f8 A MANUAL OF PHYSIOLOGY 



the chlorides and phosphates of sodium, potassium, and 

 calcium, are constantly present. 



Structure of Living Matter. The Cell. Protoplasm or living 

 substance, when examined in its most primitive, undifferentiated 

 condition in such cells as the Amoeba or the white blood-corpuscles, 

 appears a homogeneous, structureless mass, except for certain 

 granules embedded in it, and consisting either of products formed 

 by its activity or of food materials. But in many cells the 

 protoplasm presents the appearance of a honeycomb or network, 

 with granules usually situated at the nodes, arid holding in its 

 vesicles or meshes a fluid, perhaps containing pabulum, from which 

 the waste of the living framework is made good, or material upon 

 which it works, and which it is its business to transform. Some 

 observers, however, maintain that the network is an artificial appear- 

 ance produced by the precipitation of the colloid constituents of the 

 protoplasm by the fixing reagent or even by the coagulative processes 

 associated with the act of dying. How-ever this may be, in building 

 up our typical cell we start with a piece of protoplasm. Somewhere 

 in the midst of this we find a body which, if not absolutely different 

 in kind from the protoplasm of the cell, is yet marked off from it by 

 very definite morphological and chemical characters. This is the 

 nucleus, generally of round or oval shape, and often bounded by an 

 envelope. Within the envelope lies a second network of fine 

 threads, which do not themselves stain with nuclear dyes such as 

 hasmatoxylin. But in or on these 'achromatic' filaments lie small, 

 highly refractive particles, staining readily and deeply with dyes, and 

 therefore described as consisting of chromatin. This chromatin is 

 either made up of nucleins (substances composed of a sulphur-free 

 organic acid, nucleic acid, combined in various proportions with 

 proteids), or yields nucleins by its decomposition ; and it seems to 

 owe its affinity for certain staining substances to the presence of 

 nucleic acid. When the nucleus is about to divide in the manner 

 known as indirect division or karyokinesis, the chromatin granules 

 arrange themselves into one or more coiled filaments or skeins. 

 The meshes of the nuclear reticulum contain a semi-fluid material, 

 which does not readily stain. Besides the nucleus, another much 

 smaller structure, the centrosome, surrounded by the attraction sphere, 

 is differentiated from the protoplasm of the cell. 



When we carry back the analysis of an organized body as 

 far as we can, we find that every organ of it is made up of 

 cells, which upon the whole conform to the type we have 

 been describing, although there are many differences in 

 details. Some organisms there are, low down in the scale, 

 whose whole activity is confined within the narrow limits of 

 a single cell. The Amoeba sets up in life as a cell split off 

 from its parent. It divides in its turn, and each half is a 

 complete Amceba. When we come a little higher than the 



