286 - Multicellular Animals, Especially Man 



C 



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 of skeletal muscle are syncytia, 

 containing man) nuclei (Fig. 15-14). In skel- 

 etal muscle, the nuclei are found at regular 

 intervals near the surface of the fiber, just 

 subjacent to a well-marked membrane, the 

 sarcolemma. In skeletal and cardiac muscle, 

 but not in visceral muscle, one can see a 

 number of transverse striations (Fig. 15-14). 

 These striations look like a series of parallel 

 bands, passing across the fibers, but thev are 

 not actual partitions subdividing the fiber 

 into discrete sections. The nature and fine 

 structure of the striations will be discussed 

 in Chapter 21. 



Cardiac muscle is found only in the verte- 

 brate heart. The general structure of the 

 tissue is like that of skeletal muscle, except 

 that the fibers of cardiac muscle are inter- 

 rupted periodically along the length by zig- 

 zag cross membranes, called intercalated discs 

 (Fig. 15-14), and the nuclei are deeplv em- 

 bedded near the central axis of the fiber. 



Functionally, the three types of muscle are 

 also quite different. The action of skeletal 



