Page:Encyclopædia Britannica, Ninth Edition, v. 17.djvu/728

Rh 670 NUTRITION with horizontal lateral fibres, in addition to its extrinsic muscles. The tip of the tongue is then raised against the hard palate in such a manner as to form an angle in which the bolus lies. By the approximation of the tongue to the palate the angle is lessened and the bolus is, in consequence, driven backwards. This constitutes the first stage in the act of swallowing, and is a voluntary act, At the end of the first stage the morsel of food has passed beyond the level of the anterior pillars of the fauces. The acts of the second stage are very complicated, and prob ably are entirely involuntary. The posterior pillars of the fauces approach one another in the middle line, and the uvula falls into the space left between them. The fleshy curtain thus extemporized is then drawn up towards the hind wall of the pharynx, which is drawn a little forwards and upwards to meet it. Thus the passage into the nose is completely shut. Meanwhile the vocal cords of the larynx draw near to one another ; the epiglottis is pushed backwards over the larynx, and the whole larynx is drawn suddenly upwards and forwards beneath the root of the tongue. In this manner the entrance into the respiratory passages is protected. Finally, the anterior pillars of the fauces are made to meet over the tongue in order to pre vent the regurgitation of the food. There is but one way open to the bolus ; the sudden drawing forward of the larynx and the base of the tongue in fact &quot; cuts the ground&quot; from under the ball of food, which thereupon falls into the grasp of the &quot; constrictors &quot; and enters upon the third and final stage in the act of deglutition. This, even more certainly than the second stage, is purely involuntary. The constrictors contract from above down wards and force the morsel of food into the upper portion of the oesophagus. Once in the gullet, the mass of food is driven downwards by the so-called &quot;peristaltic&quot; move ments of the tube the circular fibres contract one after another from above downwards, lessening the calibre of the tube in successive stages, whilst the longitudinal fibres seem to have the function of drawing the tube over the bolus as a stocking is drawn over the foot. Deglutition is a reflex act, in so far as it is involuntary, the centre for which lies in the medulla oblongata ; de struction of this centre implies incapacity to swallow. The centre, though normally under the influence of the higher centres, may, however, act quite independently of these, as is evidenced by the fact that animals in which the cerebral hemispheres were absent have occasionally sur vived for a short time, and have still been able to suck and swallow. Although the excised gullet often exhibits a true peristalsis, which doubtless depends upon a local nervous mechanism, the normal movements in the body seem to be regulated from the medulla oblongata. At the entrance of the stomach the food meets the barrier opposed by the contracted cardiac orifice; the contraction must be overcome before the food can gain admittance. The relaxation is certainly an active process under the control of the medulla oblongata through the vagus nerve, since section of the vagi causes a block to the progress of food from the oesophagus into the stomach. In the stomach the food is detained for a period which varies very greatly with its digestibility, but which in the human subject is not often longer than four or five hours. It is subjected to a rubbing and rolling action of the stomach-walls and a modified peristalsis, which causes the food to move slowly from the cardiac orifice along the greater curvature to the pylorus, whence it returns along the small curvature to the cardiac end again, The gastric movements are slight at first, but gradually increase in vigour. The pylorus is tightly closed at the beginning of a meal, but becomes more and more relaxed as digestion proceeds, so that, whilst at first only the finer parts of the gastric contents can pass, afterwards the coarser parts and even solid lumps of imperfectly -digested aliments are permitted to escape into the duodenum. We possess little accurate knowledge as to the nervous mechanism of the stomach. Are the movements caused immediately by the local nervous ganglia situated in its walls 1 Or does the impulse to move descend directly from the encephalic centre along the pneumogastric nerves ? All that we know is ( 1 ) that movements of an excised stomach are induced with great difficulty, (2) that stimulation of the vagus will often cause movements of the stomach, and (3) that sec tion of the vagi impedes the passage of food out of the stomach. The movements of the stomach have been said to cease altogether during sleep. When the gastric contents, to which the term chyme is often applied, pass through the pylorus into the duodenum, they begin to move onward by the pure peristaltic action of the small intestines. The powerful annular fibres con tract one after another, driving the food onward, as water may be squeezed along an india-rubber tube by the com pression of the hand. The longitudinal fibres contract in such a manner that the intestine is drawn over the advanc ing mass. The movements always occur (in health at least) in a direction from the stomach to the ileo-csecal valve ; here they stop and never pass as a continuous wave to the large intestine. Peristalsis may be exhibited by an excised intestine in dependently of any extrinsic nervous apparatus. Stimu lation of the vagus nerve, as a rule, excites the intestinal movements, while excitation of the splanchnic nerves tends to still them. When the blood stagnates in the intestinal vessels active peristalsis ensues. As the splanchnic nerves are also the vaso-motor nerves of the intestines, excitation of them produces constriction of the blood-vessels and com parative bloodlessness. After passing through the ileo-csecal valve the intes tinal contents, which have been very greatly diminished in amount owing to the process of absorption that has gone on, quickly assume the characteristic appearance of faeces. The undigested and insoluble parts of the food, mixed with mucus, with epithelial debris, and with some sub stances derived from the secretions of the alimentary canal, notably with some biliary products, must be cast out ; this is effected by the act of defecation. The anus is normally kept firmly closed by the contraction of two sphincter muscles, the external, which is one of the skeletal muscles, and the internal, which is formed by a special development of the lowest rings of the circular layer of muscles of the intestine. In the act of defaecation these sphincters are relaxed, while the contraction of the rectum forces its contents downwards. The levatores ani are brought into play by the will and exert an action similar to that previously referred to as performed by the longi tudinal fibres of the intestine. Of special influence in aiding the expulsion of the contents of the bowel is the contraction of the abdominal muscles which follows a pre liminary fixation of the diaphragm by a deep inspiration. The act of defalcation is essentially a reflex act. The centre which presides over the sphincters of the anus lies in the lumbar portion of the spinal cord. This centre is under the control of the brain, under the influence of which its activity is either increased or inhibited. Vomiting, or the ejection of the contents of the stomach through the mouth, is an act of considerable complexity ; (1) the cardiac orifice of the stomach is relaxed or, to be accurate, thrown open by the operation of some reflex nervous mechanism with which the vagus is connected ; (2) contraction of the muscular coat of the stomach occurs ; (3) the abdominal walls are powerfully compressed, and the diaphragm is at the same time strongly fixed by closure of the glottis. As concurrent phenomena in ordinary vomiting may be mentioned the sense of nausea and the free flow of saliva which occurs during and indeed before the act.