CHARACTERISTICS OF ANIMALS 29 



presence of enzymes, zymins, or hormones. During the activity 

 incidental to digestion these cells give off or secrete large quanti- 

 ties of enzymes. When this process ceases, the cells, now in the 

 so-called resting stage, are storing up their product for future 

 use. Certain granules, which disappear with glandular activity, 

 are responsible for the action of the secretion. 



(c) Excreting epithelium, which has the power to pass sub- 

 stances out of the body that are of no further use. Examples 

 are the epithelium of the kidneys, sweat glands, and mammary 

 glands. It not only is capable of taking up material from the 

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

 before passing it out (milk). 



4. Ciliated epithelium, which is characterized by little hair- 

 like processes called cilia (see Fig. 2) . 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 

 is first bent down in one direction and then assumes 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 

 a*t the other. The function of this tissue is to work substances 

 from the inner parts of the body outward. It is found lining 

 the air passages and in the female genital tract, where it aids the 

 passage of the ovum from the ovary to the womb. 



II. Connective tissues develop from the mesoblast of the em- 

 bryo and are found in all paits of the body. They are piimarily 

 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 substance, which, though outside 

 the cells, has been derived from them. The activity of the con- 

 stituent 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. 

 Examples are seen in cirrhosis of the liver, where the active gland 

 cells are to some extent replaced with fibrous tissue, and in scars. 

 Connective tissues may be divided into three classes: fibrous 

 tissue, cartilaginous tissue, and bony tissue. 



1. Fibrous tissue is composed for the most part of bundles of 

 minute fibers. It may be differentiated as follows: 



(a) White fibrous tissue, which is silvery white in appearance, 



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