



[ui 



STRUCTURE AND PHYSIOLOGY OF ANIMALS 23 



the gland to form or collect) gathers in such a case in the interior 

 of the cell, and reaches the surface of the epithelium through a 

 narrow prolongation of the cell which serves as the duct of the 

 gland (B). In other cases the gland is multicellular formed of a 

 number of cells of the epithelium lining a depression or infolding, 

 simple or complex in form, of the latter (D-G). In the central 

 cavity of such a gland the secretion collects to reach the general 

 surface or cavity lined by the 

 epithelium through the passage 

 or duct. 



A series of tissues in which the 

 cells are, in most instances, sub- 

 ordinate, as regards bulk, to sub- 

 stances formed between them, is 

 the group known as the con- 

 nective tissues, including gela- 

 tinous connective tissue, retiform 

 connective tissue, fibrous connective 

 tissue, cartilage, and bone. In the 

 majority of forms of connective 

 tissue the cells lie embedded in 

 an intermediate substance called 

 the matrix or ground-substance 

 of the connective tissue. 



In the case of gelatinous con- 

 nective tissue (Fig. 10) the ground 

 substance (g) is of a gelatinous 

 character, sometimes supported 

 by systems of fibres (5 2 ), and the 

 cells are usually stellate or star- 

 shaped with radiating processes. 

 Retiform or reticulate connective 

 tissue (Fig. 11) consists of stellate 

 or branching cells with processes 

 which are prolonged into fibres 

 the fibres from neighbouring cells joining so as to form a net- 

 work. In this form of connective tissue there is no true ground- 

 substance the interspaces between the cells being filled with 

 other tissue elements. 



Fibrous connective tissue, which is a very common form, has a 

 ground-substance containing gelatin, and consisting of numerous 

 fibres, usually arranged in bundles. Thicker yellow elastic fibres 

 may be present among the others, and may be so numerous as to 

 give the entire tissue an elastic character. Associated with fibrous 

 tissue, and produced by modification of its cells is adipose tissue or 

 fat (Fig. 12). Fat consists of masses of large cells in which the 

 protoplasm has more or less completely become replaced by fat, 



FIG. 9. Diagram to illustrate the structure 

 of glands. A, unicellular glands in an 

 epithelium ; B, unicellular glands lying 

 below epithelium and communicating 

 with the surface by narrow processes 

 (ducts) ; C, group of gland-cells ; D, 

 group of gland-cells lining a depression ; 

 E and F, simple multicellular gland ; 

 G,-.braiiched multicellular gland. (From 

 Lang.) 



