The chief of the vegetal organs of the human frame, to the evolution
of which we now turn our attention, is the alimentary canal. The gut
is the oldest of all the organs of the metazoic body, and it leads us
back to the earliest age of the formation of organs--to the first
section of the Laurentian period. As we have already seen, the result
of the first division of labour among the homogeneous cells of the
earliest multicellular animal body was the formation of an alimentary
cavity. The first duty and first need of every organism is
self-preservation. This is met by the functions of the nutrition and
the covering of the body. When, therefore, in the primitive globular
Blastaea the homogeneous cells began to effect a division of labour,
they had first to meet this twofold need. One half were converted into
alimentary cells and enclosed a digestive cavity, the gut. The other
half became covering cells, and formed an envelope round the
alimentary tube and the whole body. Thus arose the primary germinal
layers--the inner, alimentary, or vegetal layer, and the outer,
covering, or animal layer. (Cf. Chapter 2.19.)
When we try to construct an animal frame of the simplest conceivable
type, that has some such primitive alimentary canal and the two
primary layers constituting its wall, we inevitably come to the very
remarkable embryonic form of the gastrula, which we have found with
extraordinary persistence throughout the whole range of animals, with
the exception of the unicellulars--in the Sponges, Cnidaria, Platodes,
Vermalia, Molluscs, Articulates, Echinoderms, Tunicates, and
Vertebrates. In all these stems the gastrula recurs in the same very
simple form. It is certainly a remarkable fact that the gastrula is
found in various animals as a larva-stage in their individual
development, and that this gastrula, though much disguised by
cenogenetic modifications, has everywhere essentially the same
palingenetic structure (Figures 1.30 to 1.35). The elaborate
alimentary canal of the higher animals develops ontogenetically from
the same simple primitive gut of the gastrula.
This gastraea theory is now accepted by nearly all zoologists. It was
first supported and partly modified by Professor Ray-Lankester; he
proposed three years afterwards (in his essay on the development of
the Molluscs, 1875) to give the name of archenteron to the primitive
gut and blastoporus to the primitive mouth.
Before we follow the development of the human alimentary canal in
detail, it is necessary to say a word about the general features of
its composition in the fully-developed man. The mature alimentary
canal in man is constructed in all its main features like that of all
the higher mammals, and particularly resembles that of the
Catarrhines, the narrow-nosed apes of the Old World. The entrance into
it, the mouth, is armed with thirty-two teeth, fixed in rows in the
upper and lower jaws. As we have seen, our dentition is exactly the
same as that of the Catarrhines, and differs from that of all other
animals (Chapter 2.23). Above the mouth-cavity is the double nasal
cavity; they are separated by the palate-wall. But we saw that this
separation is not there from the first, and that originally there is a
common mouth-nasal cavity in the embryo; and this is only divided
afterwards by the hard palate into two--the nasal cavity above and
that of the mouth below (Figure 2.311).
At the back the cavity of the mouth is half closed by the vertical
curtain that we call the soft palate, in the middle of which is the
uvula. A glance into a mirror with the mouth wide open will show its
shape. The uvula is interesting because, besides man, it is only found
in the ape. At each side of the soft palate are the tonsils. Through
the curved opening that we find underneath the soft palate we
penetrate into the gullet or pharynx behind the mouth-cavity. Into
this opens on either side a narrow canal (the Eustachian tube),
through which there is direct communication with the tympanic cavity
of the ear (Figure 2.320 e). The pharynx is continued in a long,
narrow tube, the oesophagus (sr). By this the food passes into the
stomach when masticated and swallowed. Into the gullet also opens,
right above, the trachea (lr), that leads to the lungs. The entrance
to it is covered by the epiglottis, over which the food slides. The
cartilaginous epiglottis is found only in the mammals, and has
developed from the fourth branchial arch of the fishes and amphibia.
The lungs are found, in man and all the mammals, to the right and left
in the pectoral cavity, with the heart between them. At the upper end
of the trachea there is, under the epiglottis, a specially
differentiated part, strengthened by a cartilaginous skeleton, the
larynx. This important organ of human speech also develops from a part
of the alimentary canal. In front of the larynx is the thyroid gland,
which sometimes enlarges and forms goitre.
The oesophagus descends into the pectoral cavity along the vertebral
column, behind the lungs and the heart, pierces the diaphragm, and
enters the visceral cavity. The diaphragm is a membrano-muscular
partition that completely separates the thoracic from the abdominal
cavity in all the mammals (and these alone). This separation is not
found in the beginning; there is at first a common breast-belly
cavity, the coeloma or pleuro-peritoneal cavity. The diaphragm is
formed later on as a muscular horizontal partition between the
thoracic and abdominal cavities. It then completely separates the two
cavities, and is only pierced by several organs that pass from the one
to the other. One of the chief of these organs is the oesophagus.
After this has passed through the diaphragm, it expands into the
gastric sac in which digestion chiefly takes place. The stomach of the
adult man (Figure 2.349) is a long, somewhat oblique sac, expanding on
the left into a blind sac, the fundus of the stomach (b apostrophe),
but narrowing on the right, and passing at the pylorus (e) into the
small intestine. At this point there is a valve, the pyloric valve
(d), between the two sections of the canal; it opens only when the
pulpy food passes from the stomach into the intestine. In man and the
higher Vertebrates the stomach itself is the chief organ of digestion,
and is especially occupied with the solution of the food; this is not
the case in many of the lower Vertebrates, which have no stomach, and
discharge its function by a part of the gut farther on. The muscular
wall of the stomach is comparatively thick; it has externally strong
muscles that accomplish the digestive movements, and internally a
large quantity of small glands, the peptic glands, which secrete the
gastric juice.
(FIGURE 2.349. Human stomach and duodenum, longitudinal section. a
cardiac (end of oesophagus), b fundus (blind sac of the left side), c
pylorus-fold, d pylorus-valves, e pylorus-cavity, fgh duodenum, i
entrance of the gall-duct and the pancreatic duct. (From Meyer.)
FIGURE 2.350. Median section of the head of a hare-embryo, one-fourth
of an inch in length. (From Mihalcovics.) The deep mouth-cleft (hp) is
separated by the membrane of the throat (rh) from the blind cavity of
the head-gut (kd). hz heart, ch chorda, hp the point at which the
hypophysis develops from the mouth-cleft, vh ventricle of the
cerebrum, v3, third ventricle (intermediate brain), v4 fourth
ventricle (hind brain), ck spinal canal.)
Next to the stomach comes the longest section of the alimentary canal,
the middle gut or small intestine. Its chief function is to absorb the
peptonised fluid mass of food, or the chyle, and it is subdivided into
several sections, of which the first (next to the stomach) is called
the duodenum (Figure 2.349 fgh). It is a short, horseshoe-shaped loop
of the gut. The largest glands of the alimentary canal open into
it--the liver, the chief digestive gland, that secretes the gall, and
the pancreas, which secretes the pancreatic juice. The two glands pour
their secretions, the bile and pancreatic juice, close together into
the duodenum (i). The opening of the gall-duct is of particular
phylogenetic importance, as it is the same in all the Vertebrates, and
indicates the principal point of the hepatic or trunk-gut (Gegenbaur).
The liver, phylogenetically older than the stomach, is a large gland,
rich in blood, in the adult man, immediately under the diaphragm on
the left side, and separated by it from the lungs. The pancreas lies a
little further back and more to the left. The remaining part of the
small intestine is so long that it has to coil itself in many folds in
order to find room in the narrow space of the abdominal cavity. It is
divided into the jejunum above and the ileum below. In the last
section of it is the part of the small intestine at which in the
embryo the yelk-sac opens into the gut. This long and thin intestine
then passes into the large intestine, from which it is cut off by a
special valve. Immediately behind this "Bauhin-valve" the first part
of the large intestine forms a wide, pouch-like structure, the caecum.
The atrophied end of the caecum is the famous rudimentary organ, the
vermiform appendix. The large intestine (colon) consists of three
parts--an ascending part on the right, a transverse middle part, and a
descending part on the left. The latter finally passes through an
S-shaped bend into the last section of the alimentary canal, the
rectum, which opens behind by the anus. Both the large and small
intestines are equipped with numbers of small glands, which secrete
mucous and other fluids.
For the greater part of its length the alimentary canal is attached to
the inner dorsal surface of the abdominal cavity, or to the lower
surface of the vertebral column. The fixing is accomplished by means
of the thin membranous plate that we call the mesentery.
Although the fully-formed alimentary canal is thus a very elaborate
organ, and although in detail it has a quantity of complex structural
features into which we cannot enter here, nevertheless the whole
complicated structure has been historically evolved from the very
simple form of the primitive gut that we find in our
gastraead-ancestors, and that every gastrula brings before us to-day.
We have already pointed out (Chapter 1.9) how the epigastrula of the
mammals (Figure 1.67) can be reduced to the original type of the
bell-gastrula, which is now preserved by the amphioxus alone (Figure
1.35). Like the latter, the human gastrula and that of all other
mammals must be regarded as the ontogenetic reproduction of the
phylogenetic form that we call the Gastraea, in which the whole body
is nothing but a double-walled gastric sac.
We already know from embryology the manner in which the gut develops
in the embryo of man and the other mammals. From the gastrula is first
formed the spherical embryonic vesicle filled with fluid
(gastrocystis, Figure 1.106). In the dorsal wall of this the
sole-shaped embryonic shield is developed, and on the under-side of
this a shallow groove appears in the middle line, the first trace of
the later, secondary alimentary tube. The gut-groove becomes deeper
and deeper, and its edges bend towards each other, and finally form a
tube.
As we have seen, this simple cylindrical gut-tube is at first
completely closed before and behind in man and in the Vertebrates
generally (Figure 1.148); the permanent openings of the alimentary
canal, the mouth and anus, are only formed later on, and from the
outer skin. A mouth-pit appears in the skin in front (Figure 2.350
hp), and this grows towards the blind fore-end of the cavity of the
head-gut (kd), and at length breaks into it. In the same way a shallow
anus-pit is formed in the skin behind, which grows deeper and deeper,
advances towards the blind hinder end of the pelvic gut, and at last
connects with it. There is at first, both before and behind, a thin
partition between the external cutaneous pit and the blind end of the
gut--the throat-membrane in front and the anus-membrane behind; these
disappear when the connection takes place.
Directly in front of the anus-opening the allantois develops from the
hind gut; this is the important embryonic structure that forms into
the placenta in the Placentals (including man). In this more advanced
form the human alimentary canal (and that of all the other mammals) is
a slightly bent, cylindrical tube, with an opening at each end, and
two appendages growing from its lower wall: the anterior one is the
umbilical vesicle or yelk-sac, and the posterior the allantois or
urinary sac (Figure 1.195).
The thin wall of this simple alimentary tube and its ventral
appendages is found, on microscopic examination, to consist of two
strata of cells. The inner stratum, lining the entire cavity, consists
of larger and darker cells, and is the gut-gland layer. The outer
stratum consists of smaller and lighter cells, and is the gut-fibre
layer. The only exception is in the cavities of the mouth and anus,
because these originate from the skin. The inner coat of the
mouth-cavity is not provided by the gut-gland layer, but by the
skin-sense layer; and its muscular substratum is provided, not by the
gut-fibre, but the skin-fibre, layer. It is the same with the wall of
the small anus-cavity.
If it is asked how these constituent layers of the primitive gut-wall
are related to the various tissues and organs that we find afterwards
in the fully-developed system, the answer is very simple. It can be
put in a single sentence. The epithelium of the gut--that is to say,
the internal soft stratum of cells that lines the cavity of the
alimentary canal and all its appendages, and is immediately occupied
with the processes of nutrition--is formed solely from the gut-gland
layer; all other tissues and organs that belong to the alimentary
canal and its appendages originate from the gut-fibre layer. From the
latter is also developed the whole of the outer envelope of the gut
and its appendages; the fibrous connective tissue and the smooth
muscles that compose its muscular layer, the cartilages that support
it (such as the cartilages of the larynx and the trachea), the
blood-vessels and lymph-vessels that absorb the nutritive fluid from
the intestines--in a word, all that there is in the alimentary system
besides the epithelium of the gut. From the same layer we also get the
whole of the mesentery, with all the organs embedded in it--the heart,
the large blood-vessels of the body, etc.
(FIGURE 2.351. Scales or cutaneous teeth of a shark (Centrophorus
calceus). A three-pointed tooth rises obliquely on each of the
quadrangular bony plates that lie in the corium. (From Gegenbaur.))
Let us now leave this original structure of the mammal gut for a
moment, in order to compare it with the alimentary canal of the lower
Vertebrates, and of those Invertebrates that we have recognised as
man's ancestors. We find, first of all, in the lowest Metazoa, the
Gastraeads, that the gut remains permanently in the very simple form
in which we find it transitorily in the palingenetic gastrula of the
other animals; it is thus in the Gastremaria (Pemmatodiscus), the
Physemaria (Prophysema), the simplest Sponges (Olynthus), the
freshwater Polyps (Hydra), and the ascula-embryos of many other
Coelenteria (Figures 2.233 to 2.238). Even in the simplest forms of
the Platodes, the Rhabdocoela (Figure 2.240), the gut is still a
simple straight tube, lined with the entoderm; but with the important
difference that in this case its single opening, the primitive mouth
(m), has formed a muscular gullet (sd) by invagination of the skin.
(FIGURE 2.352. Gut of a human embryo, one-sixth of an inch long,
magnified fifteen times. (From His. Showing: Epiglottis, Tongue,
Hypophysis, Hepatic duct, Tail, Allantoic duct, Tail-gut, Umbilical
cord, Larynx, Rudimentary lungs, Stomach, Pancreas, Bladder, Wolffian
duct, Rudimentary kidneys.))
We have the same simple form in the gut of the lowest Vermalia
(Gastrotricha, Figure 2.242, Nematodes, Sagitta, etc.). But in these a
second important opening of the gut has been formed at the opposite
end to the mouth, the anus (Figure 2.242 a).
We see a great advance in the structure of the vermalian gut in the
remarkable Balanoglossus (Figure 2.245), the sole survivor of the
Enteropneust class. Here we have the first appearance of the division
of the alimentary tube into two sections that characterises the
Chordonia. The fore half, the head-gut (cephalogaster), becomes the
organ of respiration (branchial gut, Figure 2.245 k); the hind half,
the trunk-gut (truncogaster), alone acts as digestive organ (hepatic
gut, d). The differentiation of these two parts of the gut in the
Enteropneust is just the same as in all the Tunicates and Vertebrates.
It is particularly interesting and instructive in this connection to
compare the Enteropneusts with the Ascidia and the Amphioxus (Figures
2.220 and 2.210)--the remarkable animals that form the connecting link
between the Invertebrates and the Vertebrates. In both forms the gut
is of substantially the same construction; the anterior section forms
the respiratory branchial gut, the posterior the digestive hepatic
gut. In both it develops palingenetically from the primitive gut of
the gastrula, and in both the hinder end of the medullary tube covers
the primitive mouth to such an extent that the remarkable medullary
intestinal duct is formed, the passing communication between the
neural and intestinal tubes (canalis neurentericus, Figures 1.83 and
1.85 ne). In the vicinity of the closed primitive mouth, possibly in
its place, the later anus is developed. In the same way the mouth is a
fresh formation in the Amphioxus and the Ascidia. It is the same with
the human mouth and that of the Craniotes generally. The secondary
formation of the mouth in the Chordonia is probably connected with the
development of the gill-clefts which are formed in the gut-wall
immediately behind the mouth. In this way the anterior section of the
gut is converted into a respiratory organ. I have already pointed out
that this modification is distinctive of the Vertebrates and
Tunicates. The phylogenetic appearance of the gill-clefts indicates
the commencement of a new epoch in the stem-history of the
Vertebrates.
In the further ontogenetic development of the alimentary canal in the
human embryo the appearance of the gill-clefts is the most important
process. At a very early stage the gullet-wall joins with the external
body-wall in the head of the human embryo, and this is followed by the
formation of four clefts, which lead directly into the gullet from
without, on the right and left sides of the neck, behind the mouth.
These are the gill or gullet clefts, and the partitions that separate
them are the gill or gullet-arches (Figure 1.171). These are most
interesting embryonic structures. They show us that all the higher
Vertebrates reproduce in their earlier stages, in harmony with the
biogenetic law, the process that had so important a part in the rise
of the whole Chordonia-stem. This process was the differentiation of
the gut into two sections--an anterior respiratory section, the
branchial gut, that was restricted to breathing, and a posterior
digestive section, the hepatic gut. As we find this highly
characteristic differentiation of the gut into two different sections
in all the Vertebrates and all the Tunicates, we may conclude that it
was also found in their common ancestors, the Prochordonia--especially
as even the Enteropneusts have it. (Cf. Chapters 1.12, 1.14 and 2.20,
and Figures 2.210, 2.220, 2.245.) It is entirely wanting in all the
other Invertebrates.
(FIGURE 2.353. Gut of a dog-embryo (shown in Figure 1.202, from
Bischoff), seen from the ventral side, a gill-arches (four pairs), b
rudiments of pharynx and larynx, c lungs, d stomach, f liver, g walls
of the open yelk-sac (into which the middle gut opens with a wide
aperture), h rectum.
FIGURE 2.354. The same gut seen from the right. a lungs, b stomach, c
liver, d yelk-sac, e rectum.)
There is at first only one pair of gill-clefts in the Amphioxus, as in
the Ascidia and Enteropneusts; and the Copelata (Figure 2.225) have
only one pair throughout life. But the number presently increases in
the former. In the Craniotes, however, it decreases still further. The
Cyclostomes have six to eight pairs (Figure 2.247); some of the
Selachii six or seven pairs, most of the fishes only four or five
pairs. In the embryo of man, and the higher Vertebrates generally,
where they make an appearance at an early stage, only three or four
pairs are developed. In the fishes they remain throughout life, and
form an exit for the water taken in at the mouth (Figures 2.249 to
2.251). But they are partly lost in the amphibia, and entirely in the
higher Vertebrates. In these nothing is left but a relic of the first
gill-cleft. This is formed into a part of the organ of hearing; from
it are developed the external meatus, the tympanic cavity, and the
Eustachian tube. We have already considered these remarkable
structures, and need only point here to the interesting fact that our
middle and external ear is a modified inheritance from the fishes. The
branchial arches also, which separate the clefts, develop into very
different parts. In the fishes they remain gill-arches, supporting the
respiratory gill-leaves. It is the same with the lowest amphibia, but
in the higher amphibia they undergo various modifications; and in the
three higher classes of Vertebrates (including man) the hyoid bone and
the ossicles of the ear develop from them. (Cf. Chapter 2.25.)
(FIGURE 2.355. Median section of the head of a Petromyzon-larva. (From
Gegenbaur,) h hypobranchial groove (above it in the gullet we see the
internal openings of the seven gill-clefts), v velum, o mouth, c
heart, a auditory vesicle, n neural tube, ch chorda.)
From the first gill-arch, from the inner surface of which the muscular
tongue proceeds, we get the first structure of the maxillary
skeleton--the upper and lower jaws, which surround the mouth and
support the teeth. These important parts are wholly wanting in the two
lowest classes of Vertebrates, the Acrania and Cyclostoma. They appear
first in the earliest Selachii (Figures 2.248 to 2.251), and have been
transmitted from this stem-group of the Gnathostomes to the higher
Vertebrates. Hence the original formation of the skeleton of the mouth
can be traced to these primitive fishes, from which we have inherited
it. The teeth are developed from the skin that clothes the jaws. As
the whole mouth cavity originates from the outer integument (Figure
2.350), the teeth also must come from it. As a fact, this is found to
be the case on microscopic examination of the development and finer
structure of the teeth. The scales of the fishes, especially of the
shark type (Figure 2.351), are in the same position as their teeth in
this respect (Figure 2.252). The osseous matter of the tooth (dentine)
develops from the corium; its enamel covering is a secretion of the
epidermis that covers the corium. It is the same with the cutaneous
teeth or placoid scales of the Selachii. At first the whole of the
mouth was armed with these cutaneous teeth in the Selachii and in the
earliest amphibia. Afterwards the formation of them was restricted to
the edges of the jaws.
Hence our human teeth are, in relation to their original source,
modified fish-scales. For the same reason we must regard the salivary
glands, which open into the mouth, as epidermic glands, as they are
formed, not from the glandular layer of the gut like the rest of the
alimentary glands, but from the epidermis, from the horny plate of the
outer germinal layer. Naturally, in harmony with this evolution of the
mouth, the salivary glands belong genetically to one series with the
sudoriferous, sebaceous, and mammary glands.
Thus the human alimentary canal is as simple as the primitive gut of
the gastrula in its original structure. Later it resembles the gut of
the earliest Vermalia (Gastrotricha). It then divides into two
sections, a fore or branchial gut and a hind or hepatic gut, like the
alimentary canal of the Balanoglossus, the Ascidia, and the Amphioxus.
The formation of the jaws and the branchial arches changes it into a
real fish-gut (Selachii). But the branchial gut, the one reminiscence
of our fish-ancestors, is afterwards atrophied as such. The parts of
it that remain are converted into entirely different structures.
(FIGURE 2.356. Transverse section of the head of a Petromyzon-larva.
(From Gegenbaur.) Beneath the pharynx (d) we see the hypobranchial
groove; above it the chorda and neural tube. A, B, C stages of
constriction.)
But, although the anterior section of our alimentary canal thus
entirely loses its original character of branchial gut, it retains the
physiological character of respiratory gut. We are now astonished to
find that the permanent respiratory organ of the higher Vertebrates,
the air-breathing lung, is developed from this first part of the
alimentary canal. Our lungs, trachea, and larynx are formed from the
ventral wall of the branchial gut. The whole of the respiratory
apparatus, which occupies the greater part of the pectoral cavity in
the adult man, is at first merely a small pair of vesicles or sacs,
which grow out of the floor of the head-gut immediately behind the
gills (Figures 2.354 C, 1.147 l). These vesicles are found in all the
Vertebrates except the two lowest classes, the Acrania and
Cyclostomes. In the lower Vertebrates they do not develop into lungs,
but into a large air-filled bladder, which occupies a good deal of the
body-cavity and has a quite different purport. It serves, not for
breathing, but to effect swimming movements up and down, and so is a
sort of hydrostatic apparatus--the floating bladder of the fishes
(nectocystis, Chapter 2.21). However, the human lungs, and those of
all air-breathing Vertebrates, develop from the same simple vesicular
appendage of the head-gut that becomes the floating bladder in the
fishes.
At first this bladder has no respiratory function, but merely acts as
hydrostatic apparatus for the purpose of increasing or lessening the
specific gravity of the body. The fishes, which have a fully-developed
floating bladder, can press it together, and thus condense the air it
contains. The air also escapes sometimes from the alimentary canal,
through an air-duct that connects the floating bladder with the
pharynx, and is ejected by the mouth. This lessens the size of the
bladder, and so the fish becomes heavier and sinks. When it wishes to
rise again, the bladder is expanded by relaxing the pressure. In many
of the Crossopterygii the wall of the bladder is covered with bony
plates, as in the Triassic Undina (Figure 2.254).
This hydrostatic apparatus begins in the Dipneusts to change into a
respiratory organ; the blood-vessels in the wall of the bladder now no
longer merely secrete air themselves, but also take in fresh air
through the air-duct. This process reaches its full development in the
Amphibia. In these the floating bladder has turned into lungs, and the
air-passage into a trachea. The lungs of the Amphibia have been
transmitted to the three higher classes of Vertebrates. In the lowest
Amphibia the lungs on either side are still very simple transparent
sacs with thin walls, as in the common water-salamander, the Triton.
It still entirely resembles the floating bladder of the fishes. It is
true that the Amphibia have two lungs, right and left. But the
floating bladder is also double in many of the fishes (such as the
early Ganoids), and divides into right and left halves. On the other
hand, the lung is single in Ceratodus (Figure 2.257).
(FIGURE 2.357. Thoracic and abdominal viscera of a human embryo of
twelve weeks, natural size, (From Kolliker.) The head is omitted.
Ventral and pectoral walls are removed. The greater part of the
body-cavity is taken up with the liver, from the middle part of which
the caecum and the vermiform appendix protrude. Above the diaphragm,
in the middle, is the conical heart; to the right and left of it are
the two small lungs.)
In the human embryo and that of all the other Amniotes the lungs
develop from the hind part of the ventral wall of the head-gut (Figure
1.149). Immediately behind the single structure of the thyroid gland a
median groove, the rudiment of the trachea, is detached from the
gullet. From its hinder end a couple of vesicles develop--the simple
tubular rudiments of the right and left lungs. They afterwards
increase considerably in size, fill the greater part of the thoracic
cavity, and take the heart between them. Even in the frogs we find
that the simple sac has developed into a spongy body of peculiar
froth-like tissue. The originally short connection of the pulmonary
sacs with the head-gut extends into a long, thin tube. This is the
wind-pipe (trachea); it opens into the gullet above, and divides below
into two branches which go to the two lungs. In the wall of the
trachea circular cartilages develop, and these keep it open. At its
upper end, underneath its pharyngeal opening, the larynx is
formed--the organ of voice and speech. The larynx is found at various
stages of development in the Amphibia, and comparative anatomists are
in a position to trace the progressive growth of this important organ
from the rudimentary structure of the lower Amphibia up to the
elaborate and delicate vocal apparatus that we have in the larynx of
man and of the birds.
We must refer here to an interesting rudimentary organ of the
respiratory gut, the thyroid gland, the large gland in front of the
larynx, that lies below the "Adam's apple," and is often especially
developed in the male sex. It has a certain function--not yet fully
understood--in the nutrition of the body, and arises in the embryo by
constriction from the lower wall of the pharynx. In many mining
districts the thyroid gland is peculiarly liable to morbid
enlargement, and then forms goitre, a growth that hangs at the front
of the neck. But it is much more interesting phylogenetically. As
Wilhelm Muller, of Jena, has shown, this rudimentary organ is the last
relic of the hypobranchial groove, which we considered in a previous
chapter, and which runs in the middle line of the gill-crate in the
Ascidia and Amphioxus, and conveys food to the stomach. (Cf. Chapter
2.16, Figure 2.246). We still find it in its original character in the
larvae of the Cyclostomes (Figures 2.355 and 2.356).
The second section of the alimentary canal, the trunk or hepatic gut,
undergoes not less important modifications among our vertebrate
ancestors than the first section. In tracing the further development
of this digestive part of the gut, we find that most complex and
elaborate organs originate from a very rudimentary original structure.
For clearness we may divide the digestive gut into three sections: the
fore gut (with oesophagus and stomach), the middle gut (duodenum, with
liver, pancreas, jejunum, and ileum, and the hind gut (colon and
rectum). Here again we find vesicular growths or appendages of the
originally simple gut developing into a variety of organs. Two of
these embryonic structures, the yelk-sac and allantois, are already
known to us. The two large glands that open into the duodenum, the
liver and pancreas, are growths from the middle and most important
part of the trunk-gut.
Immediately behind the vesicular rudiments of the lungs comes the
section of the alimentary canal that forms the stomach (Figures 2.353
d and 2.354 b). This sac-shaped organ, which is chiefly responsible
for the solution and digestion of the food, has not in the lower
Vertebrates the great physiological importance and the complex
character that it has in the higher. In the Acrania and Cyclostomes
and the earlier fishes we can scarcely distinguish a real stomach; it
is represented merely by the short piece from the branchial to the
hepatic gut. In some of the other fishes also the stomach is only a
very simple spindle-shaped enlargement at the beginning of the
digestive section of the gut, running straight from front to back in
the median plane of the body, underneath the vertebral column. In the
mammals its first structure is just as rudimentary as it is
permanently in the preceding. But its various parts soon begin to
develop. As the left side of the spindle-shaped sac grows much more
quickly than the right, and as it turns considerably on its axis at
the same time, it soon comes to lie obliquely. The upper end is more
to the left, and the lower end more to the right. The foremost end
draws up into the longer and narrower canal of the oesophagus.
Underneath this on the left the blind sac (fundus) of the stomach
bulges out, and thus the later form gradually develops (Figures 2.349
and 1.184 e). The original longitudinal axis becomes oblique, sinking
below to the left and rising to the right, and approaches nearer and
nearer to a transverse position. In the outer layer of the
stomach-wall the powerful muscles that accomplish the digestive
movements develop from the gut-fibre layer. In the inner layer a
number of small glandular tubes are formed from the gut-gland layer;
these are the peptic glands that secrete the gastric juice. At the
lower end of the gastric sac is developed the valve that separates it
from the duodenum (the pylorus, Figure 2.349 d).
Underneath the stomach there now develops the disproportionately long
stretch of the small intestine. The development of this section is
very simple, and consists essentially in an extremely rapid and
considerable growth lengthways. It is at first very short, quite
straight, and simple. But immediately behind the stomach we find at an
early stage a horseshoe-shaped bend and loop of the gut, in connection
with the severance of the alimentary canal from the yelk-sac and the
development of the first mesentery. The thin delicate membrane that
fastens this loop to the ventral side of the vertebral column, and
fills the inner bend of the horseshoe formation, is the first rudiment
of the mesentery (Figure 1.147 g). We find at an early stage a
considerable growth of the small intestine; it is thus forced to coil
itself in a number of loops. The various sections that we have to
distinguish in it are differentiated in a very simple way--the
duodenum (next to the stomach), the succeeding long jejunum, and the
last section of the small intestine, the ileum.
From the duodenum are developed the two large glands that we have
already mentioned--the liver and pancreas. The liver appears first in
the shape of two small sacs, that are found to the right and left
immediately behind the stomach (Figures 2.353 f, and 2.354 c). In many
of the lower Vertebrates they remain separate for a long time (in the
Myxinoides throughout life), or are only imperfectly joined. In the
higher Vertebrates they soon blend more or less completely to form a
single large organ. The growth of the liver is very brisk at first. In
the human embryo it grows so much in the second month of development
that in the third it occupies by far the greater part of the
body-cavity (Figure 2.357). At first the two halves develop equally;
afterwards the left falls far behind the right. In consequence of the
unsymmetrical development and turning of the stomach and other
abdominal viscera, the whole liver is now pushed to the right side.
Although the liver does not afterwards grow so disproportionately, it
is comparatively larger in the embryo at the end of pregnancy than in
the adult. Its weight relatively to that of the whole body is 1 : 36
in the adult, and 1 : 18 in the embryo. Hence it is very important
physiologically during embryonic life; it is chiefly concerned in the
formation of blood, not so much in the secretion of bile.
Immediately behind the liver a second large visceral gland develops
from the duodenum, the pancreas or sweetbread. It is wanting in most
of the lowest classes of Vertebrates, and is first found in the
fishes. This organ is also an outgrowth from the gut.
The last section of the alimentary canal, the large intestine, is at
first in the embryo a very simple, short, and straight tube, which
opens behind by the anus. It remains thus throughout life in the lower
Vertebrates. But it grows considerably in the mammals, coils into
various folds, and divides into two sections, the first and longer of
which is the colon, and the second the rectum. At the beginning of the
colon there is a valve (valvula Bauhini) that separates it from the
small intestine. Immediately behind this there is a sac-like growth,
which enlarges into the caecum (Figure 2.357 v). In the plant-eating
mammals this is very large, but it is very small or completely
atrophied in the flesh-eaters. In man, and most of the apes, only the
first portion of the caecum is wide; the blind end-part of it is very
narrow, and seems later to be merely a useless appendage of the
former. This "vermiform appendage" is very interesting as a
rudimentary organ. The only significance of it in man is that not
infrequently a cherry-stone or some other hard and indigestible matter
penetrates into its narrow cavity, and by setting up inflammation and
suppuration causes the death of otherwise sound men. Teleology has
great difficulty in giving a rational explanation of, and attributing
to a beneficent Providence, this dreaded appendicitis. In our
plant-eating ancestors this rudimentary organ was much larger and had
a useful function.
Finally, we have important appendages of the alimentary tube in the
bladder and urethra, which belong to the alimentary system. These
urinary organs, acting as reservoir and duct for the urine excreted by
the kidneys, originate from the innermost part of the allantoic
pedicle. In the Dipneusts and Amphibia, in which the allantoic sac
first makes its appearance, it remains within the body-cavity, and
functions entirely as bladder. But in all the Amniotes it grows far
outside of the body-cavity of the embryo, and forms the large
embryonic "primitive bladder," from which the placenta develops in the
higher mammals. This is lost at birth. But the long stalk or pedicle
of the allantois remains, and forms with its upper part the middle
vesico-umbilical ligament, a rudimentary organ that goes in the shape
of a solid string from the vertex of the bladder to the navel. The
lowest part of the allantoic pedicle (or the "urachus") remains
hollow, and forms the bladder. At first this opens into the last
section of the gut in man as in the lower Vertebrates; thus there is a
real cloaca, which takes off both urine and excrements. But among the
mammals this cloaca is only permanent in the Monotremes, as it is in
all the birds, reptiles, and amphibia. In all the other mammals
(marsupials and placentals) a transverse partition is afterwards
formed, and this separates the urogenital aperture in front from the
anus-opening behind. (Cf. Chapters 2.22 and 2.29.)