I STRUCTURE AND PHYSIOLOGY OF ANIMALS 25 



Glands (Fig. 12) are formed for the most part by the modifica- 

 tion of certain cells of epithelia. In many cases a single cell of the 

 epithelium forms a gland, which is then tevmeA e, itniceUular gland 

 (Vig. 12, A). Ihe secretion (or substance which it is the function of 

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

 of the cell and reaches th« 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 muUiceUular— 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 secre- 

 tion 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,- rettform 

 connective tissue, fibrous connective 

 tissue, cartilage, and hone. 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. 13) the ground- 

 substance {g) is of a gelatinous 

 character, sometimes supported 

 by systems of fibres (e/), and the 

 cells are usually stellate or star- 

 shaped with radiating processes. Retiform or reticulctte connective " 

 tissue (Fig 14) consists of stellate or branching cells with pro- 

 cesses which are prolonged into fibres — the fibres from neigh- 

 bouring cells joining so as to form a network. In this form of 

 connective tissue there is no true ground-substance — the inter- 

 spaces between the cells being filled with other tissue elements. 



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

 ground-substance containing gelatin, consisting mainly 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 ihe entire tissue an elastic character. Associated with fibrous 



Fig. 12.— 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 ; B, 

 group of gland-cells lining a depression ; 

 B and F, simple multicellular gland ; 

 G, branched multicellular gland. (From 

 Lang.) 



