i6 



NATURE 



[September 5, 1912 



voluntary muscles. These movements are actuallv the 

 result of impressions imparted to sensory or afferent 

 nerves at the periphery, e.g. in the skin or in the 

 several organs of special sense ; the effect of these 

 impressions may not be immediate, but can be stored 

 for an indefinite time in certain cells of the nervous 

 system. The regulation of movements — whether they 

 occur instantly after reception of the peripheral im- 

 pression or result after a certain lapse of time ; 

 whether they are accompanied by conscious sensation 

 or are of a purely reflex and unconscious character — 

 is an intricate process, and the conditions of their co- 

 ordination are of a complex nature involving not 

 merely the causation of contraction of certain muscles, 

 but also the prevention of contraction of others. . For 

 our present knowledge of these conditions we are 

 largely indebted to the researches of Prof. Sherrington. 

 Involuntary Movements. 



A less conspicuous but no less important part played 

 by the nervous system is that by which the contrac- 

 tions of involuntary muscles are regulated. In 

 normal circumstances these are always independent of 

 consciousness, but their regulation is brought about in 

 much the same way as is that of the contractions of 

 voluntary muscles — viz., as the result of impressions 

 received at the periphery. These are transmitted by 

 afferent fibres to the central nervous system, and from 

 the latter other impulses are sent down, mostly along 

 the nerves of the sympathetic or autonomic svstem of 

 nerves, which either stimulate or prevent contraction 

 of the involuntary muscles. Many involuntary muscles 

 have a natural tendency to continuous or rhythmic 

 contraction which is quite independent of the central 

 nervous system ; in this case the effect of impulses 

 received from the latter is merely to increase or 

 diminish the amount of such contraction. An example 

 of this double effect is observed in connection with the 

 heart, which — although it can contract regularly and 

 rhythmically when cut off from the nervous system 

 and even if removed from the body — is normally 

 stimulated to increased activity by impulses coming 

 from the central nervous system through the sym- 

 pathetic, or to diminished activity by others coming 

 through the vagus. It is due to the readiness by 

 which the action of the heart is influenced in these 

 opposite ways by the spread of impulses generated 

 during the nerve-storms which we term "emotions" 

 that in the language of poetry, and even of every 

 day, the word " heart " has become synonymous with 

 the emotions themselves. 



Effects of Emotions. 



The involuntary muscle of the arteries has its action 

 similarly balanced. When its contraction is increased, 

 the size of the vessels is lessened and they deliver less 

 blood ; the parts they supply accordingly become pale 

 in colour. On the other hand, when the contraction 

 is diminished the vessels enlarge and deliver more 

 blood ; the parts which they supply become corre- 

 spondingly ruddy. These changes in the arteries, like 

 the effects upon the heart, may also be produced undir 

 the influence of emotions. Thus "blushing" is a 

 purely physiological phenomenon due to diminished 

 action of the muscular tissue of the arteries, whilst the 

 pallor produced by fright is caused by an increased 

 contraction of that tissue. Apart, however, from these 

 conspicuous effects, there is constantly proceeding a 

 less apparent but not less important balancing action 

 between the two sets of nerve-fibres distributed to 

 lieart and blood-vessels; which are influenced in one 

 din-ction or another by every sensation which we ex- 

 perience and even by impressions of which we mav 

 be wholly unconscious, such as those which occur 

 during sleep or anaesthesia, or which affect our other- 

 wise insensitive internal organs. 

 NO. 2236, VOL. 90] 



Regulation of Secretion by the Nervous System. 



A further instance of nerve-regulation is seen in 

 secreting glands. Not all glands are thus regulated, 

 at least not directly ; but in those which are, the effects 

 are striking. Their regulation is of the same general 

 nature _ as that exercised upon involuntary muscle, 

 but it influences the chemical activities of the gland- 

 cells and the outpouring of secretion from theni. By 

 means of this regulation a secretion can be produced 

 or arrested, increased or diminished. As with muscle, 

 a suitable balance is in this way maintained, and the 

 activity of the glands is adapted to the requirements 

 of the organism. Most of the digestive glands are 

 thus influenced, as are the skin-glands which secrete 

 sweat. 



Regulation of Body Temperature. 



And by the action of the nervous system upon the 

 skin-glands, together with its effect in increasing or 

 diminishing the blood-supply to the cutaneous blood- 

 vessels, the temperature of our blood is regulated and 

 is kept at the point best suited for maintenance of 

 the life and activity of the tissues. 



Effects of Emotions on Secretion. 

 The action of the nervous system upon the secretion 

 of glands is strikingly exemplified, as in the case of 

 its action upon the heart and blood-vessels by the 

 effects of the emotions. Thus an emotion of one kind 

 — such as the anticipation of food — will cause saliva 

 to flow — "the mouth to water"; whereas an emotion 

 of another kind — such as fear or anxiety — will stop 

 the secretion, causing the "tongue to cleave unto the 

 roof of the mouth," and rendering speech difficult or 

 impossible. Such arrest of the salivary secretion also 

 makes the swallowing of dry food difficult : advan- 

 tage of this fact is taken in the " ordeal by rice " 

 which used to be employed in the East for the detec- 

 tion of criminals. 



Regulation by Cticmical Agents: Hormones. — 

 Internal Secretions. 

 The activities of the cells constituting our bodies 

 are controlled, as already mentioned, in another way 

 than through the nervous system, viz., by chemical 

 agents (hormones) circulating in the blood. Many of 

 these are produced by special glandular organs, 

 known as internally secreting glands. The ordinary 

 secreting glands pour their secretions on the exterior 

 of the body or on a surface communicating with the 

 exterior ; the internally secreting glands pass the 

 materials which they produce directly into the blood. 

 In this fluid the hormones are carried to distant 

 organs. Their influence upon an organ may be 

 essential to the proper performance of its functions or 

 may be merely ancillary to it. In the former case 

 removal of the internally secreting gland which pro- 

 duces the hormone, or its destruction by disease, may 

 prove fatal to the organism. 



Suprarcnals. 

 This is the case with the suprarenal capsules : small 

 glands which are adjacent to the kidneys, although 

 having no physiological connection with these organs. 

 .\ Guy's physician, Dr. Addison, in the middle of the last 

 century showed that a certain affection, almost always 

 fatal, since known by his name, is associated with 

 disease of the suprarenal capsules. A short time after 

 this observation a French physiologist, Brown- 

 Sequard, found that animals from whicli the supra- 

 renal capsules are removed rarely survive the opera- 

 tion for more than a few days. In the concluding 

 decade of the last century interest in these bodies 

 was revived by the discovery that they are constantly 

 yielding to the blood a chemical agent (or hormone) 

 which stimulates the contractions of the heart and 



