PHYSIOLOGICAL ANATOMY.
SECRETION.
The term _Secretion_, in its broadest sense, is applied to that process
by which substances are separated from the blood, either for the
reparation of the tissues or for excretion. In the animal kingdom this
process is less complicated than in vegetables. In the former it is
really a _separation_ of nutritive material from the blood. The process,
when effected for the removal of effete matter, is, in a measure,
chemical, and accordingly the change is greater.
Three elementary constituents are observed in secretory organs: the
cells, a basement membrane, and the blood-vessels. Obviously, the most
_essential_ part is the _cell_.
The physical condition necessary for the healthy action of the secretory
organs is a copious supply of blood, in which the nutritive materials
are abundant. The nervous system also influences the process of
secretion to a great extent. Intense emotion will produce tears, and the
sight of some favorite fruit will generally increase the flow of saliva.
The process of secretion depends upon the anatomical and chemical
constitution of the cell-tissues. The principal secretions are (1),
Perspiration; (2), Tears; (3), Sebaceous matter; (4), Mucus; (5),
Saliva; (6), Gastric juice; (7), Intestinal juice; (8), Pancreatic
juice; (9), Bile; (10), Milk.
PERSPIRATION is a watery fluid secreted in minute glands, which are
situated in every part of the skin, but are more numerous on the
anterior surfaces of the body. Long thread-like tubes, only 1/100th of
an inch in diameter, lined with epithelium, penetrate the skin, and
terminate in rounded coils, enveloped by a net-work of capillaries,
which supply the secretory glands with blood. It is estimated by Krause
that the entire number of perspiratory glands is two million three
hundred and eighty-one thousand two hundred and forty-eight, and the
length of each glandular coil being 1/16 of an inch, we may estimate the
length of tubing to be not less than two miles and a third. This
secretion has a specific gravity of 1003.5, and, according to Dr.
Dalton, is composed of
Water, 995.50
Chloride of Sodium, 2.23
Chloride of Potassium, 0.24
Sulphate of Soda and Potassa, 0.01
Salts of organic acids, with Soda and Potassa, 2.02
-------
1000.00
Traces of organic matter, mingled with a free volatile acid, are also
found in the perspiration. It is the acid which imparts to this
secretion its peculiar odor, and acid reaction. The process of its
secretion is continuous, but, like all bodily functions, it is subject
to influences which augment or retard its activity. If, as is usually
the case when the body is in a state of repose, evaporation prevents its
appearance in the _liquid_ form, it is called _invisible_ or _insensible
perspiration_. When there is unusual muscular activity, it collects upon
the skin, and is known as _sensible perspiration_. This secretion
performs an important office in the animal economy, by maintaining the
internal temperature at about 100° Fahr. Even in the Arctic regions,
where the explorer has to adapt himself to a temperature of 40° to 80°
below zero, the generation of heat in the body prevents the internal
temperature from falling below this standard. On the contrary, if the
circulation is quickened by muscular exertion, the warmer blood flowing
from the internal organs into the capillaries, raises the temperature of
the skin, secretion is augmented, the moisture exudes from the pores,
and perceptible evaporation begins. A large portion of the animal heat
is thrown off in this process, and the temperature of the skin is
reduced. A very warm, dry atmosphere can be borne with impunity but if
moisture is introduced, evaporation ceases, and the life of the animal
is endangered. Persons have been known to remain in a temperature of
about 300° Fahr. for some minutes without unpleasant effects. Three
conditions may be assigned as effective causes in retarding or
augmenting this cutaneous secretion, variations in the temperature of
the atmosphere, muscular activity, and influences which affect the
nerves. The emotions exert a remarkable influence upon the action of the
perspiratory glands. Intense fear causes great drops of perspiration to
accumulate on the skin, while the salivary glands remain inactive.
TEARS. The lachrymal glands are small lobular organs, situated at the
outer and upper orbit of the eye, and have from six to eight ducts,
which open upon the conjunctiva, between the eyelid and its inner fold.
This secretion is an alkaline, watery fluid. According to Dr. Dalton,
its composition is as follows:
Water, 882.0
Albuminous matter, 5.0
Chloride of Sodium, 13.0
Mineral Salts, a trace,
------
1000.0
The function of this secretion is to preserve the brilliancy of the eye.
The tears are spread over this organ by the reflex movement of the
eyelid, called winking, and then collected in the _puncta lachrymalia_
and discharged into the nasal passage. This process is constant during
life. The effect of its repression is seen in the dim appearance of the
eye after death. Grief or excessive laughter usually excite these glands
until there is an overflow.
SEBACEOUS MATTER. Three varieties of this secretion are found in the
body. A product of the sebaceous glands of the skin is found in those
parts of the body which are covered with hairs; also, on the face and
the external surface of the organs of generation. The _sebaceous glands_
consist of a group of flask-shaped cavities, opening into a common
excretory duct. Their secretion serves to lubricate the hair and soften
the skin. The _ceruminous glands_ of the _external auditory meatus_, or
outer opening of the ear, are long tubes terminating in a glandular
coil, within which is secreted the glutinous matter of the ear. This
secretion serves the double purpose of moistening the outer surface of
the membrana tympani, or ear-drum, and, by its strong odor, of
preventing the intrusion of insects. The _Meibomian glands_ are arranged
in the form of clusters along the excretory duct, which opens just
behind the roots of the eyelashes. The oily nature of this secretion
prevents the tears, when not stimulated by emotion, from overflowing the
lachrymal canal.
MUCUS. The mucous membranes are provided with minute glands which
secrete a viscid, gelatinous matter, called _mucus_. The peculiar animal
matter which it contains is termed _mucosin_. These glands are most
numerous in the Pharynx, Esophagus, Trachea, Bronchia, Vagina and
Urethra. They consist of a group of secreting sacs, terminating at one
extremity in a closed tube, while the other opens into a common duct.
The mucus varies in composition in different parts of the body; but in
all, it contains a small portion of insoluble animal matter. Its
functions are threefold. It lubricates the membranes, prevents their
injury, and facilitates the passage of food through the alimentary
canal.
SALIVA. This term is given to the first of the digestive fluids, which
is secreted in the glands of the mouth. It is a viscid, alkaline liquid,
with a specific gravity of about 1005. If allowed to stand, a whitish
precipitate is formed. Examinations with the microscope show it to be
composed of minute, granular cells and oil globules, mingled with
numerous scales of epithelium. According to Bidder and Schmidt, the
composition of saliva is as follows:
Water, 995.16
Organic matter, 1.34
Sulpho-cyanide of Potassium, 0.06
Phosphates of Sodium, Calcium and Magnesium, .98
Chlorides of Sodium and Potassium, .84
Mixture of Epithelium, 1.62
-------
1000.00
Two kinds of organic matter are present in the saliva; one, termed
_ptyalin_, imparts to the saliva its viscidity, and it obtained from the
secretions of the parotid, submaxillary and sublingual glands; another,
which is not glutinous, is distinguished by the property of coagulating
when subjected to heat. The saliva is composed of four elementary
secretions, derived respectively, from the mucous follicles of the
mouth, and the parotid, the submaxillary, and the sublingual glands. The
process of its secretion is constant, but is greatly augmented by the
contact of food with the lining membrane. The saliva serves to moisten
the triturated food, facilitate its passage, and has the property of
converting starch into sugar; but the latter quality is counteracted by
the action of the gastric juice of the stomach.
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.
* * * * *