CONNECTIVE TISSUES. 7 



carrying away of the fluid secreted. Some glands, however, are 

 ductless or blind (spleen, &c.) 



2. Connective Tissues are those which connect and surround 

 other tissues, and act as supporting and skeletal structures. 

 The presence of intercellular substance distinguishes this group. 

 This intercellular matter is secreted by the whole of the cells 

 which it surrounds, and is very Variable both in consistency 

 and in structure. One variety is known as fibrillar-connective 

 tissue, in which elongated cells are embedded in a solid inter- 

 cellular substance broken up into bundles of fibres. In liga- 

 ments and tendons the fibres have a wavy outline, and are 

 parallel in arrangement. When the fibrillae are treated with 

 acids, they swell up, and a second form, which resists these 

 reagents, appears. These threads are elastic fibres, and may 

 predominate so as to form elastic tissue, which branches and 

 forms a network, sometimes of great strength, such as the liga- 

 nientum nuchce of the neck — the ligament by which the head 

 of quadrupeds is held up in a horizontal posture : at other times 

 they spread out, forming the so-called " fenestrated membranes '' 

 of Henle in the arteries. The two most important skeletal 

 tissues are cartilage and bone. 



Cartilage is a true connective tissue, and may be distinguished 

 by its spherical cells and gristly intercellular substance in which 

 the cells are embedded. We can recognise three distinct kinds 

 of this cartilage — hyaline, fibrous, and elastic. The cells of 

 cartilage are placed in clear round spaces. Its varieties will 

 be pointed out when we come to more special parts. Sufiioe to 

 say here that it is found in both of the great divisions of the 

 animal kingdom, and may even constitute the entire skeleton of 

 some of the fish [Elasmohranchii). 



Osseous tissue, or bone, is hard and possesses a high degree of 

 rigidity, through the intercellular substance being hardened by 

 the deposition of carbonate and phosphate of lime, these salts 

 constituting about two-thirds of the weight of bone. The cells 

 (the bone-corpuscles) occupy spaces in this intercellular matter. 



