TISSUES, ORGANS, AND PHYSIOLOGICAL SYSTEMS 35 



9. THE REPRODUCTIVE TISSUES. These arc 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 

 female product, egg-cell or ovum, which directly develops into the 

 new human being, remains dormant until it has been fertilized, and 

 fertilization consists essentially in the fusion of its nucleus with the 

 nucleus of a cell produced by the male. 



The Combination of Tissues to Form Organs. The various 

 tissues above enumerated forming the building materials 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 functions of the 

 individual tissues would be comparable to that attained about a 

 great factory by studying separately the boilers, pistons, levers, 

 wheels, etc., found in it, and leaving cut of account altogether the 

 way in which these are combined to form various machines; for in 

 the Body the various tissues are for the most part associated to 

 form organs, each organ answering to a complex machine like a 

 steam-engine with its numerous constituent parts. And just as in 

 different machines a cogged wheel may perform very different 

 duties, dependent upon the way in which it is connected with other 

 parts, so in the Body any one tissue, although its essential proper- 

 ties are everywhere the same, may by its activity subserve very 

 various uses according to the manner in which it is combined with 

 others. For example: A nerve-fiber uniting the eye with one part 

 of the brain will, by means of its conductivity, when its end in the 

 eye is excited by the irritable tissue attached to it on which light 

 acts, cause changes in the sensory nerve-cells connected with its 

 other end and so arouse a sight sensation; but an exactly similar 

 nerve-fiber running from the brain to the muscles will, also by 

 virtue of its conductivity, when its ending in the brain is excited 

 by a change in a nerve-cell connected with it, stir up the muscle 

 to contract under the control of the will. The different results de- 

 pend on the different parts connected with the ends of the nerve- 

 fibers in each case, and not on differences in the properties of the 

 nerve-fibers themselves. 



