 stretched, the head is brought up again to its normal position by the 

 relaxation of the flexor muscles which allows the elasticity of the liga- 

 mentum nuchae to come again into play. 



(a.) Adipose Tissue. 



Distribution. In almost all regions of the human body a larger or 

 smaller quantity of adipose or fatty tissue is present; the chief exceptions 

 being the subcutaneous tissue of the eyelids, penis, and scrotum, the 

 nymphae, and the cavity of the cranium. Adipose tissue is also absent 

 from the substance of many organs, as the lungs, liver, and others. 



