18 HUMAN PHYSIOLOGY. 



MANIFESTATIONS OF CELL LIFE. 



Growth. Cells when newly formed are exceedingly small, but as they 

 approach maturity they increase in size, by the capability which the cells 

 possess of selecting and appropriating new material as food, vitalizing and 

 organizing it. The extent of cell growth varies in different tissues; in 

 some the cells remain exceedingly small, in others they attain considerable 

 size. In many instances the cell substance undergoes transformation into 

 new compounds destined for some ulterior purpose. 



Reproduction. Like all organic structures cells have a limited period 

 of life ; their continual decay and death necessitates a capability of repro- 

 duction. Cells reproduce themselves in the higher animals mainly by 

 fission. This is seen in the white blood corpuscles of the young embryos 

 of animals; the corpuscle here consists of a cell substance and nucleus. 

 When division of the cell is about to take place, the nucleus elongates, the 

 cell substance assumes the oval form, a constriction occurs, which gradually 

 deepens, until the original cell is completely divided and two new cells are 

 formed, each of which soon grows to the size of the parent cell. 



In cells provided with a cell membrane the process is somewhat different. 

 In the ova of the inferior animals, after fertilization has taken place, a 

 furrow appears on the opposite sides of the cell substance, which deepens 

 until the cell is divided into two equal halves, each containing a nucleus; 

 this process is again repeated until there are four cells, then eight, and so 

 on until the entire cell substance is divided into a mulberry mass of cells, 

 completely occupying the interior of the cell membrane. The whole pro- 

 cess of segmentation takes place with great rapidity, occupying not more 

 than a few minutes, in all probability. 



Motion. Spontaneous movement has been observed in many of the 

 cells of the body. It may be studied, for example, in the movements of 

 the spermatozoids, the waving of the cilia covering the cells of the bronchial 

 mucous membrane, the white corpuscles of the blood, etc. 



By a combination and transformation of these original structural elements, 

 and material derived from them, all the tissues are formed which enter into 

 the structure of the human body. 



CLASSIFICATION OF TISSUES. 



I. Homogeneous Substance, a more or less solid, albuminous struc- 

 ture, filling the spaces between the cells and fibres of various tissues, e.g., 

 cartilage, bone, dentine, etc. 



II. Limiting Membrane, a thin homogeneous membrane, structureless, 



