148 FUNDAMENTALS OF STRUCTURE AND FUNCTION 



stantly in motion, we must classify them as tissues, for they contain 

 living cells or corpuscles of various kinds, carried about in a fluid 

 matrix or plasma. These tissues are of the utmost importance to 

 animals, as it is only by means of them that the living cells of the body 

 receive nourishment and oxygen, and get rid of their wastes. 



The Nervous Tissues. Even in its simplest form we have seen 

 that protoplasm is sensitive and responds to stimuli. In higher 

 animals this sensitivity and conductivity of sensations is taken over 

 by the nervous tissues. The unit of structure is the neuron, or nerve 

 cell. The elongated fibers from these cells are bound together into 

 nerves or conducting ])athways for nerve impulses. All parts of the 

 vertebrate body, with the exception of the cartilages and epidermal 

 derivatives, are supplied with nervous tissue, which may be said to 

 be the master tissue of the body. 



Reproductive Tissues. These cells which, as one author puts 

 it, are "within the body though perhaps not of the body," form tis- 

 sues, eggs and sperms, that have to do with the futures of all animals. 



Why Are Living Organisms So Called? 



In the preceding pages, we have referred to living things as organ- 

 isms. The anatomist calls collections of tissues, which do specific 

 kinds of work, organs. The hand is an example of an organ which is a 

 collection of tissues. Muscles are attached to the hones by means of 

 tendons and bones are joined together by ligaments. The skm, 

 composed of several different kinds of tissue cells, is supplied with 

 blood and nervous tissues, while the whole organ is interlaced through 

 and through with other connective tissues. Living things are made up 

 of organs, and we call them organisms. The living world about us, 

 plant and animal, is a collection of organisms, some very simple, 

 others aggregates of simple cells, still others formed of untold billions 

 of differentiated cells, grouped into tissues forming an organism, 

 such as an insect, a fish, a tree, or a man. Yet all these different and 

 complex entities basically are made of the living stuff called proto- 

 plasm. In animals, this grouping of organs which are united in the 

 performance of some general function gives us a number of organ- 

 systems. There is, for example, the integumentary system, or outer 

 body covering ; the supporting system, which forms the body frame ; 

 the systems which have to do with the nutrition of the body, the 

 digestive, respiratory, circulatory, and excretory systems ; the nervous 

 system, which controls the activity of the body ; and the reproduc- 



