26 THE HORSE IN HEALTH AND DISEASE 



The cells composing the animal body are not quite so well de- 

 fined as those in plants, neither are their walls so nearly complete. 

 All cells are of a common descent, but they have acquired widely 

 different characters and functions. This specialization manifests 

 itself by a grouping of cells which have work of a similar nature to 

 perform. For instance, the fat-cells are very passive, and as they 

 perform the function of storing reserve food material they are 

 placed in those parts where least interference will be had with the 

 more active parts. The cells lining the air-passages are very 

 active and their cilia are in continual motion. Other cells, such 

 as certain kinds found in the blood, are still different, and possess 

 the power to move from place to place. By developing along one 

 definite line, cells lose their primitive power to receive and utihze 

 all kinds of food. Nature has, therefore, provided certain stand- 

 ard forms of food for their sustenance. Food is supplied to each 

 cell and waste is removed by a special transporting fluid, the blood. 

 The exact manner by which this exchange is made possible will be 

 discussed in later chapters. 



THE TISSUES OF THE ANIMAL BODY 



A tissue is a collection of more or less similar cells possessing 

 functions somewhat alike. Body tissues are divided into vegetative 

 tissues, which support, bind together, protect, and nourish; and 

 master tissues, which consist of the muscles and nerves. 



A. The vegetative tissues may be divided into (I) epithelial 

 tissues and (II) connective tissues. 



I. Epithelial tissues consist of large numbers of cells and a very 

 small amount of intercellular substance. They develop from the 

 epiblast and hypoblast of the embryo and are found on or near 

 surfaces of the body. There are four kinds of epithehal tissue, 

 namely: (1) squamous epithehum; (2) columnar epithelium; (3) 

 secreting epithelium; (4) ciliated epithelium. 



1. Squamous epithelium is primarily a protective tissue and is 

 composed of flat, scale-like cells. They may be in single layers 

 like the cells lining the air sacs of the lungs; or stratified and com- 

 posed of several layers piled one upon the other like the cells of the 

 skin, hair, horns, hoofs, and nails (see Fig. 1). The upper layers 

 are farthest from the nutrition supply, and so lose their sensitiveness 

 and become hard and act as good protective agents. Poisons can- 



