180

MOVEMENTS OF DEGLUTITION.

be even made to take place against the will. This may seem contrary to every one's daily experience; but it is nevertheless true. The movement by which the food is carried back, beneath the arch of the palate, into the pharynx, is effected by the will; but when the food has arrived there, it is laid hold of, as it were, by the muscles of the pharynx, and is then carried down involuntarily. It has several times happened, that a feather, with which the back of the mouth was being tickled to excite vomiting, having been introduced rather too far, has been thus grasped by the pharynx, and has been swallowed. Moreover, we cannot perform the act of swallowing, without carrying something backwards upon the tongue; and it is the contact of this something, even if it be only a little saliva, with the membrane lining the pharynx, that produces the muscular movement in question.

195. This action is one of the kind now denominated reflex (§ 430). It is produced through the nervous system; for if the nerves supplying the part be divided, it will not take place. But it does not depend upon the Brain; for it may be performed after the brain has been removed, or when its power has been destroyed by a blow. It is caused by the conveyance to the top of the Spinal Cord, of the impression. made on the lining of the pharynx; this impression, brought thither through one set of nerves, excites in the spinal cord a motor impulse; which, being transmitted thence through another set of nerves, calls the muscles into action.

196. This action is, therefore, necessarily connected with the impression, so long as this portion of the spinal cord, and the nerves proceeding from it, are capable of performing their functions: and it is one of those to which we may give the name of instinctive, to distinguish it from those which are effected by an effort of the Will, intentionally directed to accomplish a certain purpose. It may even take place without the animal being aware of the contact of any substance to be swallowed with the lining of the pharynx; for there is good reason to believe that when the brain has been destroyed, or paralyzed by a blow, all sensibility is destroyed; and we have also sufficient reason to consider it as suspended in profound sleep or apoplexy, in which states swallowing is still performed. In the severest cases of apoplexy, however, the power of swallowing is lost; and this is a symptom of great

Cœcum Appendix of the Cœcum

DEGLUTITION-DIGESTIVE APPARATUS.

181

danger, since it shows that not the brain alone, but the upper part of the spinal cord, is suffering from the pressure; and that the movements of respiration, which depend upon a similar action of the nervous system (§ 340), will probably soon cease, so that death must ensue.

Digestive Apparatus.

197. The food, thus propelled downwards by the action of the muscles of the pharynx and of the oesophagus (gullet),

Gall-Bladder

Large Intestine

Small Intestine

Fig. 108.-DIGESTIVE APPARATUS OF MAN.

arrives, in Man and the Mammalia, at the stomach; which is a large membranous bag, placed across the upper part of the

182

FORM OF THE STOMACH.

abdomen (fig. 108). The form of this stomach varies much, according to the nature of the aliment to be digested. Where the food is animal flesh, which is easily dissolved, the stomach is small, and appears like a mere enlargement of the alimentary tube; this is the case in the Cat tribe, for example. In Herbivorous animals, on the contrary, the stomach is very large, the food being delayed there a long time on account of the difficulty with which it is digested; and the principal part of its cavity is not a simple enlargement of the alimentary tube, but a bag or sac that bulges out, as it were, on the left side of that canal. By the degree of this bulging, we can judge of the nature of the food on which the animal is destined to live. Thus in Man (fig. 108), the large end of the stomach, situated on the left side (the right side of the figure as we look at it), is moderately developed; showing, as we might expect from the form of his teeth, as well as from his natural tastes, that he is adapted for a diet in which animal and vegetable food are mixed. In the purely carnivorous tribes, this large end of the stomach is almost deficient; whilst in the herbivorous races, it is enormously developed, and sometimes forms a distinct pouch.

Esophagus

Cardia

3d Stom.

Intestine

Pylorus 4th Stom. 2d Stom. 1st Stom.

Fig. 109.-STOMACHS OF THE SHEEP.

198. The most complex form of the stomach among Mammals, is that which we find in the animals that ruminate or chew the cud. It possesses, in fact, no less than four distinct cavities, through all of which the food has to pass during the

STOMACH OF RUMINANTS.

183

process of digestion. The external appearance of the stomach of the Sheep is seen in fig. 109; and its interior is displayed in fig. 110. The food of the Ruminant animals is not chewed by them before it is first swallowed. In their wild state, they are peculiarly exposed to the attacks of their carnivorous enemies, when they come down from their rocky heights to browse upon the rich pastures of the valleys. If they were then obliged to masticate every mouthful, they would be subjected to long-continued danger at every meal; but, by the curious construction of the digestive apparatus, this is spared to them; for they are enabled to swallow their food as fast as they can crop it, and afterwards to return it to their mouths, so as to masticate it at their leisure, when they have retreated to a place of safety. The crude unmasticated food, which is brought-down by the oesophagus, first enters the large cavity on the left side, which is commonly termed the paunch. It is there soaked, as it were, in the fluid secreted

Fig. 110. SECTION OF THE STOMACHS OF THE SHEEP.

by its walls; and is then transmitted to the second cavity, which, from the sort of network produced by the irregular folding of its lining membrane, is called the reticulum or honey-comb stomach. This stomach also has a direct communication with the oesophagus, and appears destined especially to receive the fluid that is swallowed; for this passes immediately into it, without going into the first stomach at all. The folds of its lining membrane present a large surface, through which fluid may be absorbed into the system. It is

here that we find the curious arrangement of water-cells in the stomach of the Camel, by which that animal is enabled to retain a supply of water for several days. These cells correspond with the little pits which are seen in the honey-comb stomach of the Sheep, but are much deeper, and their orifices may be closed by the action of a set of muscular fibres which pass in every direction round each, so as to form a net-work including these orifices in its meshes.

199. After the food has been macerated in the fluids of the first and second stomachs, it is returned to the mouth by a reversed peristaltic action of the oesophagus, which brings it up as a succession of globular pellets, that are formed by compression in a sort of mould at the lower end of the œsophagus. These pellets are subjected within the mouth to mastication and insalivation; and the food is then ready for the real process of digestion. It is this mastication which is commonly known as the "chewing of the cud;" and the animal, whilst performing it, seems the very picture of placid enjoyment. When again swallowed, the food is directed, by a peculiar valvular groove at the bottom of the oesophagus, into the third stomach, commonly termed the manyplies, from the peculiar manner in which its lining membrane is arranged. This presents a number of folds, lying nearly close to one another, like the leaves of a book, but all directed, by their free edges, towards the centre of the tube,—a narrow fold intervening between each pair of broad ones. The food has, therefore, to pass over a large surface, before it can reach the outlet of the cavity; and this leads to the fourth stomach, commonly termed the reed. This is the seat of the true digestive process, the gastric juice (§ 204) being formed here only; and it is from this that the rennet is taken, which is used in making cheese to cause the milk to coagulate or curdle. In the sucking animal, the milk passes directly into this fourth stomach, without entering either the first or second stomachs, and without being delayed in the third, the folds of which adhere together so as to form a narrow undivided tube. The paunch is at that time comparatively small, being of less size than the reed; and its dimensions increase, as soon as the young animal begins to distend it by swallowing solid vegetable matter.

200. In the digestive apparatus of Birds, we find a con