34 THE HUMAN BODY. 



"nervous impulse" and so can excite in turn parts distant 

 from the original exciting force. Nerve-fibres place, on 

 the one hand, the irritable tissues in connection with, the 

 automatic, co-ordinating, and sensory; and on the other 

 put the three latter in communication with the muscular, 

 secretory, and other tissues. 



9. Protective Tissues. These consist of certain cells 

 lining cavities inside the body and called epithelial cells, 

 and cells covering the whole exterior of the Body and 

 forming epidermis, hairs, and nails. The enamel which 

 covers the teeth belongs also to this group. 



The class of protective tissues is, however, even more 

 artificial than that of the nutritive tissues, and cannot be 

 defined by positive characters. Many epithelial cells are 

 secretory, excretory, or receptive; and ciliated cells have 

 already been included among the motor tissues, although 

 from the fact that the movements of their cilia go on 

 in separated cells and independently of recognizable exter- 

 nal stimuli, they might well have been put among the au- 

 tomatic. The protective tissues may be best defined as 

 including cells which line free surfaces, and whose func- 

 tions are mainly mechanical or physical. 



10. The Reproductive Tissues. These are concerned 

 in the production of new individuals, and in the Human 

 Body are of two kinds, located in different sexes. The 

 conjunction of the products of each sex is necessary for the 

 origination of offspring, since the ovum, or female pro- 

 duct which directly develops into the new human being, 

 lies, dormant until it has been fertilized or acted upon by 

 the product of the male. 



The Combination of Tissues to Form Organs. The va- 

 rious tissues above enumerated forming the building mate- 

 rials of the Body, anatomy is primarily concerned with 

 their structure, and physiology with their properties. If 

 this, however, were the whole matter, the problems 

 of anatomy and physiology would be much simpler 

 than they actually are. The knowledge about the living 

 Body obtained by studying only the forms and function? 



