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arrives in the crecum it contains 90 per cent, of water, together with
a small amount of the unabsorbed products of digestion of proteins,
fats, and carbohydrates. During its passage along the large intestine
these are absorbed, and most absorption appears to occur in the
caecum ; the normal firm consistency of the fceces, which contain



CM. XXXVll.]



INTESTINAL MOVEMENTS



661



75 per cent, of water, is not finally attained until they arrive in the
pelvic colon, where they are retained until defiBcation takes place.

Peristalsis in the colon occurs much more slowly than in the
small intestine, and the accompanying diagram gives the time in
hours after the taking of a bismuth meal that the shadow appears at
various points in man. It reaches the hepatic flexure of the colon
about two hours after it appears in the ccecum ; another two hours
approximately brings it to the splenic flexure (nine hours after the
meal). The distance from the caecum to the splenic flexure is 2
feet; the contents take as long to travel this distance as the
contents of the small intestine take to travel 22.^ feet, that is, from



splenic flexuie.



Hepatic Hexure



AscendiuL' colun /-/-



Citcum



Pelvic colon -
Pelvis




Iliac colon.



Fio. 369. — Semi-diagrammatic view of the large intestine ; the figures give in hours the average limes
after taking a meal that its debris reaches the various parts. (Hertz.) Tliis diagram shows the
transverse colon in a higher position than it occupies when the man is erect, and rather higher
than the average even in the horizontal position.



the pylorus to the caecum. A further two hours is occupied in the
journey along the descending colon, and six hours more brings it to
the end of the pelvic colon which leads at an angle into the rectum.
The total journey from the caecum to this point occupies thirteen and
a half hours. These times were confirmed by auscultation or listening
over various parts of the abdomen ; the gurgling and splashing
sounds made by the arrival of food-material are distinctly audible.
These observations were made in the daytime ; during sleep the rate
of progress may be slower.

Some observers have stated that retro-peristalsis occurs in the
colon, especially in its ascending portion. AYaves of this kind would

2 N



562 THE MKCITAXICAL PROCESSES OF DIGESTION [CH. XXXVII.

certainly mix up the caecal contents very thoroughly. They have,
however, only been seen in the exposed intestine of animals, and
therefore may be artificially produced. A study of X-ray shadows
does not reveal their existence in man. If retro-peristalsis does
occur, regurgitation is effectually prevented into the small intestine
partly by the ileo-caecal valve, and mainly by a strong band of
circular muscular fibres called the ileo-caecal sphincter; this is
normally kept in a state of tonic contraction by impulses carried by
the splanchnic nerve; it is relaxed when this nerve is cut, and then
the contents of the two intestines mix freely. (T. E. Elliott.)

Defcccation. — The rectum is a short tube about 4 or 5 inches long
in man, which is normally empty until immediately before defaeca-
tion. In a person of regular habits, a glass of cold water on rising,
the stimulus of a cold bath, the taking of breakfast, and the after-
breakfast pipe or cigarette combine to produce peristalsis of the
colon, so that a small quantity of fpeces enters the rectum, and then
arises the desire to defecate. At the end of the rectum is the anal
canal, closed by a strong internal sphincter (a thickening of the
involuntary circular fibres of the muscular coat), and by the external
sphincter, which is a voluntary muscle made of transversely striated
fibres.

The "call to defaecation" having been thus produced, the act
itself is started by the increase in intra-abdominal pressure brought
about by the voluntary contraction of the abdominal wall, the
diaphragm and the levator ani. The diaphragm is kept down by deep
inspirations, followed by closure of the glottis; this depresses the
colon., so that the shadow of its transverse portion and the flexures
may be lowered as niucli as 2 inches. The transverse colon may
not rise to its normal position until even an hour has elapsed
from the act of straining during defa3cation. Accompanying the
action of these voluntary inuscles, the whole colon from the caecum
onwards enters into powerful peristalsis; the contents of the
transverse colon are thus forced into the descending colon, from
which they are evacuated together with the faeces already present
between the splenic flexure and the anus. The entrance of more
faeces into the rectum until they reach the anal canal irritates afferent
nerves in the wall of the rectum ; the nerve impulses so generated
pass to a centre or centres in the lumbo-sacral region of the spinal
cord, where efferent impulses are set in action upon which depend
the reflex acts required to complete the process ; these are : —

1. Strong peristalsis of the whole colon.

2. Continued contraction of the abdominal muscles.

3. Eelaxation of both the anal sphincters and of the levator aui.
The last traces of faeces are expelled by voluntary contractions of

the levator ani.



CH. XXXVII.] NERVOUS MECHANISM OF LARGE INTESTINE 563

If the bowels are opened once a day, the interval between a
meal and the evacuation of its residue varies between nine and thirty-
two hours, the time depending on the hours of meals and that of
deftecation. Food taken less than nine hours previously would not
have reached far enough.

If the call to defoecation is resisted, the desire soon passes away,
and may not recur until the next regular period arrives for the
opening of the bowels, twenty-four hours later. During this time
the rectum contains faeces, there being no retro-peristalsis to carry
them back into the colon. This is* one of the commonest causes of
constipation, for the retained faeces continue to lose water, and get
harder, and more difficult to expel.

Nervous Mechanism. — The large intestine resembles the rest of
the alimentary canal in having a double nerve supply.

(1) The sympathetic. These fibres leave the cord by the lower
lumbar anterior roots ; these pass through the lateral chain, and
reach their cell-stations in the inferior mesenteric ganglion ; the post-
ganglionic fibres arising there, pass by the colonic nerves to the
colon, and by the hypogastric nerve to the rectum and internal anal
sphincter.

(2) The nervus erigens. This takes the place of the vagus,
which forms the second source of nerve supply to the stomach and
small intestine. This nerve is excitatory to both coats of the
muscular wall, whereas the sympathetic is inhibitory to the internal
sphincter.

The fibres which pass to the rectum by the pelvic nerves or nervi
erigentes arise from the third sacral nerve, and have their cell-
stations in the haemorrhoidal nerve plexus, which is the name given
to this portion of the plexus of Auerbach.

The voluntary muscles, namely, the external anal sphincter and
the levator ani, are supplied by the fourth sacral nerve, which arises
from nerve-cells in the conus terminalis of the spinal cord.

If Starling's experiment of pinching a spot in the large intestine
is performed, much the same result follows as in the small intestine ;
the wave of inhibition which travels downwards is well seen, but the
upward wave of contraction is not so marked as in the small intestine.
Stimulation of the sympathetic (hypogastric) nerve-fibres produces
movements of the colon and rectum, and inhibition of the internal
sphincter ; that is the main phenomenon in the act of defaecation. If
the lower part of the spinal cord is destroyed, defaecation still occurs,
but it is an unconscious act, and the reflex is imperfectly executed ;
the hypogastric part of the mechanism is intact, and probably the
reflex centre concerned is, as in the small intestine, in the peripheral
ganglia of Auerbach's plexus ; but the destruction of the conus
terminalis prevents the normal reflexes taking place in which the



564 THE MECHANICAL PROCESSES OF DIOESTION [CEI. XXXVIl.

levator ani and external sphincter are concerned, and the paralysis
of these voluntary muscles may lead to incontinence of faeces.

We thus see that the lowermost portion of the alimentary canal
resembles its uppermost portion (pharynx and oesophagus) in being
more under external nervous control than is the small intestine.
Autonomy at the rectal and anal portion is for obvious reasons unde-
sirable.



CHAPTER XXXVIII



THE URINARY APPARATUS




This consists of the kidneys ; from each a tube called the ureter leads
to the bladder, in which the urine is temporarily stored ; from the
bladder a duct called the urethra
leads to the exterior.

The Kidneys are situated in the
lumbar region of the abdomen on
either side of the spinal column
behind the peritoneum. In man each
is about 4 inches long, and weighs
about 4i oz.

Structure. — The kidney is covered
by a fibrous capsule, which is slightly
attached at its inner surface to the
proper substance of the organ by
means of very fine bundles of areolar
tissue and minute blood - vessels.
At the hilus of the kidney it becomes
continuous with the external coat of
the upper and dilated part of the
ureter (fig. 370).

On dividing the kidney into two
equal parts by a section carried
through its long convex border, it is
seen to be composed of two portions,
called respectively cortical and medul-
lary ; the latter is composed of about
a dozen conical bundles of urinary
tubules, each bundle forming what is called a pyramid. The upper
part of the ureter is dilated into the pelvis ; and this, again, after
separating into two or three principal divisions, is finally subdivided
into still smaller portions, varying in number from about 8 to 12,
called calyces. Each of these little calyces or cups receives the
pointed extremity or papilla of a pyramid. The number of pyramids
varies in different animals ; in some there is only one.

The kidney is a compound tubular gland, and both its cortical
and medullary portions are composed of tubes, the tuhuli uriniferi,
which, by one extremity, in the cortical portion, commence around

565



Fig. 370. — Plau of a longitudinal section
through the pelvis and sub.stance of the
right kidney, h : a, the cortical sub-
stance ; h, b, broad part of the pyramids
of Malpighi ; c, c, the divisions of the
pelvis named calyces, laid open ; c', one
of those unopened ; d, summit of the
pyramid projecting into calyces ; e, e,
section of the narrow part of two
pyramids near the calyces ; p, pelvis
or enlarged portion of the ureter
within the kidney ; v, the ureter ; s, the
sinus ; h, the hihis.



566 THE URINARY APPARATUS [CH. XXXVIII.

tufts of capillary blood-vessels, called Malpighian bodies, and, by the
other, open tlircjugh the papilLne into the pelvis of the ureter, and




Fin. 371.— A diagram of the uriniferous tubes. A, cortex limited externally by the capsule;
a, subcapsular layer not containing Malpijihian corpuscles ; a , inner stratum of cortex, also without
Malpii,'hian capsules ; B, boundary layer; C, meduUarj' part next the boundary layer; 1, Bowman's
capsuie of Malpij;hian corpuscle; 2, neck of capsule; 3, first convoluted tubule; 4, spiral tubule;
5, descending limb of Henle's loop ; G, the loop proper ; 7, thick part of the ascending limb ; S, spiral
part of ascendin;; limb ; 9, narrow ascendint: limb in the medullary ray ; 10, the zigzag tubule ; 11,
the second convoluted tubule ; 12, the j\inctional tubule ; 13, the collecting tubule of the medullary
ray ; 14, the collecting tube of the boundary layer ; 15, duct of Bellini. (Klein.)

thus discharge the urine which flows through them. They are bound
together by connective tissue.



CH. XXXVllI.]



THE KIDMKY TUBULES



5g:



In the pyramids ihc tubes are straight — uniting to form larger
tubes as they descend through these from the cortical portion ;
while in the latter region they are convoluted. But in the boundary
zone between cortex and medulla, small collections of straight tul)es
called medullary rays project into the cortical region.

Each begins in the cortex as a dilatation, called the Capsule of
Bowman ; this encloses a tuft or glomerulus of capillaries, called a
Malpighian corpuscle. The tubule leaves the capsule by a neck, and




Fig. 372. — Malpigliian corpuscle, injected through the renal artery with coloured gelatin ; a, glomerular
vessels ; b, c, capsule of Bowman ; d, att'erent vessel of glomerulus ; e, ett'erent vessels ;
/, epithelium of tubes. (Cadiat.)

then becomes convoluted {first convoluted tubule), but soon after
becomes nearly straight or slightly spiral {spiral titbule) ; then rapidly
narrowing, it passes down into the medulla as the descending tubule of
Henle ; this turns round, forming a loop {loop of Henle), and passes
up to the cortex again as the ascending tubule of Henle. It then
becomes larger and irregularly zigzag {zigzag tubule) and again con-
voluted {second convoluted tubule). Eventually it narrows into a
junctional tubule, which joins a straight or collecting tubule. This
passes straight through the medulla, where it joins with others to
form one of the ducts of Bellini that open at the apex of the pyramid.
These parts are all shown in fig. 371.

In the capsule, the epithelium is flattened and reflected over the
glomerulus.



5G8



THE URINAllV APPARATUS



[CII. XXXVIU.



In the neck tlio cpillHiliiiin is still llattenod, l)u(, in some animals,
such as frogs, where the neck is longer, the epithelium is ciliated.

In the yirs^ convoluted and spiral tubules, it is thick, and the cells
show a fibrillated structure, except around the nucleus, where the
protoplasm is granular. The cells interlock laterally and are difficult
to isolate. In the narrow descending tubule of Henle and in the
loop itself, the cells are clear and flattened and leave a considerable
lumen ; in the ascending limb they again become striated and
nearly fill the tubule. In the zigzag and second convoluted tubules
the fibrillations become even more marked. The junctional tubule
has a large lumen, and is lined by clear flattened cells; the col-
lecting tubules and ducts of Bellini are lined by clear cubical or
columnar cells (see fig. 373).








S^S^S^^''^^^*^:-^'^^*^'^



HiniiAili-. ■ '*5(>,

-iSlWEK.iiiC" ••-■•' "•''^ » ~»~ ^"' " - »'•'» »■ rS^T

Fio. 373.— From a vertical section through the kidney of a dog— the capsule of which is supposed to be
on the right, a, The capillaries of the Malpigliian corpuscle, which arearranged in lobules ; n, neck
of capsule ; c, convoluted tubes cut in various directions ; li, zigzag tubule ; d, e, and /, are straight
lubes in a medullary ray : d, collecting lube ; c , spiral tube ; /, narrow section of ascending limb.
X 380. (Klein and Noble Smith. )

The extent of the zone of clear cells in the loop of Henle varies
a good deal in dififerent animals ; a diminution of this part of the
tubule lessens the length of the total loop; in most animals there
is an admixture of long and short loops, though the proportion of the
two varies greatly in different parts of the animal kingdom. '



CH. XXXVTII.]



THE KIDSiOY TUBULKS



569



Blood-vessels of Kidney.— Tho roiial artery enters the kidney
at tho hihis, and divides into hraiudies that pass towards the cortex,

then turn over and form in-
complete arches in the region
hetween cortex and medulla.
From these arches vessels pass
to the surface which arc called
the interlobular arteries ; they
give off vessels at right angles,





Fro. 375.— Diagram showing the relation
of the Malpighiau body to the urin-
iferous ducts and blood - vessels.
n, One of the interlobular arteries;
a', atterent artery passing into the
glomerulus; c, capsule of llie Mal-
pighian body, forming the com-
mencement of and continuous with
the uriniferous tube; t', e', etl'erent
vessels which subdivide and f(jrm a
plexus, p, surrounding the tube, and
finally terminate in the branch of
the renal vein e. (After Bowman.)

which are the afferent vessels of
the glomeruli; a glomerulus is
made up of capillaries as pre-
viously stated. From each a
smaller vessel (the efferent vessel
of the glomerulus) passes out, and
like a portal vessel on a small
scale, breaks up once more into
capillaries which ramify between
the convoluted tubules. These
unite to form veins (interlobular
veins) which accompany the interlobular arteries ; they pass to venous
arches, parallel to, but more complete than, the corresponding arterial
arches ; they ultimately unite to form the renal vein that leaves the
hilus. ' These veins receive also others which have a stellate arrange-
ment near the capsule (vence stellulce).



Fio. 374.— Vascular supply of kidney, o. Part of
arterial arch ; b, interlobular artery ; c, glo-
merulus ; d, efferent vessel passing to the
medulla as false arteria recta ; <■, capillaries of
cortex ; /, capillaries of medulla ; g, venous
arch ; h, straight veins of medulla ; i, inter-
lobular vein ; j, vena steUula. (Cadiat.l



570 THE URINARY APPARATUS [CH. XXXVIII.

Tho medulla is supplied by pencils of lino straight arterioles
which arise from the arterial arches. They are called artericB rectce.
The efferent vessels of the glomeruli nearest the medulla may
also break up into similar vessels which are called false arterice
rectce. The veins {vence rectce) take a similar course and empty them-
selves into the venous arches. In the boundary zone groups of vcLsa
recta alternate with groups of tubules, and give it a striated
appearance.

The Ureters. — The duct of each kidney, or ureter, is a tube
about the size of a goose-quill, and from twelve to sixteen inches
in length, which, continuous above with the pelvis, ends below by
perforating obliquely the walls of the bladder, and opening on its
internal surface.

It is constructed of three coats : (a) an outer fibrous coat ; (b) a
middle muscular coat; and (c) a mucous membrane continuous with
that of the pelvis above, and of the urinary bladder below ; it is
composed of areolar tissue lined by transitional epithelium.

The Urinary Bladder is pyriform; its widest part, which is
situate above and behind, is termed the fundus ; and the narrow
constricted portion, by which it becomes continuous with the urethra,
is called its cervix or ueck.

It is constructed of four coats, — serous, muscular, areolar or
submucous, and mucous. The circular muscular tibres are especially
developed around the cervix of the organ and form the sphincter
vesicce. The mucous membrane is like that of the ureters. It is
provided with mucous glands, which are most numerous near the
neck of the bladder.

The bladder is well provided with blood- and lymph-vessels, and
with nerves. The latter consist of branches from the sacral and
hypogastric plexuses. Ganglion cells are found, here and there, on
the course of the nerve-fibres.

The Urethra. — This occupies the centre of the corpus spongiosum
in the male. As it passes through the prostate it is lined by transi-
tional, but elsewhere by columnar epithelium, except near the orifice,
where the epithelium is stratified like the epidermis, with which it
becomes continuous. The female urethra has stratified epithelium
throughout. The epithelium rests on a vascular cerium, and this is
severed by submucous tissue containing an inner longitudinal and
an outer circular muscular layer. Outside this a plexus of veins
passes insensibly into the surrounding erectile tissue.

Into the urethra open a number of oblique recesses or lacunce, a
number of small mucous glands (glands of Littn'), two compound
racemose glands (Cowper's glands), the glands of the prostate, and
the vas deferens. The prostate, which surrounds the commencement
of the male urethra, is a muscular and glandular mass. Its glands



CH. XXXVIII.] THE KIDNEY ONCOMETER 571

are tubular and lined by columnar epithelium ; tlieii' secretion
dilutes the semen. Very little is known of the function of the
prostate; it often enlarges and becomes calcareous in old age, and
gives rise to discomfort and difficulty in micturition. Its removal
under these circumstances is a most beneficial operation.

The Nerves of the Kidney.

These are derived from the renal plexus of each side. The renal
plexus consists of both medullated and non-medullated nerve-fibres,
with collections of ganglion cells. Fibres from the anterior roots of
the eleventh, twelfth, and thirteenth dorsal nerves (in the dog) pass
into this plexus. They are both vaso-constrictor and vaso-dilator in
function. The nerve-cells on the course of the constrictor fibres are
situated in the cceliac, mesenteric, and renal ganglia ; the nerve-cells
on the course of the dilator fibres are placed in the solar plexus and
renal ganglia. We have, at present, no knowledge of true secretory
nerves to the kidney, and the amount of urine is influenced, to a
certain extent at any rate, by the blood-pressure in its capillaries.
We shall, a few pages hence, however, see that the amount of urine
does not depend wholly on the height of the blood-pressure ; and one
very striking fact in this relation may be mentioned now, — namely,
that if the blood-pressure is increased without allowing the blood to
flow, the amount of urine formed is not increased; this can be done
by ligaturing the renal vein ; the blood-pressure within the kidney
then rises enormously, but the flow of urine stops.

The Kidney Oncometer,

This is an instrument constructed on plethysmographic principles,
by means of which the volume of the kidney is registered. The
general characters of this instrument are described in the diagrams
on p. 310. The special form introduced by Eoy for the kidney is
shown in fig. 376. Eoy's instrument, however, is but seldom used at
the present day. An air oncometer, connected with a Marey's
tambour or a bellows recorder (like that figured for the spleen on
p. 311), is much less complicated, and gives better results.

It is found that the effect on the voliune of the organ of dividing
or stimulating nerves corresponds to blood-pressure. If a rise of
pressure in the renal artery is produced by constriction of the renal
arterioles, this is accompanied by a fall of pressure in the renal
capillaries and a shrinkage of the kidney. Increase in the volume
of the kidney is produced by the opposite circumstances.

The accompanying tracing (fig. 377) shows that in a normal
oncometric curve from the kidney there is a rise of volume, due to



572



THE URINARY APPARATUS



[CH. XXXVIIL



each lieart-l)cat, and larger waves, which accompany respiration. In
some cases lander sweeping waves (Mayer curves) are S(!en as well,




Fio. 370. — Ononmetei s f^ ■



but they are absent in the tracing reproduced : if, in such a
tracing, the kidney curve is compared with the tracing of arterial




Fin. 377.- -Curvij taken by rfiial DiicuiiiBter conipareU wiUi iliat of oniiiiary biuoii-iiressure. a, Blood-
pressure curve ; 6, kidney curve. (Roy.)

pressure, it will be seen that the rise of pressure is simultaneous with
the fall in kidney volume due to constriction of the renal vessels.



The Functions of the Kidney.

The function of the kidneys is to separate the urinary con-
stituents from the blood, and by this means the blood is maintained
of constant composition. The kidney cells are remarkably sensitive,
so that alterations in the composition of the blood which are too
slight to be detected by chemical analysis (such as an increase of
water or of chlorides after a meal) are felt by the kidney, and
increased secretion (diuresis) occurs. In the case of some urinary
constituents, they are practically entirely removed by the kidney ;
urea is an example of this class. In other cases excess beyond a



CH. XXXVIII.] FUNCTIONS OF THE KIDNEY 573

certain percentage is removed from the blood ; sodium chloride is an
instance of these.

Although the glandular epithelium of the convoluted tubes is
pai' excellence the secreting mechanism of the kidney, much difference
of opinion exists as to the part played by each of the several units in
the histological complex we have already described, and this is
especially the case in relation to that unique structure, the glomerulus.
We have seen that the efferent vessel of each glomerulus has a
smaller calibre than the afferent vessel, and this produces high
pressure in the glomerular capillaries. The efferent vessel, more-



Online LibraryWilliam Senhouse KirkesHandbook of physiology.. → online text (page 56 of 93)