2 ESSENTIALS OF PHYSIOLOGY. 



modified to subserve movement, others to produce secretions, and so 

 on ; in this way the efficiency of the organism as a whole is increased. 

 Obviously in such a community of cells it is of the utmost importance 

 that the various groups should work in harmony, and to ensure this 

 they must be linked up by some controlling mechanism. 



Two such mechanisms are found in the body : (1) a system of 

 chemical messengers, or hormones, and (2) the _ nervous system. The 

 former is the more primitive of the two methods. A hormone 

 is produced in one organ and is carried by the blood to another, 

 exciting or restraining its activity. For example, as the acid contents 

 of the stomach pass into the bowel they lead to the production of a 

 hormone in the intestinal wall. The newly formed substance is taken 

 up by the blood and carried to the pancreas, which it stimulates to 

 secrete the juice required for the next stage of the digestive process. 

 Such a method of communication is comparatively slow, and where 

 rapidity of transmission is important the messages are conveyed by 

 the nervous system. The latter, in fact, bears much the same relation 

 to the blood current as the telegraphic system bears to ordinary letter 

 post. Thus if a foreign body touches the surface of the eyeball, infor- 

 mation of the fact is sent along certain nerve fibres to the brain, and 

 impulses return to the muscles of the eyelids, causing the lids to close, 

 within a small fraction of a second. 



In any living organism the unit of structure is a minute, jelly-like 

 mass known as a cell. The simplest organisms consist of a single cell, 

 those which are higher in the scale of life being composed of many 

 cells. Each cell is composed of a semi-fluid material, known as proto- 

 plasm, containing a denser circumscribed structure, the nucleus. In 

 some cases a well-defined cell-envelope exists, notably in vegetable cells 

 and in the mammalian ovum, but in many animal cells no definite 

 envelope can be demonstrated, and in these the boundary is probably 

 determined by the condensation of molecules which is known to take 

 place on the surface of colloid solutions, and which gives rise to the 

 physical condition known as surface tension. 



Protoplasm is semi-transparent, and may be homogeneous in 

 appearance, or may show traces of structure in the form of a network 

 containing hyaline fluid in its meshes. It is the working part of the 

 cell, and often contains granules which represent the products of its 

 activity. Such granules are especially seen in secreting cells, and 

 occupy corresponding spaces in the cell protoplasm. Protoplasm itself 

 varies greatly in composition, but it always contains a large proportion 

 of albuminous substances or proteins. 



The nucleus is essential to the life of the cell. When a cell is 



