SYSTEM.] PHYSIOLOGY 29 opposite side b, and contractions may occur in the muscles of the hind limbs on both sides 1, 2. Irradiation ; if the excitation be still increased in intensity, it affects higher centres c, d, and there may be contraction of the fore-limbs 3, 4. General action; if the excitation be still further increased, it may pass to a still higher reflex centre c, and the result is general convulsions. (e) Reflex centres may be so arranged in the body as to con stitute a series in which those of the cerebrum govern or con trol others in the deeper gan glia of the brain, while these, in turn, have an influence over still lower centres in the spinal cord. This arrangement is termed the "superposition of reflexes " (see fig. 9). (/) Stimulation of a sensory Fir " V:r Dia ? wn .l Illustrating the super- surface may simultaneously pro duce, by a reflex mechanism, position (if reflexes, m, m, muscles ; 1, 1, series of reflex centres on one side, under the control of 2, 2, which are muscles, secretion of saliva, and a sensation (see fig. 10). movement, secretion, and con- n s ain governed by 3. There is a cor- sciousness. Thus a condiment %S$Ui^&*&& in the mouth may cause in- over by 4. Thus a stimulus reaching voluntary twitchings of the 4 might excite the activity of all the muscles in, m , if it reached 3, only one half of the muscles ; if it reached 2, to the left, only three of the muscles ; (17) Certain substances, in ami, finally, if it affected 1, to the left, particular strychnin, increase OIll > one muscle m. reflex excitability, so that the slightest external stimulation of the sensory nerves of the skin is sufficient to cause severe convulsions. On the other hand, bromide of potas sium, hydrate of chloral, and atro pin diminish reflex excitability. (/*) Individual stimuli only excite a reflex act when they are very pow erful, but stimuli applied at frequent intervals act the more quickly and powerfully the more rapidly they FIO. 10. Diagram illustrating a complex reflex median- succeed each other. ism. The arrows indicate direction of currents. 1, To produce the re- sensor y surface ; 2, muscle ; 3, gland ; a, sensory f, L , ,1 nerve ; b, reflex centre, connected with another reflex Ilex cliange in the centre d by internuncial fibre e ; c, motor or efferent centre, therefore, a nerve ; / secretory nerve passing to gland 3. From summation or ad- the ot ler s i ( e f s sten a fibre passing to the brain, dition of centri and there exciting changes which result iu a sensation. petal excitations is required. When these reach a certain number the centre responds (Stirling). (i) Reflex actions involve time. Thus the time between the stimulation and the movement can be measured, and, if Ave take into consideration the time occupied by the passage of the nerve- current along the nerves involved, and the latent period of muscular contraction, and subtract this from the total time, the remainder will represent the time occupied by the changes in the centre or the reflex-time. This has been found to be from 0555 to 0471 of a second. It is lengthened by cold and shortened by increasing the strength of the stimulus and by strychnia. (k) In compound reflex acts the initial excitation may occur in psychical centres, as when the recollection of an odour causes nausea, or when a feeling of ennui is followed by a yawn. (1) Some reflex movements are the result of inherited peculiar ities of structure, as those made by a new-born child when it seizes the breast. Other reflex movements are acquired during life. Such are at first voluntary, but they become automatic by repetition. The following are some of the more common examples of reflex movements. Motions of the muscles in any part of the limbs or trunk under the influence of sensory impressions on the skin, such as tickling, pricking, &c. ; shud dering from cold, shuddering caused by grating noises, <fcc. ; contraction of the pupil nivler the influence of light on the retina ; winking, from irritation of the sensory nerves of the conjunctiva ; sneezing, from irritation of the Schneiderian membrane, or by a glaring light on the eye ; spasm of the glottis and coughing, from irritation of the larynx or trachea ; laughing, caused by tickling the skin ; the first respiration of the child at birth, from the im pression of cold upon the nerves of the skin, and especially those of the chest ; respiratory movements in the adult, from the impression caused by the afferent nerves of the lungs (sympathetic or vagus), by the presence of carbonic acid in the air cells and passages, or in those of the general system, also, occasional modifications of the respiratory movements from impressions of cold, &c., on the surface of the body ; sucki:ig in infancy ; deglutition or swallowing, with all the complicated movements then occurring in the tongue, fauces, larynx, and gullet ; vomiting, caused by irritation in stomach, or in fauces, or follow ing nausea ; forced contractions of the sphincter muscles of the anus, urinary bladder, and vagina, under local irritation ; erection and emission under the influence of irritation of the nerves of the penis and other parts in the vicinity ; rhythmic movements of lymphatic hearts in reptiles ; rhythmic movements of the heart by the action of cardiac ganglia ; peristaltic motions of the stomach and alimentary canal, in digestion and in defecation, &c., under the influence of impressions conveyed to the ganglia by the splanchnic and intestinal nerves ; action of the bladder in expelling urine ; expulsive action of the uterus in par turition ; contractions and dilatations of the blood-vessels under the influence of the vaso-motor system of nerves ; many obscure and complex morbid actions, such as palpitations of the heart, cramps in the limbs caused by irritation in the stomach or intestinal canal, fainting from concussion or from peculiar odours, dilatation of the pupil and grinding of the teeth from the irritation of worms, and the convulsive seizure sometimes occurring during teething. Inhibitory ami Accelerating Actions. As we have seen, stimula- Inlii- tion of a nerve may cause a sensation, a reflex action, or the direct bitory contraction of a muscle ; but in some instances when the nerve and is stimulated movements may be arrested. This occurs where accel- rhythmic and apparently spontaneous or automatic actions are erating restrained or inhibited by the activity of certain nervt-s. The most actions, striking instance of inhibition is offered by the heart. The subject will be readily understood with the aid of fig. 11, which illustrates the innervation of the heart and blood vessels and especially the inhibition and action of the depressor nerve. It is well known that the heart of a warm-blooded animal ceases to beat almost immediately after re moval from the body, but the heart of a cold-blooded animal, such as the frog, will beat for hours or even days, especiallyif it be supplied with defibrinated blood. The rhythmic beat depends to some extent on the existence in the heart of ganglia or small nerve-centres (fig. 11, I, A, R). It is quite true, however, that rhythm may go on in a portion of the heart containing no ganglionic structure. Now the heart receives nerves from two sources, from the vagus or pneumogastric nerve and from fibres derived from the spinal cord through the sympathetic. If the vagus be cut and the lower end stimulated by feeble induction- Fiaii.-The origins of pm-mnogas- i i ,1 i .%. , , trie and vaso-motor svstems are in shocks, the heart beats more slowly, me( i u ii a , that of the sympathetic and will probably be brought to a in upper portion of cord. The stand -still in a dilated condition. A strong stimulation of the vagus will invariably arrest the action of the heart, and the organ will be found dilated, or, as it is said, in a state of diastole. On removal of the stimulus the heart will soon resume its beats. It is clear that the vagus cannot be regarded as the motor nerve of the heart, because, if so, stimulation would have arrested the action of the heart in a state of contraction or systole. The question, then, is whether the fibres of the vagus possessing this remarkable power of inhibiting or restraining the action of the heart terminate in the muscular fibres, or in some intermediate structures, such as ganglia The influence of various poisons has shed light on this question. A minute dose of atropin injected into the blood paralyses this inhibitory action. After such a dose stimulation of the vagus is followed by no effect, and the heart beats as usual. "Again, in slight urari poisoning the inhibitory action of the vagus is still present ; in the profounder stages it disappears, but even then inhibition may be obtained by applying the electrodes to the sinus. In order to explain this result, it has been supposed that what we may call the inhibitory fibres of the vagus terminate in an inhibitory mechanism (probably ganglionic in nature) seated in the heart itself, and that the urari, while in large doses it may paralyse the terminal fibres of the vagus, leaves this inhibitory mechanism intact and capable of being thrown into activity by a stimulus applied directly to the sinus. After atropin has been given inhibition cannot be brought about by stimulation either of the vagus fibres or of the sinus, or indeed of any part of the heart. Hence it is inferred that atropin, unlike urari, paralyses this intrinsic inhibitory mechanism. After the application of muscarin or pilocarpin the heart stops beating, and remains in diastole in perfect stand still. Its appearance is then exactly that of a heart inhibited by profound and lasting vagus stimulation. This effect is not hindered by urari. The applica tion, however, of a small dose of atropin at once restores the beat. These facts are interpreted as meaning that muscarin (or pilocarpin) stimulates or excites the inhibitory apparatus spoken of above, which atropin paralyses or places Jiors de combat" (Foster, Text-Book of Physiology, 4th ed., p. 186). Thus physiologists are satisfied that, when the vagus is stimulated, currents of nerve-energy pass along its fibres to some of the intrinsic ganglia of the heart and inhibit or restrain these, so that the heart beats more slowly or is arrested altogether. But the centres in the heart may also be stimulated. After division of both vagi in a mammal, say a rabbit, the heart s beat may be quickened or acceler ated by stimulation of the cervical spinal cord. Fibres having this power of accelerating the action of the heart have been traced from arrows indicate direction of nerve-
centre, centre.
and A an accelerating