William H. (William Henry) Howell.

A text-book of physiology for medical students and physicians online

. (page 77 of 114)
Online LibraryWilliam H. (William Henry) HowellA text-book of physiology for medical students and physicians → online text (page 77 of 114)
Font size
QR-code for this ebook

is made in the air and the intestines are exposed to its influence, or
if the conditions of temperature and circulation are otherwise
disturbed, the movements observed are often violent and irregular.
The peristalsis runs rapidly along the intestines and may pass over
the whole length in about a minute; at the same time the con-
traction of the longitudinal muscles gives the bowels a peculiar
writhing movement. Movements of this kind are evidently
abnormal, and only occur in the body under the strong stimulation
of pathological conditions. Normal peristalsis, the object of which
is to move the food slowly along the alimentary tract, is quite a
different affair. Observers all agree that the wave of contraction
is gentle and progresses slowly, although at different rates perhaps
in different parts of the intestine. The force of the contraction
as measured by Cashf in the dog's intestine is very small. A
weight of five to eight grams was sufficient to check the onward
movement of the substance in the intestine and to set up violent,
colicy contractions which caused the animal evident uneasiness.
The time required for the passage of food through the small in-
testine must vary with its amount and character. From obser-
vations made upon man with the a:-ray, Hertz estimates that on
the average it requires about 4f hours. After a meal, therefore,
we may imagine that at about the time the stomach has finished
discharging its contents into the duodenum the first portions
have reached the ileocecal valve. That is to say, a column of
food, broken into separate segments, stretches at one time practi-
cally along the whole length of the small intestine.

Mechanism of the Peristaltic Movement. — The means by which
the peristaltic movement makes its orderly forward progression
have not been determined beyond question. The simplest explana-

* "Johns Hopkins Hospital Reports," 1, 93, 1896.

t "Proceedings of the Royal Society," London, 41, 1887.


tion would be to assume that an impulse is conveyed directly from
cell to cell in the circular muscular coat, so that a contraction started
at any point would spread by direct conduction of the contraction
change. This theory, however, does not explain satisfactorily the
normal conduction of the wave of contraction always in one direc-
tion, nor the fact that the wave of contraction is preceded by a
wave of inhibition. Moreover, Bayliss and Starling state that,
although the peristaltic movements continue after section of the
extrinsic nerves, — indeed, become more marked under these con-
ditions, — the application of cocain or nicotin prevents their oc-
currence. Since these substances may be supposed to act on the
intrinsic nerves, it is probable that the co-ordination of the move-
ment is effected through the local nerve ganglia, but our knowledge
of the mechanism and physiology of these peripheral nerve-plexuses
is as yet quite incomplete.

Rhythmical Movements. — In addition to the peristaltic wave a
second kind of movement may be observed in the small intestines.
It consists essentially in a series of local constrictions of the intes-
tinal wall, the constrictions occurring rhythmically at those points
at which masses of food lie.

Cannon * has studied these movements most successfully by

' ro O CD CD CB O '



Fig. 286. — Diagram to show the effect of the rhythmical constricting movements of
the small intestine upon tlie contained food. A string of food (1) is divided suddenly into
a series of segments (2) ; each of the latter is again divided and the process is repeated a
number of times (3 and 4). Eventually a peristaltic wave sweeps these segments forward
a certain distance and gathers them again into a long string, as in (1). The process of
segmentation is then repeated as described above. (Cannon.)

means of the Roentgen rays. He finds that as a result of these
contractions the masses or strings of food lying in the intestine are
suddenly segmented, repeatedly and in a definite manner, into a
number of small pieces, which move to and fro as the pieces combine
and are again separated (see Fig. 286). These segmentations may
proceed at the rate of thirty per minute for a certain time, and the
apparent result is that the material is well mixed with the digestive
secretions and is brought thoroughly into contact with the absorp-
tive walls. During these rhythmical contractions there is no steady
progression of the food; it remains in the same region, although

* Cannon, "American Journal of Physiology," 6, 251, 1902.


subjected to repeated divisions. From time to time the separated
pieces are caught by an advancing peristaltic wave, moved
forward a certain distance, and gathered again into a ne^^• mass.
In this new location the rhythmical contractions again segment
and churn the mass before a new peristaltic wave moves it on.
According to this description, the rhythmical movements are
local contractions (mainly of the circular muscles) which seem
to be due to the local distention caused by the food. They occur
rhythmically for a certain period and then cease until a new series
is started, and it is obvious that they must play a very important
part in promoting both the digestion and absorption of the food.
Mall* has suggested that these rhythmical contractions of the
circular coats may also act as a pumping mechanism upon the
venous plexuses in the walls and thus aid in driving the blood into
the portal system. Similar movements have been observed,
in the human being, f The curious observation is reported J
that during the period of fasting (dog) the whole gastro-intestinal
canal, although empty, shows at intervals rhythmical con-
tractions of its musculature which may last for twenty to thirty
mitiutes (see p. 790).

Cannon suggests a new and convenient nomenclature for the movements
of the stomach and intestines as follows:

(1) Rhythmic segmentations. — The rhythmic localized contractions de-
cribed in the preceding paragraph. Exhibited throughout the small intestine.

(2) Diastalsis. — Downward moving wave of contraction with a preceding
wave of inhibition (myenteric reflex). Exhibited chiefly in the small intestine.

(3) Anastalsis. — Upward moving wave of contraction without a pre-
ceding phase of inhibition Exhibited chiefly in the proximal colon.

(4) Katastalsis. — Downward moving wave of contraction without a
preceding phase of inhibition. Exhibited chiefly in the stomach.

Movements of the Intestinal Villi. — Hambleton § calls attention
to the fact that if the intestine of a living animal is opened and
spread out under suitable conditions so that the villi may be
examined under a binocular microscope, it can be shown that they
exhibit active movements of two kinds: First, lashing movements
from side to side in various directions; second, what might be
called pumping movements, in which the villi are alternately
extended and retracted. Doubtless these movements are due to
the contractions of the muscular slips, which run into the stroma of
the villi from the muscularis mucosse. If we can suppose that
they occur during normal digestion, it is evident that they add a
mechanical factor, not hitherto considered, which must help
materially in the ah-sorption of the products of digestion.

* Mall, "Johns Hopkins Hospital Reports," 1896, i., 37.

t Hertz, loc. cit.

t Boldireff, "Archives des sciences biologiques," 11, 1, 1905.

§ HamhJeton, "American Journal of Physiology," 34, 446, 1914.


The Nervous Control of the Intestinal Movements. — There

is some evidence to show that the rhythmical contractions of the
intestines are muscular in origin (myogenic), while the more co-
ordinated peristaltic movements depend upon the intrinsic nervous
mechanism. The intestine is, however, not dependent for either
movement upon its connections with the central nervous system.
Like the stomach, it is an automatic organ whose activity is simply
regulated through its extrinsic nerves.

The small intestine and the greater part of the large intestine
receive visceromotor nerve fibers from the vagi and the sympathetic
chain. The former, according to most observers, when artifically
stimulated cause movements of the intestine, and are therefore
regarded as the motor fibers. It seems probable, however, that the
vagi carry or may carry in some animals inliibitory fibers as well,
and that the motor effects usually obtained upon stimulation are
due to the fact that in these nerves the motor fibers predominate^
The fibers received from the sympathetic chain, on the other hand,
give mainly an inhibitor}^ effect when stimulated, although some
motor fibers apparently may take this path. Bechterew and
Mislawski * state that the sympathetic fibers for the small intestine
emerge from the spinal cord as medullated fibers in the sixth dorsal
to the first lumbar spinal nerves, (or lower — Bunch) and pass to the
sympathetic chain in the splanchnic nerves and thence to the
semilunar plexus. The paths of these fibers through the central
nervous system are not known, but there are evidently connections
extending to the higher brain centers, since psychical states are
known to influence the movements of the intestine, and according
to some observers stimulation of portions of the cerebral cortex
may produce movements or relaxation of the walls of the small and
large intestines.

Effect of Various Conditions upon the Intestinal Move-
ments. — Experiments have shown that the movements of the in-
testines may be evoked in many ways in addition to direct stinui-
lation of the extrinsic nerves. Chemical stimuli may be applied
directly to the intestinal wall. Mechanical stimulation — pinching,
for example, or the introduction of a bolus into the intestinal
cavity — may start peristaltic movements. Violent movements
may be produced also by shutting off the blood-supply, and again
temporarily when the supply is re-established. A condition of
dyspnea may also start movements in the intestines or in some
cases inhibit movements which are already in progress, the stimu-
lus in this case seeming to act upon the central nervous system and
to stimulate both the motor and the inhibitory fibers. Oxygen gas

* "Archiv f. Physiologie," 1889, suppl. volume.


within the bowels tends to suspend the movements of the intes-
tine, while COj, CH4, and HjS act as stimuli, increasing the move-
ments. Organic acids, such as acetic, propionic, formic, and
caprylic, which may be formed normally within the intestine as
the result of bacterial action, act also as strong stimulants.

Movements of the Large Intestine. — The opening from the
small intestine into the large is controlled both by the ileocecal
valve and by a sphincter, the ileocecal or ileocolic sphincter.
It is stated that this sphincter is normally in tonus and that
its condition of tonus is regulated through the splanchnic nerve
(Magnus). The musculature in the large intestine has the
same general arrangement as in the small, and the usual view
has been that the movements are similar, although more infre-
quent, so that the material received from the small intestine
is slowly moved along while becoming more and more solid
from the absorption of water. The contents of the ascending
colon are soft and semiliq'uid, but in the distal end of the transverse
colon they attain the consistency of the feces. Bajdiss and Starling
state that their law of intestinal peristalsis holds in this portion of
the intestine, — that is, local excitation causes a constriction above
and a dilatation below the point stimulated. Cannon,* from his
studies of the normal movements in cats, as seen by the Roentgen
rays, comes to the conclusion that the movements in the proximal
portion of the large intestine show a marked peculiarity. He
divides the large intestine into two parts; in the second, correspond-
ing roughly to the descending and distal portion of the transverse
colon, the food is moved toward the rectum by peristaltic waves.
A number of constrictions may be seen simultaneously within a
length of some inches. In the ascending colon and cecum, on the
contrary, the most frequent movement is that of antiperistalsis.
The food in this portion of the canal is more or less liquid and its
presence sets up running waves of constriction, which pass toward
the ileocecal valve. These waves occur in groups separated by
periods of rest. They seem to originate from a constricted ring
which pulsates, each contraction starting an anastaltic wave.
The presence of the ileocecal valve prevents the material from
being forced back into the small intestine. The value of this
peculiar reversal of the normal movement of the bowels at this
particular point would seem to lie in the fact that it delays the
passage of the material toward the rectum, and by thoroughly
mixing it gives increased opportunities for the completion of the
processes of digestion and absorption. In animals with a saccu-
lated colon the separate sacs or haustra may exhibit rhythmic

* Cannon, loc. cit.



contractions somewhat similar to the rhythmic segmentations in
the small intestine.* These movements (haustral churning)
would seem to favor also the processes of absorption. Hertz

SjfjnpafheAc Trunk

Kami. Zfferentei

GpneiioM inesehtei




fTfOts ffypofosfr/ajs

Fig. 287. — Schema to sliow the iniier\-ation of the rectum and internal sphincter
of the anus, and the formation of the hvpogastric plexus. (After Frankl-Hochwart and

estimates that in man the food requires about 2 hours to pass
from the ileocecal valve to the hepatic flexure and about 4j
hours to reach the splenic flexure. As the colon becomes filled
some of the material penetrates into the descending part, where
the normal peristalsis carries it very slowly toward the rectum.

The large intestine — particularly the descending colon and
rectum — receives its nerve supply from two sources (Fig. 287):
(1) Fibers which leave the spinal cord in the lumbar nerves
(second to fifth in cat), pass to the sympathetic chain, and
thence to the inferior mesenteric ganglia, which probably form
the termination of the preganglionic fibers. From this point
the path is continued by fibers running in the hypogastric nerves
and plexus. Stimulation of these fibers has given different
re.sults in the hands of various observers, but the most recent
workf indicates that they are inhibitory. (2) Fibers that leave

* Elliott and Barclay-Smith, "Journal of Phy.«iology," 31, 272, 1904.

t Langley and Anderson, "Journal of Physiologj^," 18, 67, 1895. Bay-
liss and Starling, ibid., 26, 107, 1900. Also Wischnewsky, in Hermann's
"Jahre.sbericht der Physiologie," vol. xii., 1895.


the cord in the sacral nerves (second to fourth), form part of the
nervi erigentes and enter into the h^^pogastric plexus. When
stimulated these fibers cause contractions of the muscular coats;
they may be regarded, therefore, as motor fibers. As in the
case of the small intestine and stomach, we may assume that
these motor and inhibitory fibers serve for the reflex regulation
and adaptation of the movements.

Defecation. — The undigested and indigestible parts of the
food, together with some of the debris and secretions from the
alimentary tract eventually reach the sigmoid flexure and
rectum. Authorities differ as to whether the rectum normally
contains fecal material or not. According to the observations
of Hertz,* made upon man by means of x-rays, fecal material
is normally absent from the rectum except just before defeca-
tion. It seems probable that a distinct desire to defecate
is felt only when the feces have actually entered the rec-
tum and produced some distension. The fecal material is
retained within the rectum by the action of the two sphincter
muscles which close the anal opening. One of these muscles,
the internal sphincter, is a strong band of the circular layer of
involuntary muscle which forms one of the coats of the rectum.
When the rectum contains fecal material this muscle is thrown
into a condition of tonic contraction until the act of defecation
begins, when it is relaxed. The external sphincter ani is com-
posed of striated muscle tissue and is under the control of the
will to a certain extent. It is supplied by a motor nerve, the
Nn. hemorrhoidales inferiores, arising from the N. pudendus
and eventually from the sacral spinal nerves. This muscle,
therefore, like striated muscle in general, is innervated, directly
from the spinal cord, but it possesses properties which are to
some extent intermediate between those of plain and of striated
muscle. For example, it differs from the latter and resembles
the former in the fact that it does not atrophy after section of
its motor nerve; it is much less sensitive to the paralyzing action
of curare than the typical striated muscle, and it is stated that
its curve of contraction, when it is stimulated through its nerve,
exhibits a long latent period and a slow contraction and relaxa-
tion. Both the internal and the external sphincter are normally
in tonus and unite in protecting the anal opening. The force
of the tonic contraction of the internal is somewhat less (30 to
60 per cent.) than that of the external sphincter.! The innerva-
tion and control of the internal sphincter is better understood

* Hertz, "Guv's Hospital Reports," 61, 389, 1907.

t Consult Frankl-Hochwart and Frohlich, "Archiv f. de ges Physiologie,"
81, 420.


than that of the external. Like the rest of the rectum, it receives
motor fibers from the hypogastric plexus by way of the nervus
erigens, and inhibitory fibers from the same plexus by way of the
hypogastric nerve. It has been possible to show experimentally
that each of these sets of fibers may be acted upon reflexly, for
example, by stimulation of the sensory nerves in the sciatic.
The reflex takes place in this case through the lower portion of
the cord. Both the hypogastric nerve and the N. erigens con-
tain also afferent fibers. Stimulation of the central end of the
severed N. erigens gives a reflex inhibition through the hypo-
gastric nerve, and stimulation of the central stump of the cut
hypogastric causes a reflex contraction through the N. erigens.
The act of defecation as it occurs normally is partly a voluntary
and partly an involuntary act. The involuntary act consists in
peristaltic contractions of the rectum or, indeed, of the whole
colon, together with an inhibition of the sphincters. Whether
the inhibition of the sphincters is normally entirely an involuntary
reflex cannot be stated definitely. No doubt the sensory stimuli
arising from the accumulation of fecal material would eventually
cause in this way a relaxation of the sphincters, but the act of
defecation usually takes place before such a strong necessity arises.
It is initiated usually by a voluntary act, and it is possible that in
such cases the relaxation of both sphincters may be effected by
voluntary inhibition acting upon the spinal centers.

The voluntary factor in defecation consists mainly in the
contraction of the abdominal muscles. When these latter
muscles are contracted and at the same time the diaphragm is
prevented from moving upward by the closure of the glottis,
the increased abdominal pressure is brought to bear upon the
abdominal and pelvic viscera, and aids strongly in pressing the
contents of the descending colon and sigmoid flexure into the
rectum. The pressure in the abdominal cavity is still further
increased if a deep inspiration is first made and then maintained
during the contraction of the abdominal muscles. Hertz, on
the basis of his skiagraphic observations, insists that simul-
taneously with the contraction of the abdominal muscles and
the closure of the glottis the diaphragm is also contracted and
thus aids in bringing pressure to bear upon the pelvic organs.
Although the act of defecation is normally initiated by voluntary
effort, it may also be carried out as a purely involuntary reflex
when the sensory stimulus is sufficiently strong. Goltz* has
shown that in dogs in which the spinal cord had been severed
in the lower thoracic region defecation was performed normally.
In later experiments, in which the entire spinal cord was removed

* "Archiv f. die gesammte Physiologie," 8, 160, 1874; 63, 362, 1896.


except in the cervical and upper part of the thoracic region, it
was found that the animal, after it had recovered from the
operation, had normal movement once or twice a day, indicating
that the rectum and lower bowels acted by virtue of their
intrinsic mechanism. An interesting result of these experi-
ments was the fact that the external sphincter suffered no
atrophy, although its motor nerve was destroyed, and that it
eventually regained its tonic activity.

It would seem that the whole act of defecation is, at bottom,
an involuntary reflex. The physiological center for the move-
ment probably lies in the lumbar cord, and it has sensory and
motor connections with the rectum and the muscles of defecation.
As stated above, the inhibitory fibers to the internal sphincter
pass by way of the hypogastric nerve, the motor fibers through
the nervus erigens, and both of these nerves contain afferent
fibers which may reflexly excite inhibition or contraction. But
this center is probably provided also with intraspinal con-
nections with the centers of the cerebrum, through which the
act may be controlled by voluntary impulses and by various
psychical states; the effect of emotions upon defecation being
a matter of common knowledge. In infants the essentially in-
voluntary character of the act is well known.

Vomiting. — The act; of vomiting causes an ejection of the con-
tents of the stomach through the esophagus and mouth to the
exterior. It was long debated whether the force producing this
ejection comes from a strong contraction of the walls of the stom-
ach itself or whether it is due mainly to the action of the walls of
the abdomen. A forcible spasmodic contraction of the abdominal
muscles takes place, as may easily be observed by any one upon
himself, and it is now believed that the contraction of these muscles
is the principal factor in vomiting. Magendie found that if the
stomach was extirpated and a bladder containing water was sub-
stituted in its place and connected with the esophagus, injection
of an emetic caused a typical vomiting movement with ejection of
the contents of the bladder. Gianuzzi showed, on the other hand,
that upon a curarized animal vomiting could not be produced by an
emetic — because, apparently, the muscles of the abdomen were
paralyzed by the curare. There are on record a number of ob-
servations which tend to show that the stomach is not passive
during the act. On the contrary^ it may exhibit contractions, more
or less violent in character. According to Openchowski,* the
pylorus is closed and the pyloric end of the stomach firmly con-
tracted so as to drive the contents toward the dilated cardiac por-
tion. Cannon states that in cats the normal peristaltic waves pass
* Openchowski, "Archiv f. Physiologie," 1889, p. 552.


over the pyloric portion in the period preceding the vomiting and
that finally a strong contraction at the "transverse band" com-
pletely shuts off the pyloric portion from the body of the stomach,
which at this time is quite relaxed. The act of vomiting is, in fact,
a complex reflex movement into which many muscles enter. The
following events are described : The vomiting is usually preceded by
a sensation of nausea and a reflex flow of saliva into the mouth.
These phenomena are succeeded or accompanied by retching move-
ments, which consist essentially in deep, spasmodic inspirations with
a closed glottis. The effect of these movements is to compress the
stomach by the descent of the diaphragm, and at the same time to
increase decidedly the negative pressure in the thorax, and therefore
in the thoracic portion of the esophagus. During one of these
retching movements the act of vomiting is effected by a convulsive
contraction of the abdominal wall that exerts a sudden additional
strong pressure upon the stomach. At the same time the cardiac

Online LibraryWilliam H. (William Henry) HowellA text-book of physiology for medical students and physicians → online text (page 77 of 114)