538 



NATURE 



[December 22, 192 1 



pump, the heart, would be the distribution of 

 water in pipes; but everyday experience reminds 

 us that there is also a postal system by means of 

 which actual material bodies convey messages. 

 Thus a letter can order a supply of coal or cut 

 off the electric current. The "chemical messen- 

 gers " or " hormones " are the letters of the living 

 organism. When the acid contents of the stomach 

 pass into the intestine a chemical compound of 

 unknown constitution is formed in the cells lining 

 this tube, passes into the blood, and ultimately 

 arrives at the pancreas, the cells of which alone 

 of all those with which the blood comes into con- 

 tact are able to respond to its stimulus. This 

 they do by secreting pancreatic juice. The hor- 

 mone, called "secretin," merely to give it a name, 

 is a letter sent to the pancreas by the food asking 

 for the enzymes needed to digest it. It does 

 not serve in any sense as food for the cells, it is 

 not a part of the secretion, but sets the machinery 

 to work. 



The fact must not be overlooked, however, that 

 these various means of integration act together. 

 Just as coal can be ordered by telegram, so a 

 supply of sugar or oxygen can be ordered by a 

 nervous message sent by an active organ, and we 

 have already seen that a chemical messenger ( = a 

 letter) can excite the respiratory centre to send 

 nervous impulses ( = telegraphic messages) to the 

 muscles producing respiratory movements. In 

 general, co-ordination by chemical means is apt 

 to be of a more prolonged or continuous nature 

 than that by nervous reflexes. Even in the latter 

 case, however, it is very common to find that an 

 organ is under the continuous control of both ex- 

 citatory and inhibitory nerves, the two opposing 

 influences being in action at the same time. Any 

 particular state is thus the expression of a pre- 

 ponderance on one or the other side of the 

 balance. The fact has been made clear in the 

 tone of the stomach of some reptiles, but it ap- 

 pears to be of wide occurrence, if not a general 

 law. 



The internal secretions of the endocrine glands 

 are to be included in the category of chemical 

 messengers. An interesting and instructive article 

 by Prof. Strohl, of Zurich, was published in the 

 Revue genSrale des Sciences (May 15, 192 1), and, 

 in point of fact, suggested the present remarks. 

 His article deals with the general biological sig- 

 nificance of the internal secretions, and contains 

 a full list of papers, including those concerning 

 lower animals. The author compares hormones 

 to the electric waves of wireless telegraphy, since 

 only certain tissues are tuned to respond to them. 

 I venture to think that the postal system is a 

 better illustration. Actual material bodies are 

 carried, and these "letters" are accepted only 

 at the correct addresses. In many cases, such as 

 those of the suprarenal bodies and the thyroid 

 gland, we are able to assign the function in ques- 

 tion definitely to particular organs, which, indeed, 

 appear to possess no other function. 



In other cases, more difficulty is met with, 

 NO. 2721, VOL. 108] 



owing to the fact that the cells producing the 

 internal secretion are mixed up with cells having 

 other known functions. Such, for example, is the 

 case with the "islets of Langerhans," embedded 

 in the pancreatic tissue which secretes the diges- 

 tive juice. There is evidence that these " islets " 

 produce a hormone which is required for the due 

 utilisation of sugar. This co-existence, side by 

 side, of cells with different functions is only one 

 of the many difficulties met with in the study of 

 internal secretions. It appears that removal of 

 one particular endocrine organ aftects the func- 

 tions of those that are left. Indeed, the com- 

 plexity of the phenomena may be some excuse 

 for the contradictory and hazy statements so often 

 made by workers on the problem, and for the 

 hasty and sensational "discoveries" announced 

 from time to time in the Press. This aspect of 

 the subject is rightly emphasised by Gley in his 

 valuable lectures on " Les Secretions Internes," 

 where it is pointed out that clear and definite state- 

 ments are much to be desired. Names are too 

 often given which, although they profess to ex- 

 plain, are nothing but descriptions of the facts 

 in other words. No advance is possible in this 

 way. 



The existence of certain organs or tissues 

 which, so far as can be made out, have no func- 

 tion other than that of making chemical com- 

 pounds for the purpose of acting on other tissues 

 suggests the question whether those tissue cells 

 which have obvious functions of another kind do 

 not also give out to the blood substances which 

 act in other places — whether the view of Brown- 

 Sequard that all tissues make internal secretions 

 may not fee correct. In the simple case of carbon 

 dioxide this is, of course, the case ; but it is 

 possible that many of the chemical products of 

 the activity of cells may be inert for want of 

 receptive cells sensitive to them. Many substances 

 found in animals and plants appear to be waste 

 products of metabolism — as, for example, the 

 great variety of alkaloids met with in plants. 

 There is evidence that some influence is exerted 

 by cells on the growth and nature of cells of other 

 kinds in their neighbourhood. The experiments 

 of Champy may be cited as examples of this in- 

 fluence ; moreover, fragments of differentiated 

 tissue grown on a slide under the microscope are 

 apt to lose their distinctive marks as they pro- 

 liferate, and to become of an embryonic type. If, 

 however, the fragment contains both epithelium 

 and connective tissue, the new epithehal cells re- 

 tain the characters of those from which they arise, 

 so long as they remain in proximity to the con- 

 nective tissue; but if they wander away they lose 

 their identity and become generalised in aspect. 

 Similarly, the excessive outgrowth of connective 

 tissue in retinal fragments is held in check by 

 the presence of nerve cells. It seems that the 

 normal growth of tissue cells is only possible 

 when under the "control" of other cells. This 

 circumstance is significant in relation to the prob- 

 lem of malignant growths. Champy describes a 



