GASTRIC JUICE. The minute tubes, or follicles, situated in the mucous membrane of the stomach, secrete a colorless, acid liquid, termed the gastric juice. This fluid appears to consist of little more than water, containing a few saline matters in solution, and a small quantity of free hydrochloric acid, which gives it an acid reaction. In addition to these, however, it contains a small quantity of a peculiar organic substance, termed _pepsin_, which in chemical composition, is very similar to ptyalin, although it is very different in its effects. When food is introduced into the stomach, the peristaltic contractions of that organ roll it about, and mingle it with the gastric juice, which disintegrates the connective tissue, and converts the albuminous portions into the substance called chyme, which is about the consistency of pea-soup, and which is readily absorbed through the animal membranes into the blood of the delicate and numerous vessels of the stomach, whence it is conveyed to the portal vein and to the liver. The secretion of the gastric juice is influenced by nervous conditions. Excess of joy or grief effectually retard or even arrest its flow.
INTESTINAL JUICE. In the small intestine, a secretion is found which is termed the _intestinal juice_. It is the product of two classes of glands situated in the mucous membrane, and termed respectively, the _follicles of Lieberkuhn_ and the _glands of Brunner_. The former consist of numerous small tubes, lined with epithelium, which secrete by far the greater portion of this fluid. The latter are clusters of round follicles opening into a common excretory duct. These sacs are composed of delicate, membranous tissue, having numerous nuclei on their walls.
The difficulty of obtaining this juice for experiment is obvious, and therefore its chemical composition and physical properties are not known. The intestinal juice resembles the secretion of the mucous follicles of the mouth, being colorless, vitreous in appearance, and having an alkaline reaction.
PANCREATIC JUICE. This is a colorless fluid, secreted in a lobular gland which is situated behind the stomach, and runs transversely from the spleen across the vertebral column to the duodenum. The most important constituent of the pancreatic juice is an organic substance, termed _pancreatin_.
THE BILE. The blood which is collected by the veins of the stomach, pancreas, spleen, and intestines, is discharged into a large trunk called the portal vein, which enters the liver. This organ also receives arterial blood from a vessel called the _hepatic artery_, which is given off from the aorta below the diaphragm. If the branches of the portal vein and hepatic artery be traced into the substance of the liver, they will be found to accompany one another, and to subdivide, becoming smaller and smaller. Finally, the portal vein and hepatic artery will be found to terminate in capillaries which permeate the smallest perceptible subdivisions of the liver substance, which are polygonal masses of not more than one-tenth of an inch in diameter, called the _lobules_. Every lobule rests upon one of the ramifications of a great vessel termed the _hepatic vein_, which empties into the inferior vena cava. There is also a vessel termed the _hepatic duct_ leading from the liver, the minute subdivisions of which penetrate every portion of the substance of that organ. Connected with the hepatic duct, is the duct of a large oval sac, called the _gall-bladder_.
Each lobule of the liver is composed of minute cellular bodies known as the _hepatic cells_. It is supposed that in these cells the blood is deprived of certain materials which are converted into bile. This secretion is a glutinous fluid, varying in color from a dark golden brown to a bright yellow, has a specific gravity ranging from 1018 to 1036, and a slightly alkaline reaction. When agitated, it has a frothy appearance. Physiologists have experienced much difficulty in studying the character of this secretion from the instability of its constituents when subjected to chemical examination.
[Illustration: Fig. 52.
Section of the Liver, showing the ramifications of the portal vein. 1. Twig of portal vein. 2, 2', 2", 2"'. Interlobular vein. 3, 3', 3", Lobules.]
_Biliverdin_ is an organic substance peculiar to the bile, which imparts to that secretion its color. When this constituent is re-absorbed by the blood and circulates through the tissues, the skin assumes a bright yellow hue, causing what is known as the jaundice. _Cholesterin_ is an inflammable crystallizable substance soluble in alcohol or ether. It is found in the spleen and all the nervous tissues. It is highly probable that it exists in the blood, in some state or combination, and assumes a crystalline form only when acted upon by other substances or elements.
Two other constituents, more important than either of the above, are collectively termed _biliary salts_. These elements were discovered in 1848, by Strecker, who termed them _glycocholate_ and _taurocholate of soda_. Both are crystalline, resinous substances, and, although resembling each other in many respects, the chemist may distinguish them by their reaction, for both yield a precipitate if treated with subacetate of lead, but only the glycocholate will give a precipitate with acetate of lead. In testing for biliary substances, the most satisfactory method is the one proposed by Pettenkoffer. A solution of cane-sugar, one part of sugar to four parts of water, is mixed with the suspected substance. Dilute sulphuric acid is then added until a white precipitate falls, which is re-dissolved in an excess of the acid. On the addition of more sulphuric acid, it becomes opalescent, and passes through the successive hues of scarlet, lake, and a rich purple. Careful experiments have proved that it is a _constant_ secretion; but its flow is mere abundant during digestion. During the passage through the intestines it disappears. It is not eliminated, and Pettenkoffer's test has failed to detect its existence in the portal vein. These facts lead physiologists to the conclusion, that it undergoes some transformation in the intestines and is re-absorbed.
After digestion has been going on in the stomach for some time, the semi-digested food, in the form of chyme, begins to pass through the _pyloric orifice_ of the stomach into the duodenum, or upper portion of the small intestine. Here it encounters the intestinal juice, pancreatic juice, and the bile, the secretion of all of which is stimulated by the presence of food in the alimentary tract. These fluids, mingling with the chyme, give it an alkaline reaction, and convert it into chyle. The transformation of starch into sugar, which is almost, if not entirely, suspended while the food remains in the stomach, owing to the acidity of the chyme, is resumed in the duodenum, the acid of the chyme, being neutralized by the alkaline secretions there encountered.
Late researches have demonstrated that the pancreatic juice exerts a powerful effect on albuminous matters, not unlike that of the gastric juice.
Thus, it seems that while in the mouth only starchy, and while in the stomach only albuminous substances are digested, in the small intestine all kinds of food materials, starchy, albuminoid, fatty and mineral, are either completely dissolved, or minutely subdivided, and so prepared that they may be readily absorbed through the animal membranes into the vessels.
MILK. The milk is a white, opaque fluid, secreted in the lacteal glands of the female, in the mammalia. These glands consist of numerous follicles, grouped around an excretory duct, which unites with similar ducts coming from other lobules. By successive unions, they form large branches, termed the _lactiferous ducts_, which open by ten to fourteen minute orifices on the extremity of the nipple. The most important constituent of milk is _casein_; it also contains oily and saccharine substances. This secretion, more than any other, as influenced by nervous conditions. A mother's bosom will fill with milk at the thought of her infant child. Milk is sometimes poisoned by a fit of ill-temper, and the infant made sick and occasionally thrown into convulsions, which in some instances prove fatal. Sir Astley Cooper mentions two cases in which terror instantaneously and permanently arrested this secretion. It is also affected by the food and drink. Malt liquors and other mild alcoholic beverages temporarily increase the amount of the secretion, and may, in rare instances, have a beneficial effect upon the mother.
They sometimes affect the child, however, and their use is not to be recommended unless the mother is extremely debilitated, and there is a deficiency of milk.
CHAPTER XI.
PHYSIOLOGICAL ANATOMY.
EXCRETION.
The products resulting from the waste of the tissues are constantly being poured into the blood, and, as we have seen, the blood being everywhere full of corpuscles, which, like all living things, die and decay, the products of their decomposition accumulate in every part of the circulatory system. Hence, if the blood is to be kept pure, the waste materials incessantly poured into this fluid, or generated in it, must be as continually removed, or excreted. The principal sets of organs concerned in effecting the separation of excrementitious substances from the blood are the lungs, the skin, and the kidneys.
The elimination of carbonic acid through the lungs has already been described on page 66, and the excretory function of the skin on page 70.
[Illustration: Fig. 53.
View of the kidneys, ureters, and bladder. ]
The kidneys are two bean-shaped organs, placed at the back of the abdominal cavity, in the region of the loins, one on each side of the spine. The convex side of each kidney is directed outwards, and the concave side is turned inwards towards the spine. From the middle of the concave side, which is termed the _hilus_, a long tube of small caliber, called the _ureter_, proceeds to the bladder. The latter organ is an oval bag, situated in the pelvic cavity. It is composed principally of elastic muscular fibers, and is lined internally with mucous membrane, and coated externally with a layer of the _peritoneum_, the serous membrane which lines the abdominal and pelvic cavities. The ureters enter the bladder through its posterior and lower wall, at some little distance from each other. The openings through which the ureters enter the bladder are oblique, hence it is much easier for the secretion of the kidneys to pass from the ureters into the bladder than for it to get the other way. Leading from the bladder to the exterior of the body is a tube, called the _urethra_, through which the urine is voided.
The excretion of the kidneys, termed the _urine_, is an amber-colored or straw-colored fluid, naturally having a slightly acid reaction, and a specific gravity ranging from 1,015 to 1,025. Its principal constituents are _urea_ and _uric acid_, together with various other animal matters of less importance, and saline substances, held in solution in a proportionately large amount of water. The composition of the urine and the quantity excreted vary considerably, being influenced by the moisture and temperature of the atmosphere, by the character of the food consumed, and by the empty or replete condition of the alimentary tract.
On an average a healthy man secretes about fifty ounces of urine in the twenty-four hours. This quantity usually holds in solution about one ounce of urea, and ten or twelve grains of uric acid. In the amount of other animal matters, and saline substances, there is great variation, the quantity of these ranging from a quarter of an ounce to an ounce.
The principal saline substances are common salt, the sulphates and phosphates of potassium, sodium, calcium, and magnesium. In addition to the animal and the saline matters, the urine also contains a small quantity of carbonic acid, oxygen and nitrogen.
CHAPTER XII.
PHYSIOLOGICAL ANATOMY.
THE NERVOUS SYSTEM.
Hitherto, we have only considered the anatomy and functions of the organs employed in Digestion, Absorption, Circulation, Respiration, Secretion and Excretion. We have found the vital process of nutrition to be, in all its essential features, a result of physical and chemical forces; in each instance we have presupposed the existence and activity of the nerves. There is not an inch of bodily tissue into which their delicate filaments do not penetrate, and form a multitude of conductors, over which are sent the impulses of motion and sensation.
[Illustration: Fig. 54.
The Nervous System.]
Two elements, _nerve-fibers_ and _ganglionic corpuscles_, enter into the composition of nervous tissue. Ordinary nerve-fibers in the living subject, or when fresh, are cylindrical-shaped filaments of a clear, but somewhat oily appearance. But soon after death the matter contained in the fiber coagulates, and then the fiber is seen to consist of an extremely delicate, structureless, outer membrane, which forms a tube through the center of which runs the _axis-cylinder_. Interposed between the axis-cylinder and this tube, there is a fluid, containing a considerable quantity of fatty matter, from which is deposited a highly refracting substance which lines the tube. There are two sets of nerve-fibers, those which transmit sensory impulses, called _afferent_ or _sensory_ nerves, and those which transmit motor impulses, called _efferent_ or _motor_ nerves. The fibers when collected in bundles are termed nerve trunks. All the larger nerve-fibers lie side by side in the nerve-trunks, and are bound together by delicate connective tissue, enclosed in a sheath of the same material, termed the _neurilemma_. The nerve-fibers in the trunks of the nerves remain perfectly distinct and disconnected from one another, and seldom, or never, divide throughout their entire length. However, where the nerves enter the nerve-centers, and near their outer terminations, the nerve-fibres often divide into branches, or at least gradually diminish in size, until, finally, the axis-cylinder, and the sheath with its fluid contents, are no longer distinguishable. The investing membrane is continuous from the origin to the termination of the nerve-trunk.
[Illustration: Fig. 55.
Division of a nerve, showing a portion of a nervous trunk (_a_) and separation of its filaments (_b, c, d, e_.)]
In the brain and spinal cord the nerve-fibers often terminate in minute masses of a gray or ash-colored granular substance, termed _ganglia_, or _ganglionic corpuscles_.
The ganglia are cellular corpuscles of irregular form, and possess fibrous appendages, which serve to connect them with one another. These ganglia form the cortical covering of the brain, and are also found in the interior of the spinal cord. According to Kolliker, the larger of these nerve-cells measure only 1/200 of an inch in diameter. The brain is chiefly composed of nervous ganglia.
Nerves are classified with reference to their origin, as _cerebral_--those originating in the brain, and _spinal_--those originating in the spinal cord.
There are two sets of nerves and nerve-centers, which are intimately connected, but which can be more conveniently studied apart. These are the _cerebro-spinal_ system, consisting of the cerebro-spinal axis, and the cerebral and spinal nerves; and the _sympathetic_ system, consisting of the chain of sympathetic ganglia, the nerves which they give off, and the nervous trunks which connect them with one another and with the cerebro-spinal nerves.
THE CEREBRO-SPINAL SYSTEM.
THE CEREBRO-SPINAL AXIS consists of the brain and spinal cord. It lies in the cavities of the cranium and the spinal column. These cavities are lined with a very tough fibrous membrane, termed the _dura mater_, which serves as the periosteum of the bones which enter into the formation of these parts. The surface of the brain and spinal cord is closely invested with an extremely vascular, areolar tissue, called the _pia mater_. The numerous blood-vessels which supply these organs traverse the pia mater for some distance, and, where they pass into the substance of the brain or spinal cord, the fibrous tissue of this membrane accompanies them to a greater or less depth. The inner surface of the dura mater and the outer surface of the pia mater are covered with an extremely thin, serous membrane, which is termed the _arachnoid_ membrane. Thus, one layer of the arachnoid envelopes the brain and spinal cord, and the other lines the dura mater. As the layers become continuous with each other at different points, the arachnoid, like the pericardium, forms a shut sac, and, like other serous membranes, it secretes a fluid, known as the _arachnoid fluid_. The space between the internal and the external layers of the arachnoid membrane of the brain is much smaller than that enclosed by the corresponding layers of the arachnoid membrane of the spinal column.
[Illustration: Fig. 56.
Cross-section of spinal cord.]
THE SPINAL CORD is a column of soft, grayish-white substance, extending from the top of the spinal canal, where it is continuous with the brain, to about an inch below the small of the back, where it tapers off into a filament. From this nerve are distributed fibers and filaments to the muscles and integument of at least nine-tenths of the body.
The spinal cord is divided in front through the middle nearly as far as its center, by a deep fissure, called the _anterior fissure_, and behind, in a similar manner, by the posterior _fissure_. Each of these fissures is lined with the pia mater, which also supports the blood-vessels which supply the spinal cord with blood. Consequently, the substance of the two halves of the cord is only connected by a narrow isthmus, or bridge, perforated by a minute tube, which is termed the _central canal_ of the spinal cord.
Each half of the spinal cord is divided lengthwise into three nearly equal parts, which are termed the anterior, lateral, and posterior columns, by the lines which join together two parallel series of bundles of nervous filaments, which compose the roots of the spinal nerves. The roots of those nerves, which are found along that line nearest the posterior surface of the cord, are termed the posterior roots; those which spring from the other line are known as the anterior roots.
Several of these anterior and posterior roots, situated at about the same height on opposite sides of the spinal cord, converge and combine into what are called the _anterior_ and _posterior bundles_; then two bundles, anterior and posterior, unite and form the trunk of a spinal nerve.
The nerve trunks make their way out of the spinal canal through apertures between the vertebra, called the _inter-vertebral foramina_ and then divide into numerous branches, their ramifications extending principally to the muscles and the skin. There are thirty-one pairs of spinal nerves, eight of which are termed cervical, twelve dorsal, five lumbar, and six sacral, with reference to that part of the cord from which they originate.