TRANSPORT OF MATERIALS 137 



of the muscles, for example, in order to discover whether this is the 

 case with them. 



The most typical of all chemical messengers actually obtained 

 in solution is the " secretin " produced by the action of acid on the 

 lining cells of the first part of the small intestine. This is formed 

 for the express purpose of setting the pancreas into activity, as we 

 have seen. 



Chemical messengers are also to be met with in plants. There 

 are, as it appears, chemical substances which diffuse from some 

 parts and, circulating in the sap, favour the growth of other parts. 

 But these may possibly be of the nature of food materials. There 

 are others, however, which check or inhibit the growth of certain 

 parts in a definite manner. Thus, the apical shoot of a fir tree 

 produces some substances that prevent the lower shoots from grow- 

 ing vertically upwards ; so that when this shoot is cut off or 

 injured, one of the lower ones which is growing more vigorously 

 than its neighbours begins to grow vertically. As it grows 

 upwards, it, in its turn, produces the inhibiting material and pre- 

 vents others from turning upwards. It would appear that the 

 exciting cause must be the action of gravity on some constituents 

 of the cells, analogous to starch grains in the more common form 

 of response to the stimulus of gravity. 



The Kidneys. In addition to carbon dioxide, there are other 

 waste products of tissue activity. The most important of these is 

 urea, resulting from the deamination of amino-acids in the liver. 

 There are also other compounds of nitrogen, arising either as by- 

 products of chemical reactions, or from wear and tear of cell 

 structures. Amongst these are uric acid and creatine. Then 

 again, there are substances taken in with the food, which are either 

 useless or only wanted in small quantity, such, for example, as 

 phosphates and sulphates. We notice that all these substances 

 which we want to get rid of are crystalloids of small molecular size, 

 present in true solution, so that if we filter the blood through a 

 membrane which has pores small enough to prevent the passage 

 of the blood corpuscles and the colloids, we can effect a separation. 

 Such a membrane is parchment paper, as we saw in the first 

 chapter, and the wall of the small blood vessels is a membrane of 

 similar properties as regards permeability. But in filtering off the 

 waste products in such a way, we remove from the blood other con- 

 stituents which are of value, namely, water, salts, glucose, and 

 amino-acids. Nevertheless, this is the method adopted by the 

 animal body to rid itself of its waste products. How the dis- 

 advantage is remedied will be seen later. 



The kidney is the organ in which the process takes place, and 

 the structures in it where filtration occurs are coils of small blood 



