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COLLEGE ZOOLOGY 



Muscle or contractile tissue 



Muscle tissue (Fig. 43) is composed of 

 cells specialized for contraction. Muscle cells 

 possess fibrils which are able to contract 

 with great force. The fibrils are usually of 

 two kinds: (1) striated and (2) smooth, 

 without striations. The latter are found in 

 smooth muscles of the simpler inactive 

 animals, and in those internal organs of 

 higher organisms not under voluntary con- 

 trol, such as the walls of the blood vessels 

 and the urinary bladder; they are therefore 

 also known as involuntary muscles. Striated 

 muscles are of two types : ( 1 ) skeletal mus- 

 cles, which are, for the most part, under 

 control of the brain and are hence called 

 voluntary muscles; and (2) cardiac muscle, 

 which occurs in the heart and is involun- 

 tary. 



Nervous tissue 



Nervous tissue is composed of cells spe- 

 cialized for the reception of stimuli and the 

 transmission of impulses (Fig. 43). All pro- 

 toplasm is irritable, as in the amoeba, but 

 in the metazoans certain cells are specialized 

 for the sole purpose of performing nervous 

 functions. This is the most highly special- 

 ized tissue in animals. 



Vascular tissue 



Vascular tissue (Fig. 43) is a fluid tissue 

 which is composed of white blood cells 

 (leukocytes), red blood corpuscles (erythro- 

 cytes), blood platelets, liquid plasma, and 

 lymph. Blood plasma transports various 

 substances to the cells of the body; erythro- 

 cytes carry oxygen to the tissues; and the 

 leukocytes may move about somewhat like 

 amoebas and engulf bacteria and other parti- 

 cles that get into the blood plasma. The 

 tissue fluid is an accessory to the blood 

 proper; it arises from the blood by diffusion 

 through the walls of the capillaries into the 



tissue spaces, and it is the fluid medium in 

 which the individual cells live. Lymph con- 

 sists of tissue fluid and leukocytes which 

 have entered the lymphatic vessels; these 

 vessels return the lymph to the blood 

 stream. 



ORGANS AND SYSTEMS 

 OF ORGANS 



An organ is an aggregate of tissues ar- 

 ranged in a characteristic structural plan, 

 which performs one or more special func- 

 tions. For example, the human intestine 

 (Fig. 43) is a digestive organ; it consists of 

 a variety of tissues, including epithelial, con- 

 tractile (muscles), nervous (nerves), vas- 

 cular (blood), and fibrous connective. How- 

 ever, the lining of epithelium is of primary 

 importance, for the digestive glands are an 

 outgrowth of it. 



Protozoa carry on physiologic processes 

 without the presence of definite organs, but 

 in most of the metazoans many organs are 

 usually necessary for the performance of a 

 single function; for example, the proper di- 

 gestion and absorption of food in man re- 

 quire a large number of organs collectively 

 known as the digestive system. Similarly, 

 other sets of organs are associated for carry- 

 ing on other functions. The principal sys- 

 tems of organs in man and in other higher 

 animals, and their chief functions, are as 

 follows. 



Digestive system (digestion 

 and absorption) 



In the mouth, the teeth, assisted by the 

 tongue, masticate food. Salivary glands and 

 glandular cells of the mouth furnish saliva. 

 The food passes through the pharynx and 

 esophagus into the stomach where mucin 

 and gastric juices are added to it; here it 

 undergoes mechanical and chemical changes. 

 Digestion continues in the small intestine. 



