. Principles of modern biology. Biology. D EHm. Fig. 15-13. Structure of glands; the actual gland cells are shaded in each figure. A, single gland cells scattered among ordinary epithelial cells; B, simple multicellular gland, consisting of a group of gland cells lining a slight invagina- tion of the epithelium; C, simple tubular gland; D, simple alveolar gland; E, compound tubular gland (large glands, such as the liver and pancreas, are of this type, with thousands of secreting alveoli); d, duct. epithelial outpocketings (Fig. 15-13). Some multicellular glands are simple glands, like the gast
. Principles of modern biology. Biology. D EHm. Fig. 15-13. Structure of glands; the actual gland cells are shaded in each figure. A, single gland cells scattered among ordinary epithelial cells; B, simple multicellular gland, consisting of a group of gland cells lining a slight invagina- tion of the epithelium; C, simple tubular gland; D, simple alveolar gland; E, compound tubular gland (large glands, such as the liver and pancreas, are of this type, with thousands of secreting alveoli); d, duct. epithelial outpocketings (Fig. 15-13). Some multicellular glands are simple glands, like the gastric glands of the stomach, being com- posed of epithelial cells (Fig. 15-13C); but others are compound glands (Fig. 15-13E), which contain a significant amount of con- nective tissue between the epithelial chan- nels. Likewise compound glands are inde- pendently supplied with nerves and blood vessels, and each is covered by an epithelial membrane. Muscle Tissues. The three kinds of muscle tissue of vertebrate animals are: (1) visceral muscle, which is found in most of the in- ternal organs, or viscera; (2) skeletal muscle, which occurs mainly in the both wall, con- nected to the bones and other skeletal pails; and (3) cardiac muscle, which is found only in the heart. All muscle tissues are character- ized by elongate cells or fibers, which gener- ate movement by shortening, or contracting, in a forcible manner. No force is developed during the relaxation ol a muscle fiber; in other words, muscle fibers work, not by push- ing, but by pulling. All muscle tissues appear to generate a contractile force by the shorten- ing of numerous delicate protoplasmic fibrils, the myofibrils, which are present in the fibers. The myofibrils run lengthwise through the fluid protoplasm, or sarcoplasm, of the fibers, as can be demonstrated by proper staining, in all varieties of muscle tissue. In visceral muscle, each fiber is a single spindle-shaped cell (Fig. 15-14), but the indi- vidual fibers o
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