362 PRINCIPLES OF GENERAL PHYSIOLOGY 



Luminous Substances. There are many organisms known which are capable of 

 secreting substances which give off light. Very little is known of the chemical 

 nature of the reaction concerned, but it is evidently an oxidation process of some 

 kind, since the luminosity disappears in the absence of oxygen. According to 

 the work of Raphael Dubois (1913 and literature cited therein), there are two 

 substances concerned, neither of which is luminous alone. The one is of the nature 

 of an oxidising enzyme, or peroxidase, the nature of which we shall have to discuss 

 in Chapter XX. This can be replaced by solution of a permanganate, or by some 

 other oxidising enzyme. It is called " luciferase." The substance oxidised is 

 called " luciferine " ; its chemical nature is unknown, but it appears to have some 

 of the properties of proteins. A remarkable fact about the light produced is that 

 the radiation contains only a very small percentage of the longer wave lengths, 

 known as heat rays, and is almost entirely composed of "light" rays. It has 

 hence been designated " cold light " and indicated as the ideal illuminant. 

 Further details will be found in Chapter XIX. 



The mollusc, Pholas dactylus, which bores its way into hard mud on the sea 

 coast, is frequently to be found and has a brightly luminous secretion. 



The work of Molisch (1904) on luminous bacteria will be found of much 

 interest. The article by Mangold (1910) on the production of light by organisms 

 may also be consulted. 



Electrical Organs. In the electrical fish, Malapterurus, found in the Nile and 

 known to the ancient Egyptians, the electrical organ is evidently developed from 

 skin glands. We have seen that the process of secretion is accompanied by 

 electrical changes and it is curious to note how this has been made use of for the 

 purpose of defence and perhaps of benumbing prey. In other electrical fish, the 

 organ seems to have been formed from muscular tissue and will be referred to in 

 Chapter XXII. 



SUMMARY 



In a general way, all living cells may be said to give off to the surrounding 

 medium products of the chemical reactions taking place within them. But the 

 name of secretion is especially given to those cases in which the products are made 

 use of for purposes of importance to the organism as a whole. 



Under the name are also included processes in which the function of the cells 

 is to separate from the blood products of the metabolism of the organism as a 

 whole. These waste products would be deleterious if allowed to accumulate, and 

 there are arrangements produced in order to reject them to the exterior of the 

 organism. This process is sometimes called "excretion," and is the particular 

 function of the kidney, although the epithelium of the alimentary canal takes part 

 in the excretion of foreign substances under certain conditions. 



Secreting organs, or glands, may either discharge their products by means of 

 a special channel, the duct, into a cavity such as the alimentary canal, which 

 cavity is, in a sense, outside the organism itself ; or their products may diffuse 

 into the blood vessels and in this way affect distant organs. Glands of this latter 

 kind are known as those with internal secretion. 



The products of glands with external secretion, such as the pancreas, are given 

 out, for the most part, dissolved in water, so that the first problem is the way in 

 which the cells produce a current of water through their substance in order to 

 wash out, as it were, the chemical compounds which they have formed. 



The filtration of pure water from a solution of the molar concentration of 

 blood is impossible by pressures directly available in living organisms. If, 

 however, the liquid to be filtered off consists of blood minus its colloids only, the 

 arterial pressure is higher than the osmotic pressure of these colloids and can filter 

 off a solution of this kind. The process actually occurs in the glomerulus of 

 the kidney. 



Since it is found that the pressure under which secretion is possible is higher, 

 in some cases, than that of the arterial blood, osmotic forces are indicated as the 



