112 PRIiXrrPLES (W ANIMAL BIOLOGY 



It is not uncommon that glands produce two or more secretions. If 

 so, there are usually as many kinds of cells as there are secretions produced. 

 When stained the different types of cells can usually be distinguished by 

 their form and appearance. Glands producing two secretions are to 

 be found in the cardiac end of the frog's stomach, and the pancreas pro- 

 duces three secretions. 



Glands may be of microscopic size or they may be large. Examples 

 of large glands are the spleen, pancreas, kidneys and liver, the last of 

 which is the largest gland of the body in vertebrates. The large size 

 of these organs is not due, however, entirely to the amount of secreting 

 tissue present, for most large glands as the liver and kidneys are com- 

 posed of several kinds of tissue in addition to the epithelium. The addi- 

 tional portions include connective tissue, blood vessels and nerves. These 

 tissues have only an accessory function, for in each case the secreting 

 tissue is epithelium. 



In view of the morphology of secreting organs, as described above, 

 a gland may be defined as a structure which is derived from an epithelial 

 surface and whose function is the production of a secretion or an excretion. 



Fig. 74. — Smooth muscle cells. 



Sustentative Tissues. — The sustentative tissues are all those which 

 give support to the body and its parts, or connect and bind together cell 

 layers, organs, and systems. They may take the form of rigid supporting 

 tissues such as cartilage or bone; dense connective tissue such as tendons 

 or ligaments; spongy, fibrous, or elastic connective tissues; or adipose 

 tissue for storage of fat. In lower animals simpler types of rigid support- 

 ing tissues may be noted but they are omitted from this account. The 

 sustentative tissues find their highest development among the vertebrates. 



Contractile Tissues. — Under the term contractile tissues are included 

 all tissues which by their contraction serve to produce motion of an 

 organ or part, or locomotion of the animal as a whole. Among lower 

 animals many individual cells, as the porocytes which control the size 

 of the pores through which water enters a sponge, have the power of 

 contraction. Such scattered cells do not form tissues. 



Muscle. — In manj'- invertebrates and in all vertebrates contractile 

 cells are aggregated into sheets, layers or groups called muscles. Two 

 general structural types of muscles may be recognized, namely, smooth 

 and striated. Smooth muscle cells. Fig. 74, are somewhat spindle shajTed 

 with undivided or with branching ends, and possess one nucleus to 



