256 



THE POPULAR EDUCATOR. 



the heart gives the explanation of the difficulty of breath- 

 ing and palpitation sometimes experienced after a full meal, 

 and of the frequent pain in the region of the heart com- 

 plained of by persons suffering from stomach derangement, 

 for when it ia distended either by a full meal or by flatulence, 

 it pushes upwards the diaphragm, and thus contracting the 

 capacity of the chest, presses upon the heart, and embarrasses 

 the respiration. The muscular coat of the stomach consists of 

 three distinct layers, which, by the peculiarity of their arrange- 

 ment, enable the walls to perform three distinct offices to keep 

 the orifices of the stomach closed, save when it is necessary 

 to admit food, or allow of the escape of the digested food into 

 the intestine ; to keep the walls of the stomach in constant 

 contact with its contents, so ac to exercise some amount of 

 pressure upon them ; to impart to those contents a peculiar 

 kind of double motion, in order that they shall'be intimately 

 mixed and every portion brought in contact with the gastric 

 juice. 



If the mucous membrane of the stomach be examined 

 when the organ is distended, it is found to present a smooth, 

 velvety appearance ; but when the organ is empty it is drawn 

 into rugae, or folds. If it be examined with a slightly magnifying 

 power, it is seen to have a kind of honeycomb look, the re- 

 sult of the presence of a number of shadow depressions, or 

 cells, at the bottom of which minute orifices are visible. These 

 are the mouths of the tubular glands, which secrete the 

 gastric juice, the office of which will be presently described. 



The right upper extremity of the stomach is placed on a 

 lower level and more anteriorly than the one which communi- 

 cates with the oesophagus. It opens by a narrow valvular kind 

 of orifice into the first part of the small intestine, which is a 

 tube of about twenty feet in length, gradually diminishing in 

 size from its commencement to its termination. It is arbi- 

 trarily divided into three parts the first, into which the 

 stomach opens, is called the duodenum, and is about ten inches 

 in length. From its junction with the stomach it first of all 

 ascends, coming in relation with the under surface of the liver ; 

 it then descends and crosses to the left side of the body, where 

 it terminates in the second portion, called the jejunum. These 

 constitute about two-fifths of the small intestines. There is 

 no characteristic mark to distinguish where the jejunum ter- 

 minates and the ileum begins. The ileum is the lowermost 

 portion of the small intestine, and is twisted into many coils or 

 convolutions. The large intestine extends from the termina- 

 tion of the ileum to the lower opening of the canal. Like the 

 small intestine, it may be divided into three parts. The caecum, 

 into which the ileum opens, is a blind pouch, shut off from the 

 small intestine by a valve, and is the most dilated portion of 

 the large intestine. It is situated in the lower portion of the 

 right side of the abdomen. From the caecum springs the colon, 

 which first of all passes directly upwards along the right side 

 of the abdomen, then curves across, lying immediately below 

 the liver and stomach to the left side, where it becomes 

 much narrower, and is twisted into many coils, from which 

 cause it here receives the name of the sigmoid flexure. From 

 the termination of the colon the rectum, so called on account 

 of its comparative straightness, continues the intestine. This 

 is the terminating portion of the alimentary canal. 



The structure of the intestines resembles that of other parts 

 of the alimentary canal, being externally composed of muscular 

 fibre, which is here arranged in two layers, and internally of 

 mucous membrane. The characters of the mucous membrane 

 vary according to the part of the intestine which is examined. 

 In the small intestine, especially the upper part, are found ap- 

 pearances which do not occur elsewhere. In the first place, 

 owing to its much greater extent than the muscular walls, the 

 mucous membrane hangs in transverse folds, which partly 

 obstruct the canal, and perform a double office, serving to retard 

 the passage of the food through the intestines, and to present 

 a larger surface for absorption. Another of the peculiarities of 

 this membrane is caused by another provision for the per- 

 formance of the same function of absorption. When the mucous 

 membrane is examined, it presents a fleecy, velvety appearance. 

 This, when magnified, is found to be due to the presence and 

 projection from its surface of great numbers of little club-like 

 processes, called villi. These are exclusively confined to the 

 small intestines, and are so closely set that it is calculated that 

 there are not fewer than four millions of them. Each villus 



consists of a fold of mucous membrane, enclosing a network of 

 blood-vessels, and also of another set of vessels, called lacteala ; 

 these latter convey the prepared elements of the food into the 

 circulation, and will be further described when we have traced 

 the food to its destination. Numerous little glanda are found 

 scattered throughout the whole extent of the intestines, the 

 secretions of which perform minor parts in the function of 

 digestion. 



We must now consider those organs which, though not a 

 portion of the alimentary canal, act important parts in preparing 

 the food for assimilation by the system ; these are two in 

 number and have this in common, that they are both glan- 

 dular structures the liver and the pancreas. 



The liver, which is the largest gland in the body, weighing 

 generally from three to four pounds, in external appearance is a 

 large flat substance of a dark-brown colour, and is divided into 

 two principal lobes, the right being much larger than the left. 

 It measures from ten to twelve inches transversely, and from five 

 to six inches from before backwards ; it is situated on the right 

 side of the abdomen, in contact with the diaphragm, but 

 stretches across beyond the middle line, so that the left lobe is 

 in relation with the stomach and duodenum. The anterior sur- 

 face of the liver is convex, and the upper and back part much 

 thicker than the lower edge. Its under surface is concave, and 

 presents about the centre of its lower edge, near the junction of 

 the right and left lobes, a hollow containing a conically-shaped 

 bag the gall-bladder. This acts as a reservoir for the bile, and 

 from it springs a duct which, after joining a similar one from 

 the pancreas, opens inbo the duodenum. Along this duct the bile 

 is poured into the intestines. 



If a section of the liver is made, it is found to be composed of 

 an immense aggregation of small lobules, each of which consists 

 of branches of blood-vessels, both vein and artery, bile-ducts, and 

 the secreting cells, which have the power of extracting the bile 

 from the blood. All these elements are bound together by con- 

 nective tissue, which is a continuation inwards of the external 

 capsule of the liver. 



The liver, in common with all other organs, has special vessels 

 to supply it with nutritive blood. This it derives from the 

 hepatic artery ; but, in addition, it has another supply peculiar 

 to itself. The portal vein, which carries the whole of the blood 

 distributed to the stomach, spleen, and small intestines back 

 towards the heart, instead of going direct to that organ, enters 

 the liver, and splits up into innumerable small branches, thus 

 bringing this blood, which contains the products of digestion, 

 into immediate contact with the secreting cells of the liver; 

 and it is from this blood that the bile is mainly secreted. 



In addition to secreting the bile, the liver has another rather 

 peculiar duty to fulfil that of changing some of the elements 

 contained in the blood into sugar, the ultimate destination of 

 which is probably the lungs, where it is consumed in the respira- 

 tory process, and contributes to the production of animal heat. 



The pancreas, or sweetbread, is a long thin gland, in shape 

 something like a dog's tongue, lying deep in the abdomen, imme- 

 diately below the stomach, its left extremity being in connection 

 with the spleen, and the right coming into relation with the left 

 lobe of the liver. In structure it closely resembles the salivary 

 glands, and its secretion, the pancreatic juice, has many points 

 of resemblance to saliva. Along the centre of the gland runs its 

 duct, which, as has been said, joins that from the gall-bladder, 

 and empties itself into the duodenum. 



In describing the alimentary canal we have spoken of it all the 

 way through as having two coats, but that part of it which is 

 contained in the abdomen has, in common with the rest of the 

 abdominal organs, an external covering formed by a membranous 

 closed sac, which lines the abdominal walls, and is reflected on 

 to the organs contained within them. This sac is called the 

 peritoneum, and is one of the serous membranes, which differ 

 from the mucous in being closed sacs, containing a certain 

 quantity of fluid. Other serous membranes are the pleurae, the 

 pericardium, and the arachnoid ; also the various bursaa scat- 

 tered about the body, the office of which has been already 

 described. The peritoneum sends out processes which act as 

 ligaments, holding the various organs in their places, and it also 

 serves to prevent any undue amount of friction between them. 



We have now examined all the parts that are concerned in the 

 function of digestion, and must pass on to examine the nature 

 and sources of the different varieties of food. 



