INTRODUCTORY. 3 



divided into a part which contains the nucleus and a part which does 

 not, an experiment which may be performed with the larger unicellular 

 organisms, the part separated from the nucleus becomes inactive and 

 dies. Further, when cell-division occurs, the nuclear changes which 

 lead to the formation of two daughter nuclei precede the division of the 

 protoplasm. The nucleus also differs in chemical composition and in 

 staining reactions from the rest of the cell. It contains a substance 

 called nuclein, which is a compound of nucleic acid with protein. 

 Nucleic acid is distinguished by containing a considerable proportion 

 of phosphorus in its molecule. 



In the human body all the tissues and organs subserve, directly or 

 indirectly, the production of movement, whether that takes the form 

 of locomotion, work, speech, or writing. The nervous and muscular 

 tissues are the master tissues of the body, the remaining tissues and 

 organs being designed for their protection and nutrition. The bony 

 skeleton forms a framework which is necessary for the carrying out of 

 movements, and which shields the brain and other important structures 

 from injury. The skin is also protective in function, and, with the eye, 

 ear, and other sense organs, it receives impressions from the outer 

 world which are of service in determining the bodily activities. The 

 digestive tract converts insoluble food substances into soluble bodies 

 which are then absorbed into the blood. The circulatory system 

 conveys the blood to all the cells and tissues of the body so that they 

 receive nourishment. The blood also receives from the cells and tissues 

 the waste products formed by their activity and carries these to the 

 lungs and kidneys, by which they are excreted. Besides excreting one 

 of the waste products, carbonic acid, the lungs take up oxygen from the 

 air and convey it to the blood, from which it passes to the cells and 

 tissues. Further, in addition to the glands which secrete the digestive 

 and other juices, there are in the body certain glands whose function it 

 is to produce various hormones. All these subsidiary systems are only 

 of importance in that they serve to sustain the muscular and nervous 

 structures. The muscular system, in its turn, is to be looked upon as 

 the organ of expression of the nerve centres. 

 therefore, consists ultimately in its nervous activities. 



