106 TEXTBOOK OF ZOOLOGY 



from these various glands lubricate the surfaces, contain enyzmes 

 for digestion of food, supply regulatory substances directly to the 

 blood, serve as poison to other animals, and some are repellent to 

 enemies. 



Sustentative Tissue. — This type comprises all tissues whose func- 

 tion is to bind together or support the various parts of the body. 

 Connective tissue is, in most cases, composed of slender cells with 

 an abundance of intercellular material. This tissue is almost uni- 

 versally present in the various organs throughout the body. Ten- 

 dons, the tough cords that connect muscles to bones, of which the 

 "hamstring" is a good example, and much of the dermis of the skin 

 are composed of connective tissue. Bone and cartilage, which make 

 up the framework of the body and support the other tissues, are 

 called supporting tissues. In crayfishes and grasshoppers the sup- 

 porting tissue is chitin instead of bone or cartilage. Cartilage is com- 

 posed of scattered cells interspersed with abundant, homogeneous, 

 granular, semisolid matrix or intercellular substances. Bone is some- 

 what similar, except that the matrix has been replaced by a heavy 

 deposit of calcium phosphate and calcium carbonate, two solid salts. 

 The scattered cells are present as bone cells. 



Muscular or Contractile Tissue. — This is distinctive because of 

 its ability to contract and in that way produce movements. Cells 

 adapted to this function are more or less elongated and fiberlike. 

 There are three types of muscular tissue : smooth, involuntary, and 

 nonstriated, as found in the wall of the intestine ; striated, volun- 

 tary, skeletal, as found in the muscle of the arm ; and striated, in- 

 voluntary, cardiac, as found in the wall of the heart. Skeletal, 

 voluntary muscle is made up of large multinucleate (many nuclei) 

 fibers, each composed of many fibrils (myofibrils) along which are 

 evenly distributed dense and light areas, which give the general 

 appearance of stripes across the cell, because the dense areas on the 

 adjacent fibrils come at the same level. The smooth involuntary 

 muscle is composed of individual, spindle-shaped (fusiform) cells, 

 the cytoplasm of which is largely myofibrils but without striations 

 and therefore smooth. There is a single oval nucleus, centrally lo- 

 cated. The outer membrane of a muscle cell is the sarcolemma. 

 The cardiac involuntary muscle is said to be made up of individual 

 cells, highly modified in arrangement. The definition of cells in 



