13. Is gastric juice acid or alkaline in reaction during digestion?
QUESTIONS 285
14. What is the acidity due to? What percentage of hydrochloric
acid is normally present in the gastric juice?
15. What is the function of pepsin in the gastric secretion?
Rennin?
16. Is the intestinal secretion acid or alkaline in reaction?
17. Name the enzymes found in the pancreatic juice.
18. Give briefly the function of the enzymes trypsin, amylopsin,
steapsin, as regards their action during intestinal digestion.
19. Name the enzyme present in the intestinal juice and give its
function.
20. What cells secrete the bile and how does it leave the liver?'
21. Name the organ in which bile is stored.
22. How does the bile reach the intestine from the liver and
gall-bladder?
23. Give the physiologic functions of bile.
24. Give the contents of the colon following intestinal digestion.
25. Give the capacity of the infant's stomach at birth; two, four,
six, eight, ten, twelve, fourteen to eighteen weeks; five to six, seven
to eight, twelve to fourteen months.
26. Describe the functions of the stomach during gastric digestion
in infants.
27. In which portion of the alimentary canal is the nourishment
absorbed from during infant digestion?
CHAPTER XIII
ABSORPTION
ABSORPTION is the process whereby the nutritive
material, lymph, is transferred from the tissues; the
serous cavities pericardium, peritoneum, etc.; and
mucous membranes into the blood. The lymph is
absorbed from the mucous membrane of the alimen-
tary canal, as it is the principal source of nutritive
material used by the body for the maintenance of
the quantity and quality of the blood; while the
lymph absorbed from the serous cavities and tissues
represents a reabsorption of the nutritive materials
which have escaped through the capillary walls, and
are returned to the veins through the lymphatic
vessels. Were this lymph allowed to collect in the
tissues, there would occur an excessive accumulation,
and this condition would be readily accounted for in
the swelling of the subcutaneous tissue and organs
giving rise to a pathologic condition termed edema.
Under the chapter on digestion it was shown how
the food we eat is reduced to a liquid condition by
the action of the various gastric, pancreatic, and
intestinal juices and their ferments. This nutritive
material is taken up by the mucous membrane of
the intestine and absorbed, then reaches the blood-
current by way of the lymph channels lacteals, and
finally reaches the thoracic duct; or by way of the
venous capillaries of the mesenteric veins, and is carried
to the liver, and thence to the right side of the heart.
Before understanding the methods of food absorp-
tion, a description of the mucous membrane of the
intestines is necessary.
THE FUNCTION OF THE V1LLI 287
Structure of the Villi. The mucous membrane of
the small intestine is covered by tiny conical processes
which extend from the end of the pylorus of the stomach
to the end of the ileum. These, when examined under
the microscope, show a conical process covered with
columnar epithelial cells, each cell containing a nucleus,
and here and there goblet cells which secrete mucus.
The epithelial cells rest upon a basement membrane.
In the body of the villus that portion within the
mucous membrane contains a net-work of connective
tissue supporting arterioles, capillaries, venules, and
lymphatic vessels. In the centre of the villus is a
lymph capillary, usually single, with epithelial cells in
its wall.
The Function of the Villi. The action of the cells
of the villi during digestion is to absorb the nutritive
products of digestion. These products are taken
from the intestinal canal and transferred into the
lymph spaces within the body of the villi, from which
they are finally taken up by the blood capillaries and
lymphatics. There are two routes by which the nutri-
tive material passes into the general blood stream.
The capillaries which enter the villi and are in inti-
mate relation with the lymph space give up their
nutritive materials (blood plasma) by a transudation
through their walls, which forms the lymph; mean-
time the nutritive material absorbed from the intes-
tines by the cells of the villi undergoes metabolic
changes and enters the arterial capillaries from the
lymph spaces. This fenourished blood in the arterial
capillaries passes into the venules and then into the
larger veins of the intestines, to be conveyed to the
liver, which uses it to maintain its functions. The
lymph contained in the lymph spaces within the villi
is called chyle (a milk-white fluid), which is absorbed
by the lymphatic vessels called lacteals, and these
empty into the large mesenteric lymphatics to drain
288
ABSORPTION
into the thoracic duct, and the latter communicates
with the left subclavian vein, and thus returns the
excess of lymph back to the blood-stream.
The wonderful aspect of absorption is the apparently
unsolvable problem in regard to how these numbers
of cells in the villi of the intestinal mucous membrane
FIQ. 107
PLANE OF MU-
COUS SURFACE
Mucosa of small intestine in ideal vertical cross-section. (Testut, after
Heitzmann.)
can absorb the different constituents of the nutritive
materials from the alimentary canal and transfer
them into the lymph spaces to be absorbed by the
blood capillaries and lymphatics.
It is supposed to be due to a "selective action"
based on their organization and living condition, an
fiction which is to a great extent conditioned and
QUESTIONS 289
limited by the degree of diffusibility of the substances
to be absorbed (Brubake'r's Physiology, p. 225).
QUESTIONS
1. Describe absorption.
2. By what two sources does lymph reach the blood current?
3. What is the function of the cells in the villi of the mucous
membrane of the small intestines?
4. Where are the lymph spaces found?
5. What do you understand by the term lacteals?
6. What is chyle?
7. How does the nutritive material absorbed by the cells of
the villi reach the arterial capillaries? Where is this material
conveyed?
8. Where is the lymph absorbed by the lacteals from the lymph
spaces conveyed?
9. Name the vessels found in a villi.
19
CHAPTER XIV
SECRETION
SECRETION is a term applied to a process by which
a portion of the constituents of the blood are separ-
ated from the blood-stream, by the activities of the
endothelial cells of the capillary walls, as the blood
flows through the capillaries. In this process the
endothelial cell is aided by the physical forces
diffusion, osmosis, and filtration. The materials thus
separated are collectively termed lymph (Brubaker).
These secretions are utilized and adapted to many
and complex functions, dependent upon the secre-
tory organ which secretes the fluid and the membrane
it is poured out upon. They enable the tissues of
the body to repair, grow, and produce heat and
energy. Others are to promote digestion, etc., remove
foreign bodies (dust, etc.) from membranes, as the
conjunctiva, to prevent friction between the serous
membranes, as the pericardium, pleura, and peri-
toneum; and to prevent friction between the ex-
tremities of the bones entering into the formation of
the joints, as the fluid in synovial membranes.
Secretions are divided into internal and external
secretions. Internal secretions are fluids secreted by
the epithelial cells of certain organs of the body which
do not possess any ducts by which their secretion
is poured into any cavity or organ, but is reabsorbed
into the blood, and the contained specific constituents
aid in the nutrition of the body. These organs are:
the thyroid, thymus, adrenal, spleen, pituitary glands,
hypophysis, etc. (See description of the Ductless
Glands, page 303.)
THE SECRETING MEMBRANES 291
External secretions are fluids of a definite consist-
ency and known function which, when secreted by
the epithelial cells and poured from the organ by
means of a duct or ducts on to the membrane they are
to bathe, etc., perform this given activity. Such
secretions are: the saliva, mucus, milk, gastric juice,
sebaceous matter, etc.
The epithelium lining the secretory organs have
a general similar histologic arrangement, and resem-
blance; however, the difference in the constituents of
the secretion is supposed to be based upon their ulti-
mate chemic structure.
The epithelial secretory organ consists of a thin,
delicate membrane lined on its functionating surface
by a layer of epithelial cells and on the outer side
by a net- work of capillary bloodvessels, lymph vessels,
and nerves.
The epithelial secretor organs are subdivided into:
(1) secreting membranes; (2) secreting glands.
THE SECRETING MEMBRANES
These are the membranes lining the pulmonary
and gastro-intestinal tracts, the genito-urinary tracts,
and the serous membranes lining closed cavities, such
as the pleural, pericardial, peritoneal, and synovial
membranes.
The secretion from the various epithelial cells
lining mucous membranes possesses different com-
position, according to the locality. It is called mucus,
a pale, semitransparent, alkaline fluid containing
white cells and epithelial cells. Chemically it consists
of water, mineral salts, and a protein mucin. Most
of the mucus is secreted by the goblet cells. Mucus
is classified according to where secreted, as nasal,
bronchial, vaginal, urinary, and gastro-intestinal.
The serous membranes are practically large lymph
292 SECRETION
spaces and the contained fluid is practically lymph.
It diminishes friction when the organs they enclose
rub against one another.
Synovial membranes secrete a fluid resembling lymph,
but it also possesses a protein a mucin-like sub-
stance, which renders it viscid. Synovial membranes
prevent friction between adjacent surfaces of bone
entering into the formation of joints.
The other secretions of the body, as the aqueous
humor of the eye, gastric secretions, etc., will be
described under the physiology of the parts.
THE SECRETING GLANDS
These are a group of cells given off as a pouch from
the mucous membrane or skin, and communicating
with the lining membrane or surface the secretion
is to act upon by means of an open passageway,
called a duct. Their epithelial cells vary in their
structure and function dependent on their location.
The epithelial cells of the secretory glands are sur-
rounded by a net-work of blood capillaries, lymph
vessels, and nerves; the nerves are in direct connec-
tion with the epithelial cells and bloodvessels.
How these epithelial cells absorb from the lymph
and blood plasma their essential constituents of the
secretions and change them into their different chemic
and physiologic fluids is not definitely known, except
that they are the result of metabolic processes going
on within the cells.
All secretory glands are controlled by nerve centres
situated in the central nerve system. Normal
secretions of glands are brought about by a reflex
action. In all reflexes there must be a sensitive
surface to receive the impression (skin, mucous mem-
brane, etc.), an afferent nerve (one which transmits
the impression to the centres in the brain), an efferent,
nerve (one which transmits the return stimulus to
THE SECRETING GLANDS 293
a responsive organ in this case the cells of the secre-
tory organ or gland).
The active discharge of the secretion from the
cells is interrupted by periods of rest, during which
time, if they be examined under the microscope, after
the absorption of lymph, they will show accumula-
tions within themselves of their characteristic prod-
ucts as globules of mucin granules which are the
basic formation of the digestive ferments or enzymes,
granules of glycogen, globules of fat, sugar, and
protein, as in the case of the mammary gland.
Excretion is a process similiar to secretion, the only
difference being that the fluids removed are the
waste products from the cells formed as a result of
metabolism.
The Mammary Glands. These are accessory to
the reproductive system and secrete the milk. They
are two rounded eminences, one on each side of the
thorax, between the sternum and axilla and the
third and seventh ribs. Just below the centre is a
conical eminence, the nipple, which is dark, and is
surrounded by a pinkish areola which darkens in
pregnancy. It presents the orifices of the lactiferous
ducts, and consists of vessels mixed in with plain
muscular fibers,, and by friction may be made to
undergo erection.
The mamma consists of a number of lobes separated
by fibrous tissue and some adipose tissue. The lobes
are divided and subdivided into smaller lobules,
which are in turn made up of alveoli. Each lobe has
an excretory (galactophorous) duct, and these, about
sixteen in number, converge to the areola, there
dilating into ampullae or sinuses. They then become
smaller again, and surrounded by areolar tissue and
vessels, pass through the nipple to empty on the
surface by separate orifices.
Milk. Milk as obtained from the breast during
active secretion or lactation is an opaque, bluish-
294
SECRETION
white fluid, without any oder, sweetish in taste,
alkaline in reaction, and has a specific gravity of
from 1.025 to 1.040. Examined microscopically it
presents a clear fluid called the plasma, which holds
in suspension great numbers of oil globules. The
FIG. 108
Nipple
Lactiferous
duct.
Ampulla.
Loculi in connective tissue.
Dissection of the lower half of the female breast during the period of
lactation. (From Luschka.)
amount of milk secreted each day by a healthy woman
averages about 1200 c.c. Milk is the most important
of the animal foods, containing all the elements
necessary to properly nourish and develop the body,
and is used as a food.
THE SECRETING GLANDS 295
Differences in chemical composition of human and
cows' milk (Holt):
Woman's milk Cows' milk
average average
per cent per cent.
Fat 4.00 4.00
Sugar 7.00 4.50
Proteins 1.50 3.50
Salts 0.20 0.75
Water . 87.30 87.25
100.00 100.00
By the above it will be seen that cows' milk has an
excess of proteins and salts, and is deficient in sugar.
The secretion of milk is influenced by emotional
states, both as to quantity and quality, due to some
connection between the nerve centres and the gland
cells.
Colostrum. This is the first fluid secreted by the
breasts after the birth of the infant. It is a liquid
which resembles milk, and contains epithelial cells,
fat globules, colostrum corpuscles. Colostrum is
richer than the milk, containing more lactose and
inorganic salts. According to some authors it contains
compounds which act as a laxative to the newborn.
The Liver (Hepar). Description of the Liver. The
liver is the largest gland of the body, and fills the
entire hypochondrium, the greater portion of the
epigastrium, sometimes extending into the left hypo-
chondrium. It weighs from 50 to 60 ounces in the
male; 40 to 50 ounces in the female. Constitutes
one-eighteenth of the body weight in the adult, and
one-thirty-sixth of the body weight in the fetus. It
measures, transversely, from 8 to 9 inches; antero-
posterior, 4 to 5 inches, and vertically, near its right
surface, about 6 or 7 inches. Its specific gravity is
1.05.
The liver presents a superior surface which includes
the right and left lobes; an inferior surface, including
the right, left, caudate, spigelian, and quadrate lobes;
296
SECRETION
THE SECRETING GLANDS 297
anterior and posterior surfaces comprising the right
and left lobes; a lateral surface of the right lobe, only.
It has an inferior border or margin which is thin and
sharp, and notched opposite the falciform ligament,
for the round ligament (umbilical notch), and opposite
the cartilage of the ninth rib by a second notch for
the fundus of the gall-bladder.
The left extremity of the inferior margin of liver is
thin and flattened from above downward.
The ligaments of the liver are all peritoneal folds,
except the round ligament, which is a fetal remnant
of the umbilical vein. The ligaments hold the liver
in position, and are as follows:
Falciform or suspensory. Left lateral.
Coronary. Round.
Right lateral.
The lobes of the liver are also five in number. The
right is the largest, being six times as large as the
left. The left lobe is flattened, lies in the epigastrium,
and is in relation below with the stomach. The lobus
quadratus is on the under surface of the right lobe.
The Spigelian lobe lies behind and above the preceding.
The caudate lobe, or tuberculum caudatum, runs outward
from the base of the Spigelian lobe to the under surface
of the right lobe.
The fissures of the liver are five. The longitudinal
separates the right and left lobes. The fissure of the
ductus venosus is the part of the longitudinal fissure
behind the transverse. The transverse is the point
of exit (hepatic ducts) and entrance of the portal vein,
hepatic arteries, nerves, and lymphatic vessels. The
fissure for the gall-bladder is on the under surface of
the right lobe, parallel to the longitudinal fissure, sepa-
rated from it by the quadrate lobe. The fissure for
the inferior vena cava, sometimes a complete canal, lies
to the right of the Spigelian lobule.
The Structure of the Liver. It is covered by a serous
layer derived from the peritoneum, except the posterior
298 SECRETION
surface, which is in relation with the diaphragm for
about 3 inches, included between the reflections of the
coronary ligaments. Beneath this serous covering is
a fibrous or areolar capsule (capsule of Glisson), which
passes into the transverse fissure around the vessels
and blends with the areolar tissue which holds the liver
lobules together.
FIG. 110
Trunk of infralobt
vein.
ntralobular vein.
Horizontal section of injected liver (dog).
The lobules compose the main mass of the liver
substance, and consist of irregular chains of hepatic
cells, which secrete the bile, and are surrounded by a
capillary net-work of intralobular veins, which are the
minute terminations of the portal vein; they course
toward the centre of the lobule, opening into a
central intralobular vein; also small arteries, branches
of the hepatic artery, lie between the cells.
In addition, within the chain of cells are the minute
biliary ducts, or capillaries, which are the commence-
ment of the hepatic duct that conveys the bile formed
THE SECRETING GLANDS 299
by the liver cells to the intestinal canal and gall-
bladder.
The Functions of the Liver. (1) The liver secretes
the bile; (2) produces and stores glycogen until needed
to aid in the nutrition of the tissues; (3) aids in the
formation and excretion of urea. The production of
bile and its physiologic actions have been described.
(See Part Played by the Bile in Digestion, page 281.)
The Formation and Function of Glycogen. Glycogen is
derived from the dextrose resulting from the action of
the intestinal juices upon the food. It represents the
products of the carbohydrates absorbed as dextrose
and carried in the blood by the branches of the portal
vein to the liver, when it undergoes chemical changes,
due to the action of the liver cells, and is deposited
as a non-diffusible body. Glycogen is stored in the
liver until needed by the body tissues.
The Formation of Urea. Urea is believed to be
produced and excreted by the liver cells. It is formed
from ammonium salts, as carbonate, lactates, which
are formed as a result of tissue metabolism upon the
proteins contained in the food we eat, and these
salts are absorbed from the tissues or from the intes-
tines, and conveyed by the blood to the liver cells,
where they are converted into urea and eliminated as a
waste product. It is excreted by the kidneys and
found in the urine.
The Gall-bladder. This is a pear-shaped sac lying
in the impression of the right lobe of the liver. It is
the reservoir for the bile. It is 4 inches long and 1J
inches broad, holding 8 to 12 drams, and is held in
place by areolar tissue and the peritoneum. Its
relations are as follows: Above, liver; below, ascending
duodenum, pyloric end of stomach, hepatic flexure of
colon; in front, abdominal wall (ninth or tenth costal
cartilages) .
The hepatic duct is formed by the junction at an
obtuse angle of a branch from each lobe of the
300
SECRETION
liver and runs downward and to the right for nearly
2 inches and joins the cystic duct to form the common
bile duct. The cystic duct of the gall-bladder is 1| inches
FIG. Ill
The gall-bladder and bile ducts, opened up. (Spalteholz.)
long, and descends toward the left and joins the above
as described. The common bile duct is nearly 3 inches
long and 3 lines in diameter. It runs along the right
border of the lesser omentum, behind the first part
THE SECRETING GLANDS
301
of the duodenum, and between the pancreas and
descending duodenum, then to the right of the pan-
creatic duct, with which it communicates by a common
orifice, at the summit of a papilla situated just below
the middle of the inner wall of the second portion of
the duodenum. The cystic artery and veins comprise
the blood-supply of the gall-bladder and its duct.
FIG. 112
The pancreas and its relations. (Gray.)
The Pancreas. The pancreas is a compound race-
mose gland, of a pinkish-white color. Situated at the
back of the epigastrium and left hypochondrium ;
connected to the posterior abdominal wall by connected
tissue, and lies posterior to the stomach and behind
the peritoneum. It is 5 or 6 inches long; its breadth
is 1| inches; its thickness J to 1 inch, being greater
at its right extremity and upper border. The pancreas
is divided into a head, a neck, a body, and a tail.
The duct of the pancreas is called the pancreatic
duct or canal of Wirsung. It extends transversely
302 SECRETION
through the substance of the gland to drain the
lobules by means of small ducts which open into it.
Increasing in size it reaches the neck, passes down-
ward, backward, and obliquely to the right, piercing
the muscular and mucous coat of the second portion
of the duodenum where it opens into the ampulla of
Vater, common to it and the bile duct; the latter opens
into the canal of the duodenum.
The Structure of the Pancreas. It is similar in
structure to the salivary glands, consisting of numbers
of lobules, forming lobes, and all held together by
connective tissue. Each lobule contains one of the
branches of the main duct, which terminates in the
grape-like alveoli. The alveoli are lined by cylindric
cells, which differ in their appearance. They are
divided into a central set, in the end of the alveoli,
which are dark and granular, and a peripheral set,
in the outside of the former, which are clear. During
digestion the granular area becomes broader and the
cells show an increase in granules; in the interval of
rest following active digestion the clear zone increases
in width, showing an absence of granules.
The Areas or Islands of Langerhans are groups of
globular cells arranged in columns situated between
the alveoli; surrounded by connective tissue, which
separates them from the alveoli and each other. The
connective tissue contains large, twisted, capillary
bloodvessels. These groups of goblet cells are supposed
to secrete an internal secretion, which is absorbed by
the blood and carried to the different tissues. Metab-
olism of the carbohydrates is interfered with, if any
diseased condition or removal of the pancreas takes
place. The secretion from the cells of the alveoli,
on the other hand, secretes the pancreatic juice.
The pancreatic secretion 1 leaves the pancreas by
way of the duct of Wirsung; it is supposed to create
1 See page 279 for action of pancreatic secretion during digestion.
THE SECRETING GLANDS
303
an internal secretion which regulates the production
of glycogen by the liver, thus possessing both an
internal and external secretory function.
FIG. 113
Interlobular duct.
Interlobular
connective
tissue.
Capillary.
Area of
Langerhans.
Intralobular
duct.
Section of human pancreas, showing pancreatic islands. (Radasch.)
Ductless Glands. The ductless glands of the body
are:
Thyroid.
Parathyroids.
Spleen.
Carotid.
Thymus.
Suprarenal capsules.
Pituitary body.
Coccygeal.
The above glands aid in the nutrition of the whole
body as well as in that of individual organs by means
of an internal secretion which is absorbed by the blood