108 THE BODY AT WORK 



some sugar is produced, but the process of conversion is very 

 slow. It is almost impossible to digest raw starch in the 

 mouth sufficiently to render it insusceptible to the colouring 

 action of iodine. The sugar produced by the action of ptyalin 

 is of the same nature as that which appears during the malting 

 of barley. It is therefore termed " maltose." It closely re- 

 sembles grape-sugar, but is not identical with it. 



The Secretion of Saliva. The accessibility of the salivary 

 glands, and especially of the submaxillary, has led to their 

 being used for a very large number of experiments. They have 

 been studied with the aim of coming to an understanding of the 

 mechanism of secretion in general. The glands consist of tubes 

 of gland-cells, each tube suspended in a basket of connective 

 tissue, in a bath of lymph (cf. Fig. 3). Innumerable capillary 

 bloodvessels traverse the lymph-bath. The arteries which carry 

 blood to the gland are supplied with nerves, which regulate 

 their calibre, and therefore determine the amount of blood 

 which passes through the capillaries into which they break up. 

 The glands also are supplied with nerves which influence their 

 functional activity. Nutrient substances and oxygen pass out 

 of the blood into the lymph. Carbonic acid passes into the blood 

 from the lymph. Waste products are either carried away in 

 the lymph-stream, or make their way through the walls of the 

 capillaries into the blood. Many problems present themselves 

 for solution. How does the amount of work done by the 

 gland affect its supply of blood ? Does the quantity of saliva 

 secreted vary directly with the pressure of lymph in the spaces 

 by which the gland is surrounded ? Is this pressure wholly 

 dependent upon the pressure of the blood ? Are the sub- 

 stances secreted by the gland supplied as such by the blood, 

 or does the gland make the ptyalin and mucus which it secretes ? 

 If it makes its secernable products, what materials does it 

 abstract from the blood for the purpose of their manufacture ? 

 Does it use the whole of these materials, whatever they may 

 be, or does it use part only and return the residue to the 

 lymph ? Does it make its products only when it is actively 

 secreting, or is it always making them, and storing them in its 

 cells in order that it may have a supply to discharge when 

 called upon by the stimulation which results from the presence 

 of food in the mouth ? Is their discharge merely a washing-out 



