METABOLISM IN VERTEBRATES 



may become compound by the formation of outpocketings along their lengths. 

 The glands related to digestion, and many others, are divided into secretory 

 portions and the tubes, or ducts, by way of which the secretion passes out to 

 the cavity or surface where it is used. Not all glands possess ducts; those 

 that do not, the ductless glands, will be discussed in the next chapter (p. 95). 

 The gastric glands, present in the mucous membrane of the stomach, and the 

 intestinal glands that occur in the mucous membrane of the duodenum are 

 simple tubular glands. The liver is a compound tubular gland; the pancreas, 

 a compound alveolar gland. 



Sustentative Tissue. The sustentative tissues, often called the connec- 

 tive tissues, are a very heterogeneous group, classed together because they are 

 all derived during development from the same source — the stellate mesen- 

 chyme cells (Fig. 3.155). In general, sustentative tissues function in sup- 

 porting the body and connecting or binding together its parts. This group of 

 tissues is characterized by the large amount of intercellular material produced 

 by the cells. In the vertebrates this intercellular material is responsible for 

 the supporting and connecting qualities. Sustentative tissue may be divided 

 into four subclasses: connective tissue in the restricted sense, cartilage, bone, 

 and adipose tissue. 



Connective tissues are of three kinds: mucous connective tissue, in which the 

 intercellular material is gelatinous, is found in the umbilical cords of mam- 

 mals (Fig. 3.15/1); reticular connective tissue, in which there is a meshwork of 

 connective tissue cells with the interspaces filled with other types of cells, forms 

 the framework of organs like the spleen (Fig. 3.156*); and fibrous connective 

 tissue, in which the intercellular material is composed of fibers, is distributed 

 widely as a binding tissue in many organs. The intercellular fibers of fibrous 

 connective tissue are of two kinds, collagenous and elastic. The collagenous 

 or white fibers are very fine and occur in bundles, whereas elastic or yellow 

 fibers are thicker and occur singly. Fibrous connective tissue in which both 

 collagenous and elastic fibers occur is found in the submucosa of the digestive 

 tract and in the dermis of the skin (Figs. 3.15/) and 3.25). Fibrous con- 

 nective tissue in which collagenous fibers predominate is found in tendons, 

 and that containing chiefly elastic fibers is found in the walls of larger 

 arteries and in certain ligaments (Fig. 3.15£'). The cells of fibrous connective 

 tissues are spindle-shaped or irregular in outline and possess relatively little 

 cytoplasm. 



The second subclass of sustentative tissues is cartilage, which is a supporting 

 tissue. The intercellular material in cartilage is sometimes hardened by 

 impregnation with inorganic salts, chiefly those of calcium. Here the cells 

 are more or less rounded and lie in spaces known as lacunae. When the 

 matrix between the cells is translucent and apparently structureless, the tissue 

 is called hyaline cartilage or gristle (Fig. 3.15G). Hyaline cartilage is found 

 at the ends of long bones, at the ends of ribs, and in the cartilages of the nose 

 and trachea. The cartilage of the external ear contains elastic fibers in its 

 matrix and, therefore, is known as elastic cartilage (Fig. 3.15//); that found 



61 



