108 TEXTBOOK OF ZOOLOGY 



the brushlike ending of the axone of one comes into close proximity 

 with a dendrite of another. In this way an impulse can be trans- 

 mitted from one part of the body to other parts. The chief function 

 of the nervous tissue is to relate the organism to its environment. 



Vascular Tissue. — This is fluid tissue consisting of cells known as 

 corpuscles in a fluid medium called plasma. The cells are the red 

 corpuscles (erythrocytes) and white corpuscles (leucocytes), while 

 the plasma or fluid is the intercellular substance. Blood and lymph 

 are the two common vascular tissues. Lymph has no red corpuscles. 

 In the blood of mammals the red corpuscles are without nuclei ; 

 while in fish, frogs, turtles, and birds these cells are nucleated. The 

 chief function of this tissue is the transportation of digested food 

 and oxygen to the cells of the body and the removal of waste by- 

 products of metabolism from them. 



An organ is an arrangement of two or more tissues as a part of 

 the body which performs some specific function or functions. Some 

 organs are made up of all of the different types of tissues just de- 

 scribed. For example the stomach is an organ with an internal 

 cavity. It is covered and lined with epithelium; the wall contains 

 two strong layers of muscular tissue ; blood vessels carrying blood, 

 and lymph spaces bearing lymph, branch through the wall ; nervous 

 tissue reaches all parts of the organ to receive stimuli and distribute 

 impulses; and connective tissue serves to bind all the others in 

 proper relation. 



A system is an aggregation of organs properly associated and 

 related to perform some general function of life. There are ten 

 different systems usually recognized: 



a. The Integumentary System is composed of the skin and its out- 

 growths, such as hair, nails, scales, horns, hoofs, and similar struc- 

 tures. Its principal purposes are protection, primarily, with some 

 degree of excretion and respiration, some absorption, and regulation 

 of body temperature. 



b. The Skeletal System composes the supporting framework of the 

 body. The bony and cartilaginous tissues make up the material of 

 this system. The vertebral column, skull, ribs, sternum, and bones 

 of the limbs are the general parts of the vertebrate skeletoji, and 

 they serve for the support of the body as a whole and for the pro- 

 tection of the internal, vital organs. 



