THE STRUCTURE OF THE ELEMENTARY TISSUES. 7 



cell-substance, but even the nuclei may disappear. The embryonic tis- 

 sue, from which elastic fibres are developed, is composed of fusiform 

 cells, and a structureless intercellular substance by the gradual fibrilla- 

 tion of which elastic fibres are formed. The fusiform cells dwindle in 

 size and eventually 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 trabeculse 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 with- 

 out 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 re- 

 laxation of the flexor muscles which allows the elasticity of the ligamentum 

 nuchaa to come again into play. 



(d.) 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 excep- 

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

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

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



Fatty matter, not in the form of a distinct tissue, is also widely 

 present in the bod} T , e.g., in the liver and brain, and in the blood and 

 chyle. 



Adipose tissue is almost always found seated in areolar tissue, and 

 forms in its meshes little masses of unequal size and irregular shape, to 

 which the term lobules is commonly applied. 



Structure. Under the microscope adipose tissue is found to consist 

 essentially of little vesicles or cells which present dark, sharply-defined 

 edges when viewed with transmitted light: they are about ^-J-g- or -g-fg- of 

 an inch in diameter, each composed of a structureless and colorless 

 membrane or bag, filled with fatty matter, which is liquid during life, 



