CONNECTIVE TISSUES. 7 



or case for the organism, an exoskeleton, as seen in the 

 Crustacea and insects. 



Glandular epithelium is that epithehum in which some cells 

 secrete not a solid but a liquid substance. In the most rudi- 

 mentary cases the gland is formed by a single epithelial cell, 

 the secretion passing out by either a special opening or through 

 the superficial membrane. Several of these cells may arrange 

 themselves around a central space and pour their secretion into 

 it ; the gland then forms a blind invaginated sac opening to the 

 exterior or interior by the neck of the whole glandular mass. 

 From this simple gland a compound gland is built up by re- 

 peated regular or irregular outgrowths. The terminal portion 

 of each gland is converted into a duct in most glands, for the 

 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 fiJ)rillar-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 fibrillaj are treated with 

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

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

 predominate so as to form elastic tissue, which branches and 

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

 mentum nucJiw 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. 



Cartilarje is also a true connective tissue, and may be told by 



