272 THE TISSUES. 



fibrous tissue to form the areolar tissue, wherever found ; and of the 

 third form of it the middle coat of the large arteries is almost ex- 

 clusively, and that of the smallest in part, formed. It also abounds 

 between the air-cells of the lungs. 



None of the preceding structures are, however, formed of the 

 elastic tissue alone. E.g. between the yellow fibres of the liga- 

 menta subflava, "which are not collected into either fasciculi or lamellce, 

 but are continuously connected throughout the entire thickness of 

 each yellow ligament, there is interposed some white fibrous tissue ; 

 upon the whole in small quantity, but demonstrable in every pre- 

 paration, and occurring in the form of lax, undulating fasciculi, 

 which are arranged parallel with the principal direction of the 

 elastic fibres." (KolUker, p. 284.) 



Distribution of the Elastic Tissue in the Lower Animals. 



In all the vertebrated classes this tissue is found in the same 

 localities as in man, and in some places besides as in the ligaments 

 of the cat's claw, and in the alary membrane of the mammals. 



In large animals, as the elephant and rhinoceros, the yellow fibrous 

 tissue is employed in the form of a belt to support the abdominal 

 parietes. (QuecJcett.) In the ligamentum nuchse of the giraffe, before 

 alluded to, the fibres are marked with transverse striae, extending 

 through about the central third of their width. The internal por- 

 tions were, however, made up entirely of the common plain fibres. 

 Similar striated fibres also are found in the rhinoceros, and the 

 sheep ; and even in arteries. 



A variety of this tissue constitutes the ligament supporting the 

 expanded wings of the larger birds, as the eagle, crane, heron, &c. 

 It also exists in the lungs of birds. 



It is a ligament of elastic tissue which in the bivalve mollusca 

 keeps the valves open whenever the adductor muscle ceases to close 

 them by its contraction. In the oyster this tissue is placed within the 

 hinges, and therefore is compressed whenever the valve is closed. 

 Hence the compressed elastic tissue forces the valves apart when 

 the muscle ceases to keep them closed. In the cockle the elastic 

 tissue is placed externally to the hinge, and being stretched when 

 the valves are closed, pulls them open by its elasticity whenever the 

 adductor muscle ceases to act. 



The form of this tissue occurring in the middle coat of arteries, 

 was found in that of the aorta of a whale to be 1J inch thick; the 

 diameter of the vessel being 12 inches, and its length over 50 feet. 



Development of Elastic Tissue. 

 The supposition of Schwann, that this tissue is developed from 



