251. What is said of the arrangement of the fibres of the muscular coat of the large intestine? 252. What are the lacteals? Give their course from the mucous coat of the intestine to the thoracic duct.

253. Describe the course of the thoracic duct. How is the venous blood prevented from pa.s.sing into this duct?

[Ill.u.s.tration: Fig. 62. A portion of the small intestine, lacteal vessels, mesenteric glands, and thoracic duct. 1, The intestine. 2, 3, 4, Mesenteric glands, through which the lacteals pa.s.s to the thoracic duct.

5, 6, The thoracic duct. 7, The point in the neck where it turns down to enter the vein at 8. 9, 10, The aorta. 11, 12, Vessels of the neck. 13, 14, 15, The large veins that convey the blood and chyle to the heart. 16, 17, The spinal column. 18, The diaphragm, (midriff.)]

Explain fig. 62. What is said respecting the mesenteric glands?

_Observation._ The mesenteric glands, which are situated between two layers of serous membrane (mesentery) that connects the small intestine with the spinal column, occasionally become diseased in childhood, and prevent the chyle from pa.s.sing to the thoracic duct.

Children thus affected have a voracious appet.i.te, and at the same time are becoming more and more emaciated. The disease is called mesenteric consumption.

254. The LIVER, a gland appended to the alimentary ca.n.a.l, is the largest organ in the system, and weighs about four pounds. It is situated in the right side, below the diaphragm, and is composed of several lobes. Its upper surface is convex; its under, concave. This organ is retained in its place by several ligaments. It performs the double office of separating impurities from the venous blood, and of secreting a fluid (bile) necessary to chylification. On the under surface of the liver is a membranous sac, called the _gall-cyst_, which is generally considered as a reservoir for the bile.

[Ill.u.s.tration: Fig. 63. The under surface of the liver. 1, The right lobe. 2, The left. 3, 4, Smaller lobes. 10, The gall-bladder, or cyst, lodged in its depression. 17, The notch on the posterior border, for the spinal column.]

_Observation._ A good idea of the liver and intestines can be obtained by examining these parts of a pig. In this animal, the sacs, or pouches, of the large intestine are well defined.

255. The PANCREAS is a long, flattened gland, a.n.a.logous to the salivary glands. It is about six inches in length, weighs three or four ounces, and is situated transversely across the posterior wall of the abdomen, behind the stomach. A duct from this organ opens into the duodenum.

254. Describe the liver. 255. What is said of the pancreas?

256. The SPLEEN, (milt,) so called because the ancients supposed it to be the seat of melancholy, is an oblong, flattened organ, situated in the left side, in contact with the diaphragm, stomach, and the pancreas. It is of a dark, bluish color, and is abundantly supplied with blood, but has no duct which serves as an outlet for any secretion. Its use is not well determined.

[Ill.u.s.tration: Fig. 64. The pancreas with its duct, through which the pancreatic secretion pa.s.ses into the duodenum.]

257. The OMENTUM (caul) consists of four layers of the serous membrane, which descends from the stomach and transverse colon. A quant.i.ty of adipose matter is deposited around its vessels, which ramify through its structure. Its function is twofold in the animal economy. 1st. It protects the intestines from cold. 2d. It facilitates the movements of the intestines upon each other during their vermicular, or worm-like action.

258. Every part of the digestive apparatus is supplied with arteries, veins, lymphatics, and nervous filaments, from the ganglionic system of nerves.

256. Why is the spleen so called? What is peculiar to this organ? 257.

Of what is the omentum composed? What is its use? 258. With what is every part of the digestive apparatus supplied?

CHAPTER XIV.

PHYSIOLOGY OF THE DIGESTIVE ORGANS.

259. Substances received into the stomach as food, must necessarily undergo many changes before they are fitted to form part of the animal body. The solid portions are reduced to a fluid state, and those parts that will nourish the body are separated from the waste material.

260. The first preparation of food for admission into the system, consists in its proper mastication. The lips in front, the cheeks upon the side, the soft palate, by closing down upon the base of the tongue, retain the food in the mouth, while it is subjected to the; process of _mas-ti-cation_, (chewing.) The tongue rolls the ma.s.s around, and keeps it between the teeth, while they divide the food to a fineness suitable for the stomach.

261. While the food is in process of mastication, there is incorporated with it a considerable amount of _sa-liva_, (spittle.) This fluid is furnished by the salivary glands, situated in the vicinity of the mouth. The saliva moistens and softens the food, so that, when carried into the pharynx. it is pa.s.sed, with ease, through the oesophagus into the stomach.

262. When the food has been properly masticated, (and in rapid eaters when it is not,) the soft palate is raised from the base of the tongue backward, so as to close the posterior opening through the nostrils.

By a movement of the muscles of the tongue, cheeks, and floor of the mouth, simultaneous with that of the soft palate, the food is pressed into the upper part of the pharynx.

259-272. _Give the physiology of the digestive organs._ 259. What is necessary before food can nourish the body? 260. Describe how mastication is performed. 261. Of what use is the saliva in the process of mastication? 262. How is the food pressed into the pharynx?

263. When in the pharynx, the food and drink are prevented from pa.s.sing into the trachea by a simple valve-like arrangement, called the _ep-i-glottis_. The ordinary position of this little organ is perpendicular, so as not to obstruct the pa.s.sage of air into the lungs; but in the act of swallowing, it is brought directly over the opening of the trachea, called the _glottis_. The food, being forced backward, pa.s.ses rapidly over the epiglottis into the oesophagus, where the circular band of muscular fibres above, contracts and forces the food to the next lower band. Each band relaxes and contracts successively, and thus presses the alimentary ball downward and onward to the stomach.[8]

[8] The process of deglut.i.tion may be comprehended by a.n.a.lyzing the operation of swallowing food or saliva.

_Observation._ If air is inhaled when the food or drink is pa.s.sing over the glottis, some portions of it may be carried into the larynx or trachea. This produces violent spasmodic coughing, and most generally occurs when an attempt is made to speak while masticating food; therefore, never talk when the mouth contains food.

263. When the food is in the pharynx, how is it prevented from pa.s.sing into the trachea, or windpipe? Describe how it is pa.s.sed into the stomach? Give the observation. 264. Describe how the food in the stomach is converted into chyme.

264. When the food reaches the stomach, the gastric glands are excited to action, and they secrete a powerful solvent, called gastric juice.

The presence of food in the stomach also increases a contractile action of the muscular coat, by which the position of the food is changed from one part of this cavity to another. Thus the aliment is brought in contact with the mucous membrane, and each portion of it becomes saturated with gastric juice, by which it is softened, or dissolved into a pulpy h.o.m.ogeneous ma.s.s, of a creamy consistence, called _Chyme_. The food is not all converted into chyme at the same time; but as fast as it is changed, it pa.s.ses through the pyloric orifice into the duodenum.

_Observation._ The gastric juice has the property of coagulating liquid alb.u.minous matter when mixed with it. It is this property of rennet, which is an infusion of the fourth stomach of the calf, by which milk is coagulated, or formed into "curd."

265. The CHYME is conveyed through the pyloric orifice of the stomach into the duodenum. The chyme not only excites an action in the duodenum, but also in the liver and pancreas. _Mucus_ is then secreted by the duodenum, _bile_ by the liver, and _pancreatic fluid_ by the pancreas. The bile and pancreatic fluid are conveyed into the duodenum, and mixed with the chyme. By the action of these different fluids, the chyme is converted into a fluid of a whitish color, called _Chyle_, and into residuum.

_Observation._ The bile has no agency in the change through which the food pa.s.ses in the stomach. In a healthy condition of this organ, no bile is found in it. The common belief, that the stomach has a redundancy of this secretion, is erroneous. If bile is ejected in vomiting, it merely shows, not only that the action of the stomach is inverted, but also that of the duodenum. A powerful emetic will, in this way, generally bring this fluid from the most healthy stomach. A knowledge of this fact might save many a stomach from the evils of emetics, administered on false impressions of their necessity, and continued from the corroboration of these by the appearance of bile, till derangement, and perhaps permanent disease, are the consequences.

266. The CHYLE and residual matter are moved over the mucous surface of the small intestine, by the action of its muscular coat. As the chyle is carried along the tract of the intestine, it comes in contact with the villi, where the lacteal vessels commence. These imbibe, or take up, the chyle, and transfer it through the mesenteric glands into the thoracic duct, through which it is conveyed into a large vein at the lower part of the neck. In this vein the chyle is mixed with the venous fluid. The residual matter is conveyed into the large intestine, through which it is carried and excreted from the system.

(Appendix E.)

What peculiar property has gastric juice? 265. Where and how is chyme converted into chyle? What is said in regard to the bile? 266. What becomes of the chyle? Of the residuum?