28 THE HORSE IN HEALTH AND DISEASE 



and passing it out (urine), but they may even alter it greatly before 

 passing it out (milk). 



4. Ciliated epithelium is characterized by little hair-like proc- 

 esses called cilia (see Fig. 1) . These cilia vary in length, depending 

 upon their location. In the living state they are in constant motion 

 like a grain field waving in the wind, each cilium being bent down in 

 one direction and then again assuming the erect position. All 

 cilia of an organ work harmoniously in the same direction, the 

 motion begins at one end of the surface and ends at the other. 

 The function of this tissue is to work foreign material from the 

 inner parts of the body out and finally to expel it. 



II. Connective tissues are derived from the mesoblast of the 

 embryo. They are primarily to support and hold more highly 

 specialized, active tissues in position. They are composed of but 

 few cells and a large amount of intercellular or cementing sub- 

 stance, which, though outside the cells, has been derived from them. 

 The activity of the constituent cells is confined chiefly to producing 

 and maintaining the tissues and to restoring them when injured. 

 After injury to the physiologically more active tissues with less 

 regenerative power, connective tissues may be produced to take 

 their place. An example is seen in cirrhosis of the liver, where the 

 active gland cells are to some extent replaced with fibrous tissue. 

 Connective tissues may be divided into three classes : fibrous tissue, 

 cartilage tissue, and bone tissue. 



1. Fibrous tissue is composed of bundles of minute fibers. It is 

 of two varieties: white fibrous and yellow elastic. 



(A) The white fibrous is found as a connecting tissue framework 

 in various parts of the body. Tendons and ligaments are composed 

 of it. 



(B) Yellow elastic tissue is firmer than the white fibrous and 

 has more elasticity. It forms the large ligamentum nuchse that 

 stretches from the head to the withers. 



2. Cartilage tissue is glass-hke in appearance, translucent, homo- 

 geneous, tough, and of elastic consistency. It cuts Hke cheese with 

 a knife and is commonly named "gristle." Cartilage cells are 

 peculiar in character and relatively few in number. All cartilage 

 in the body is surrounded by a membrane known as the perichon- 

 drium, a fibrous tissue which carries blood-vessels to nourish the 

 cartilage cells. In the embryo most bones are preceded by carti- 

 lage; later this is usually replaced by bone tissue. The rings of the 



