942 INFLUENCE OF THE NERVOUS SYSTEM 



it is incumbent, therefore, to offer some more unexceptionable proof of their 

 position than has yet been given; since their doctrine is opposed by so many 

 considerations of great weight. 



946. That many of the Organic Functions, however, are directly influenced 

 })j the Nervous System, is a matter which does not admit of dispute ; and this 

 influence, exerted sometimes in exciting, sometimes in checking, and sometimes 

 in otherwise modifying them, may well be compared to that which the hand 

 and heel of the rider have upon his horse, or which the engine-driver exerts 

 over a locomotive. It is most remarkably manifested in the result of severe 

 injury of the Nervous centres, such as concussion of the Brain or of the Solar 

 plexus ( 321); for this does not merely bring about a suspension of the re- 

 spiratory and other movements which minister to the Organic functions, hence 

 inducing a gradual stagnation of the latter ; but occasions a sudden and complete 

 cessation of the whole train of action, which cannot be attributed to any other 

 cause than a positive depressing influence of some kind, propagated through- the 

 Nervous System. In such cases, even the vitality of the Blood is often affected ; 

 the usual coagulation not taking place after death, so long, at least, as the blood 

 remains within the vessels. A similar general depression may result from 

 Mental Emotion, operating through the same channel ; but this more commonly 

 has rather a local action, or operates more gradually ( 797). 



947. The influence of the Nervous System is often especially exerted in 

 giving temporary excitement to a Secreting process which need not be kept in 

 constant activity, or of which circumstances may occasionally require an in- 

 crease. This is the case, for example, in regard to the secretions connected 

 with the process of digestion the Saliva, Gastric fluid, Bile, Pancreatic fluid, 

 &c. ; all of these being excited by the contact of the substances on which they 

 act, with the surfaces on which their respective ducts open. The secretion of 

 Milk, again, in a nursing female, may be excited by irritation of the nipple; 

 and the determination of blood to the Mammae during pregnancy arises from an 

 increased action in the part, which is probably excited sympathetically by the 

 changes occurring in the Uterus; and for this kind of communication, the 

 Sympathetic System seems to afford the readiest channel, since the organs in 

 question are for the most part supplied by it. There is an apparent exception, 

 however, in the case of the Salivary and Lachrymal glands, which are supplied 

 by the 5th Pair : but this nerve contains so many organic filaments, and is so 

 intimately connected with the Sympathetic, as evidently to supply (in the head) 

 the place of a separate ganglionic system. It is by Nervous influence, that the 

 mucous secretion covering the membranes is caused to be regularly formed for 

 their protection; for it is shown by pathological facts, that when this influence 

 is interrupted, and the secretion is no longer supplied, the membrane, losing 

 its protection, is irritated by the air or the fluids with which it may be in con- 

 tact, and passes into an inflammatory condition. This is partly the explanation 

 of the fact, which has been well ascertained, that the eye is liable to suppurate 

 when the 5th pair has been divided; and that the mucous membrane of the 

 bladder becomes diseased in paraplegia. 



948. The influence of particular conditions of the Mind, in exciting various 

 Secretions, is a matter of daily experience. The flow of Saliva, for example, 

 is stimulated by the idea of food, especially that of a savory character. Lach- 

 rymal secretion, again, which is continually being formed to a small extent for 

 the purpose of bathing the surface of the eye, is poured out in great abundance 

 under the moderate excitement of the emotions, either of joy, tenderness, or 

 grief. It is checked, however, by violent emotions; hence in intense grief, the 

 tears do not flow ; and it is a well-known proof of moderated sorrow when the 

 gush takes place, this very act affording a further relief ( 797). Violent emo- 

 tion may also suspend the Salivary secretion ; as is shown by the well-known 



