throughout except possibly for a slight enlargement of the anterior
region where it leaves the pharynx. The two “olivary enlargements”
mentioned by Chaffanjon (15) are not always present, and when seen were
found to contain either food or small stones or both.
The outside of the œsophagus is smooth and muscular while the lining
is thrown into numerous longitudinal folds that in the empty œsophagus
nearly obliterate the lumen; where distended by food or pebbles the
longitudinal folds may be almost obliterated. In a thirty-inch animal
the œsophagus is about six inches long, and opens suddenly, but
without any apparent valve, into the large chamber of the stomach. The
histology of the œsophagus and the other regions of the digestive tract
will be described later.
[Illustration: FIG. 34. DIGESTIVE SYSTEM OF A. MISSISSIPPIENSIS.
_bd_, bile duct; _bs_, bile sac; _c_, cloaca; _e_, œsophagus; _f_,
larger or fundic region of stomach; _h_, hyoid apparatus; _l_, liver;
_p_, smaller or pyloric region of the stomach; _pa_, pancreas; _r_,
rectum; _s_, small intestine; _t_, tongue; _tr_, trachea.]
THE STOMACH
The stomach, as is well known, is made up of two distinct parts;
that on the animal’s left, into which the œsophagus opens, is many
times larger than the part from which the small intestine leads. The
larger or _fundic region_, Fig. 34, _f_, has, as will be described,
very heavy muscular walls. When empty the lining of this part of
the stomach is thrown into a few comparatively large folds, but when
greatly distended with food, as it sometimes is, the internal folds are
completely obliterated and the muscular layers are stretched until they
have scarcely an eighth of their original thickness. In Figure 34 the
stomach is considerably distended.
The large region of the stomach frequently contains a number of stones,
and for that reason, probably, is sometimes spoken of as the gizzard.
In one thirty-inch alligator fourteen pebbles of irregular shape,
varying in largest diameter from four to seventeen mm. and aggregating
six grams in weight, were found. Voeltzkow (78) says that gastroliths
of two to three cm. diameter are found in the stomach of the adult
Madagascar crocodile.
Neither the transverse fold nor the smooth, lateral disks (or shields)
described by Chaffanjon could be seen in either the empty or in the
distended stomach.
The _smaller part_ of the stomach, Fig. 34, _p_, lies to the right
and somewhat ventrad to the anterior region of the larger part, near
the entrance to the œsophagus. It connects by a fairly large opening
with the larger part of the stomach, and by a smaller opening with the
duodenum. The former opening apparently has no valve, unless it be a
slight sphincter muscle; the latter is guarded by a pair of thickened
lips, called by Chaffanjon “semilunar valves.”
The walls of the smaller part of the stomach are, as might be
expected, much thinner than those of the larger region, but they are
proportionately fairly thick and are internally thrown into numerous
folds.
THE INTESTINE
In the intestine three regions may be distinguished: a long,
considerably coiled small intestine; a wide, nearly straight rectum;
and a short, wide cloaca.
The _small intestine_, Fig. 34, _s_, is of moderate and rather uniform
diameter, though somewhat thicker near the stomach, and is not coiled
so extensively as figured by Chaffanjon. Near the stomach it receives
the ducts of the liver and pancreas. The bile duct, Fig. 34, _bd_,
is a continuation of an elongated bile sac, _bs_, which lies between
the large right and smaller left lobes of the liver, _l_. The two
main lobes of the liver, which appear smaller than in reality because
of foreshortening in drawing, are connected, across the base of the
œsophagus, by a narrow transverse band.
The pancreas, _pa_, which is of fair size, lies partly dorsal to and
partly in a narrow loop of the intestine, so that it is not very
evident in a ventral view of the animal.
The small intestine has heavy muscular walls whose histological
structure will be described elsewhere. It opens abruptly, without any
indication of a cæcum, into the large intestine or rectum.
The _rectum_, _r_, is of about twice the diameter of the small
intestine, though this, of course, varies with the amount of fecal
matter it contains; it is nearly straight and possesses much thinner
walls than the small intestine, though this, again, varies with the
state of collapse or distention.
At the posterior end of the rectum is a heavy sphincter valve
separating that part of the intestine from the cloaca.
The _cloaca_, _c_, is widest anteriorly where it is about as wide as
the rectum; it gradually diminishes in diameter caudad, and appears
flattened laterally. Its wall has the same general structure as the
rectum, as will be described below. The mucous membrane posterior
to the openings of the genital ducts is thrown into a more or less
complete, ring-like transverse fold (Fig. 55 g.). In some species there
may be a second, half-ring-like fold in the dorsal wall caudad to the
more complete ring. The cloaca is divided by this fold into a larger
anterior portion, g, and a shorter posterior portion, h; in the former
the mucous membrane is thrown into a large number of small folds that
in places form a network; in the latter the mucous membrane has a hard,
thick epithelium, with a smooth surface and only a few longitudinal
folds.
The _ureters_ open, Fig. 55, _d_, _e_, at a moderate distance from each
other, into the anterior region of the cloaca (about where the dorsal
and lateral walls of this region come together). The _genital ducts_
(oviducts or vasa deferentia), _c_, _f_, on the other hand, open close
together through the ventral wall of the posterior half of the cloaca,
just in front of the copulatory organ.
Into the cloaca, very near the anus, open two glands of fairly large
size that Rathke called _musk glands_. These glands lie outside of
the pelvis between the side walls of the cloaca and a large muscle
that surrounds this part of the body. They have an oval form and open
usually from their anterior end, sometimes just caudad to this, by a
short, fairly wide, slit-like opening which has an anteroposterior
direction. The walls of the glands are made up of three closely
associated layers of connective tissue, the inner one being thrown into
folds. Since these layers contain no muscle fibers the secretion of the
gland is probably squeezed out by contraction of the circular muscles
of the cloaca. Usually the cloacal glands are stretched full by a
thick, yellowish mass that smells strongly of musk.
The part of the cloaca caudad to the pelvic opening has a differently
arranged musculature from the more anterior region. It consists of two
separate pairs of striped muscles that surround the musk glands on
the outer side. The first pair form a fairly broad, moderately thick
ring muscle next to the anus that is attached anteriorly to the pubis
and posteriorly to the second hæmal process. When these muscles draw
together they narrow or completely close the anal slit. The muscles
of the other pair are broader but thinner, and extend in a general
dorso-ventral direction. Anteriorly, above the cloaca, they are united
with each other, but posteriorly they separate and, with the above
ring muscle, are inserted on the second hæmal arch. Judging from their
attachment they widen the anal opening laterally.
THE HISTOLOGY OF THE ENTERON OF THE FLORIDA ALLIGATOR
It has long been known that the sea lamprey, _Petromyzon marinus_,
during the spawning season, when the body is distended with eggs, takes
no food, and that the digestive tract during this period shrivels up
until it is reduced to a mere thread. This condition doubtless obtains
in other forms as well, though it has not been actually observed by the
writer elsewhere.
A number of small alligators that were kept alive in the laboratory
for a year or more caused the writer to wonder whether any very marked
change had taken place in their digestive tracts during the months they
took no food.
In captivity, especially if the water in their tank be kept cold,
alligators may refuse food for five or sixth months. Whether, during
the winter months, in their native haunts, they entirely cease feeding,
the writer has had no opportunity to observe, though it is popularly
reported that such is the case.
The first alligator from which tissues were taken was about a year and
a half old, and measured eighteen inches in length. It was killed in
March after a fast of several months, probably four or five, possibly
more, though it was not in the writer’s possession for so long a time.
[Illustration: FIG. 35. OUTLINE OF DIGESTIVE TRACT
Fig. 35. A diagrammatic outline of the digestive tract of the
alligator from the beginning of the œsophagus to the cloaca, to
show the planes of the sections that were studied, _a.oes._,
anterior œsophagus; _a.r._, anterior rectum; _a.s.i._, anterior
small intestine; _c.st._, cardiac stomach; _f.st._, fundic stomach;
_m.s.i._, middle small intestine; _p.oes._, posterior œsophagus;
_p.r._, posterior rectum or cloaca; _p.s.i._, posterior small
intestine; _p.st._, pyloric stomach.]
Although carefully fixed in the usual fluids, the epithelial
structures from this animal were not as clearly defined in most cases
as could be desired; this rather unsatisfactory fixation may have
been due to some physiological condition characteristic of the period
of hibernation. That this was the case seems likely from the better
fixation obtained by the same methods in the case of animals killed
during the feeding season.
The other animals from which tissues were taken were considerably
smaller than the one mentioned above. They were killed early in the
fall, after having been fed regularly for about five months upon bits
of meat, both raw and cooked.
_The Tongue._ The covering of the tongue was studied in two regions,
near the free end, and towards the base.
A section of the former region, drawn under high power, is shown in
Figure 36. It consists of a dense mass of fibrous tissue, _a_, and
small scattered cells, overlaid by a stratified epithelium of eight
or ten layers. Only a small part of the fibrous base, just beneath
the epithelium, is here shown. It is a dense areolar tissue with the
elastic fibers apparently predominating.
The epithelium, _e_, consists, as has just been said, of about eight
or ten layers of cells, those at the base being generally cuboidal
in shape, while towards the surface the cells become more and more
flattened until at the surface they form a thick horny layer, _h_,
in which no nuclei can be seen. The cells of the horny layer are
flattened into mere fibers, which, at places, are seen projecting from
the surface. The boundary between the horny cells and those beneath
is quite distinct, though perhaps not quite so sharp as shown in the
figure under discussion.
[Illustration:
FIG. 36. The covering of the anterior region of the tongue of the
hibernating animal, under fairly high magnification; the plane of
this section is not shown in Figure 35; _a_, areolar tissue; _e_,
epithelium; _h_, horny layer of epithelium.]
In a previous paper, the writer noted that the dorsum of the tongue
is covered with small, evenly distributed papillæ, easily seen by aid
of a hand lens. These so-called papillæ are here seen to be hardly
papillæ at all, but small folds or wrinkles, although the epithelium
is somewhat thickened at intervals. No glands are to be seen in this
region of the tongue.
The only difference between the anterior region of the tongue during
hibernation and during the feeding season seems to be in the scaly
layer of the epithelium. Instead of the compact, sharply differentiated
layer of scaly cells seen in Figure 36, the anterior region of the
tongue during feeding is covered with a layer of rather loose, scaly
cells, in most of which the nuclei may be seen. No difference in
the amount of sloughing off can be noticed as is the case with the
epithelium of the roof of the mouth.
Figure 37 represents a section, under very low magnification, of the
covering of the base of the tongue. The areolar tissue, _a_, is about
the same as in the preceding section, except that it is more compact
just under the epithelium than it is in its deeper regions. It seems
also more vascular than in the preceding section.
[Illustration:
FIG. 37. Covering of the posterior region of the tongue of the
hibernating animal showing glands, under low magnification; _a_,
areolar tissue; _bv_, blood-vessels; _g_, glands; _e_, epithelium.]
The epithelium, _e_, is of the stratified squamous variety, but
consists of many more layers of cells than in the preceding section and
is hence several times as thick. While its cells are flattened towards
the surface, after the manner of this kind of epithelium, they do not
form the definite horny layer described above.
[Illustration:
FIG. 38. One of the glands from the posterior region of the tongue
of the hibernating animal, under high magnification; _a_, areolar
tissue; _av_, alveolus.]
The most marked difference between the two regions of the tongue is
the presence, in the posterior or basal region, of numerous glands,
_g_, probably mucous- or slime-secreting. They are thickly scattered
through the areolar base, close beneath the epithelium. Two large
glands and one small one are shown in the figure under discussion.
Each gland opens to the surface by an apparently wide duct, but since
no good section of such a duct was obtained it is not shown in the
figure. Although the rest of the tissue was well preserved and showed
cell structure clearly, it was with difficulty that the details of the
glands could be determined.
A high-power drawing of a portion of one of the glands is shown
in Figure 38. The large alveolus, _av_, to the left, is from the
peripheral region of the gland and is surrounded, on its free side, by
the areolar tissue described above. The inter-alveolar spaces, which
are somewhat exaggerated in the figure, are filled with fibers which
are arranged more or less in layers and hence appears different from
the surrounding areolar tissue. The alveoli are circular or elongated
in section, and have fairly wide lumina. They are lined with a single
layer of columnar or cuboidal cells which are very granular, so that
their walls are difficult to determine. Each cell contains, near its
base, a very large, usually spherical nucleus. These nuclei stain
darkly and give the dark appearance to the glands as seen under low
magnification, especially in rather thick sections.
During feeding the epithelium of this region of the tongue consists
of fewer layers of cells than during hibernation but is otherwise
unchanged from what is described above. The glands consist, at least in
all of the material examined, of much fewer alveoli than are shown in
Figure 37. One of these glands is shown in Figure 39.
Although no more care was used in fixation than in the corresponding
tissue of the hibernating animal the glands here show their cell
details far more clearly than in the former tissue; this may have been
partly due to the latter sections being thinner.
The glands are of a compound, tubulo-alveolar type; although numerous
sections through ducts were obtained (as in Fig. 39), no details of
these ducts could be seen.
[Illustration:
FIG. 39. One of the glands from the posterior region of the tongue
of the feeding animal, under somewhat higher magnification than used
in Figure 37; _av_, alveolus; _d_, duct of gland; _e_, stratified
epithelium.]
As noted above, and as may be seen by comparing Figures 37 and 39, the
gland during hibernation, at least in the animals studied, consists
of many more alveoli than during the feeding season; this, of course,
might not prove to be always the case if larger numbers of animals were
studied; the difference in the ages of the animals might have caused
this difference in the glands. In the material studied the largest
glands from the hibernating animals consist of more than twice as many
alveoli as the glands in the feeding animals. As seen under high
magnification there is no noticeable difference in the glands at the
two seasons.
Rathke has given the name of “Geschmackwärzchen” to the conical
projections found on the dorsum of the crocodilian tongue; they are
distinguished by their softness and thinner epithelial covering from
the cones that, in many of these animals, bear the openings of the
mucous glands.
These taste papillæ generally have the form of a truncated cone and
often are surrounded by a shallow circular pit, outside of which, in
turn, is sometimes a small low wall. They are distributed over the
entire dorsum of the tongue, usually at considerable distance from each
other in comparison to the size of the tongue, and are not so numerous
as the taste papillæ of the Mammalia. Rathke found their absolute
number greatest in _A. lucius_.
Rathke mentions other larger and harder projections on the tongue of
certain Crocodilia which, though not perforated by a mucous duct, he
thinks are of questionable relation to the sense papillæ. They usually
have more the form of a flattened than of a truncated cone, and are
very numerous in some species.
_The Roof of the Mouth._ In the paper mentioned above the author notes
that the papillæ on the roof of the mouth are evenly distributed and
are more distinct than those of the dorsum of the tongue. One of these
papillæ as seen under fairly high magnification is shown in Figure 40.
[Illustration:
FIG. 40. The covering of the roof of the mouth of the hibernating
animal, under fairly high magnification; _a_, areolar tissue; _e_,
epithelium; _h_, horny layer; _f_, fibers of horny layer.]
The areolar tissue, _a_, forming the base of the section is of about
the same character as seen in the section of the tongue. Less than one
tenth of the thickness of the entire areolar base is shown in this
section.
The epithelium, _e_, where not thrown into papillæ, has also about the
same character as that of the anterior region of the tongue--the same
number of cell layers and the same distinct horny layer.
At intervals the thickness of the cellular part of the epithelium
is greatly increased, and at the same time the horny layer is also
thickened, to form distinct papillæ like the one shown in the figure.
These, as has been said, are comparatively small and have the shape
of a blunt cone. The center of the cone is, of course, made up of the
cellular epithelium, while the outside is covered with the thickened
horny layer from which fibers, _f_, are often seen projecting. Near the
apex of the cone the nuclei are larger and more widely scattered than
those at the base.
No glands were seen in the roof of the mouth of the hibernating animal,
but since the entire roof was not sectioned it is probable that they
may exist in some regions; in fact, as noted below, sections through
the posterior region of the roof of the mouth of the feeding animal do
show numerous glands.
As might be expected there is comparatively little difference between
this region of the enteron during hibernation and during the feeding
season. The only noticeable difference is in the stratified epithelium;
that of the feeding animal not only has less sharp papillæ but has also
a much thinner scaly layer of cells. As is seen in the figure of the
roof of the mouth during hibernation the scaly cells make up, except on
the papillæ, nearly or quite half of the thickness of the epithelium,
while in the feeding animal they make up not more than one fourth or
one third of the entire epithelium. Very few cells are seen sloughing
off as in Figure 40; possibly the act of feeding keeps the superficial
scaly cells rubbed off smooth.
In the extreme posterior region of the roof of the mouth the epithelium
consists of a greater number of layers (though the number is very
variable) than in the region shown in Figure 40. In this posterior
region, as noted above, glands are found. These glands have the same
structure as those described in connection with the posterior region of
the tongue.
_The Œsophagus._ Sections of the œsophagus were made from two regions,
an anterior, half-inch caudad to the pharynx, and a posterior region,
half-inch cephalad to the opening of the œsophagus into the stomach
(Fig. 35).
The general structure of the wall of the œsophagus, as seen under a
low magnification, will first be described, after which the minute
structure of the epithelium, as seen under high magnification, will be
discussed.
In the _anterior region_ the usual layers of the vertebrate enteron are
present, except, possibly, the muscularis mucosa.
The epithelium, to be described later, is, together with the submucosa,
thrown into complicated folds; its closely arranged and darkly stained
nuclei cause it to stand out in strong contrast to the other tissues of
the section (Fig. 41, _e_).
The submucosa, _sm_, is of considerable thickness. It is composed
of a fairly dense mass of connective tissue, mainly elastic fibers,
through which are scattered small blood-vessels, _bv_, and small dark
areas, _mb_, that are apparently longitudinal bundles of involuntary
muscle fibers. These few and scattered fibers probably represent the
muscularis mucosa that is so well developed in the posterior region of
the œsophagus.
Outside of the mucosa is a thick circular layer of involuntary muscle
fibers, _cm_, the fibers being collected into irregular bundles,
between which are narrow spaces filled with connective tissue that
contains a few small blood-vessels.
Surrounding the circular layer is a thinner and less clearly defined
layer of longitudinal muscle fibers, _lm_. The muscle bundles are
more definite than in the circular layer and are separated from each
other by a considerable amount of connective tissue with a few small
blood-vessels.
[Illustration:
FIG. 41. A transsection through the anterior region of the
œsophagus of the hibernating animal under low magnification; _bv_,
blood-vessels; _mb_, muscle bundles; other letters as in Figure 42.]
The serosa, _s_, is here quite indistinct. It consists of a slightly
vascular connective tissue which cannot be distinctly differentiated
from the connective tissue of the longitudinal layer.
[Illustration:
FIG. 42. A transsection through the posterior region of the œsophagus
of the hibernating animal, under low magnification; _e_, epithelium;
_cm_, circular muscles; _lm_, longitudinal muscles; _mm_, muscularis
mucosa; _sm_, submucosa; _s_, serosa.]
In the _posterior region_ of the œsophagus, as may be seen by
comparison of figures 41 and 42, the wall as a whole is about one
third thicker than in the anterior region just described, though how
much of this difference is due to different degrees of distension or
contraction it is hard to say.
The epithelium, _e_, is in the tissue studied thrown into less
complicated folds than in the anterior region, and is not so thick.
The submucosa, _sm_, if the entire layer may be so called, has about
the same thickness and structure as in the more anterior region; but
instead of the small and widely scattered bundles of longitudinal
muscle fibers there is a distinct layer of muscle which may be called
the muscularis mucosa, _mm_, lying about midway between the epithelium
and the circular muscle layer.
The muscularis mucosa is somewhat variable in thickness and is thrown
into folds that correspond to the larger folds of the epithelium and
the submucosa; one of these folds is shown in Figure 42. The fibers of
the muscularis mucosa are apparently all longitudinal in position.
Outside of the submucosa is a layer of circular muscle fibers, _cm_; it
is here somewhat wider and more dense than in the anterior region.
The longitudinal muscle layer (Fig. 42, _lm_) is much wider and more
compact than in the anterior region. The fibers are indistinctly
divided into large irregular masses as shown in the figure.
The serosa (Fig. 42, _s_) is a varying but fairly thick layer that is
quite distinct from the longitudinal muscle layer. It consists of the
usual connective tissue groundwork with scattered blood-vessels.
The epithelium, as was said above, is thicker and somewhat more folded
in the anterior than in the posterior region, and in the former region
is partially ciliated while in the latter cilia are entirely wanting.
With these exceptions the epithelium is practically the same in the two
regions.
Figure 43 represents the epithelium from the anterior region as seen
under high magnification. The outlines of all the cells could not be
determined but if each nucleus represents a cell there are twenty-five
or thirty layers of cells. The nuclei are arranged in two dense,
irregular groups, one along the base of the epithelium, the other about
two thirds of the distance from the base to the free border. The basal
nuclei are perhaps slightly larger and more rounded than those of the
distal group. Between these two groups are numerous more scattered
nuclei; while scattered through the epithelium, except near the free
border, are smaller, round nuclei that stain somewhat darker than the
rest; these, from their size and appearance, seem possibly to belong
to an invisible network of connective tissue that has penetrated the
epithelium from the surrounding mucosa.
[Illustration:
FIG. 43. The epithelium of the anterior region of the œsophagus of
the hibernating animal, under high magnification.]
The free border of the epithelium consists of long, ciliated, columnar
cells in which the cell walls may be easily seen. The cilia are of
average length and even in this anterior region are not everywhere
present; possibly they are arranged in bands, but the material at hand
was not sufficient to determine this. As was noted above, cilia are
wanting in the posterior region.
The only differences noted in the anterior region of the œsophagus
between the feeding and the hibernating conditions are in the
muscularis mucosa and the epithelium. As was noted above, the
muscularis mucosa is practically absent in the hibernating stage, being
represented only by a few small, scattered bundles of longitudinal
muscle fibers; while in the feeding stage there is a narrow but fairly
distinct layer to represent the muscularis mucosa.
[Illustration:
FIG. 44. The epithelium of the anterior region of the œsophagus of
the feeding animal, under high magnification.]
The difference in the appearance of the epithelium is not striking. The
nuclei are somewhat larger in the feeding stage and, instead of being
crowded into a basal and a median zone, as noted in the hibernating
conditions, they form a dense basal zone, but show no indication of
medial zone. From the dense basal zone the nuclei become more scattered
towards the free surface and are rarely found closer to the surface
than is shown in Figure 44. The smaller nuclei scattered among the
larger ones, noted in connection with the hibernating stage, are not
here seen.
As in the hibernating stage cilia are present on some but not all cells
of this region.
The only noticeable difference between the feeding and hibernating
conditions of the posterior region of the œsophagus is in the
epithelium, which, as in the feeding condition of the anterior
œsophagus, exhibits but one zone of closely set nuclei, that at the
base of the epithelium.
_The Stomach._ The stomach was sectioned in three regions, as shown
in Figure 35: (1) in the cardiac region very near the opening of the
œsophagus; (2) in the middle or fundic region; and (3) in the region
near the opening of the pylorus. The first two sections are in the
first or large region of the stomach; the third section is in the
second or small region of the stomach (Fig. 35).
The wall as a whole is thickest in the fundus, being there practically
twice as thick as in the pyloric and half again as thick as in the
cardiac region. This great thickening is due mainly to a thickening
of the middle or oblique layer of muscle, which is here remarkably
developed. The mucosa is of nearly uniform thickness in the different
regions and will be described later.
Since there is no striking difference beside that of thickness in the
general structure of the wall of the different regions, the pyloric
region, as seen under low magnification, will now be described (Fig.
45).
The mucosa, _m_, consists of fairly long glands underlaid by a
well-marked muscularis mucosa, _mm_, the latter exhibiting a compact
circular layer over a wider but more scattered layer of longitudinal
fibers. A considerable amount of fibrous connective tissue lies among
the muscle fibers. The circular layer of the muscularis mucosa sends
towards the surface numerous strands or septa between the glands;
six or eight of these are seen in the figure. These strands are not
nearly so numerous in the large region of the stomach. As was said, the
outer or longitudinal layer of the muscularis mucosa is wider but less
compact than the circular and its bundles of fibers are seen in the
figure as a layer of large, scattered dots just beneath the circular
layer.
The submucosa, _sm_, is of average thickness and density. In the fundic
and cardiac regions it seems to extend between the circular and oblique
layers; at any rate, there is a considerable layer of connective tissue
between these two muscular layers.
The circular muscular layer, _cm_, is of only moderate thickness and
is of rather a loose character. In the pyloric region it is not very
distinct from the underlying oblique layer, but in the other regions,
as has just been said, it is separated from the oblique layer by a
considerable layer of connective tissue like that of the submucosa.
The oblique layer, _om_, even in this section of the pyloric region
is the thickest of the three muscle layers; while in the cardiac, and
especially in the fundic, regions it is of great thickness, as was
noted above, and is made up of larger bundles with less intervening
connective tissue.
[Illustration:
FIG. 45. A transsection through the wall of the pyloric region of the
stomach of the feeding animal, under low magnification; _m_, mucosa;
_om_, oblique muscles; other letters as in Figure 42.]
The outer or longitudinal muscle layer, _lm_, is comparatively
little developed and consists of small rather scattered bundles of
muscles with a correspondingly large amount of connective tissue.
This connective tissue passes insensibly into that of the surrounding
serosa, _s_, a loose, vascular layer of varying thickness and density,
shown very thick in Figure 45, but often much thinner.
So far as could be determined, the mucous membrane has the same
structure in both anterior and middle regions of the stomach. That of
the pyloric or small region, although fixed, stained, et cetera, just
as carefully as the rest, did not show cell details sufficiently well
to draw; the ducts of the glands in this region are fairly distinct but
the deeper parts of the glands have the appearance of series of alveoli
or large adipose cells. What the significance of this condition may be
the writer is not able to say, but since the structure of this region
of the gastric mucous membrane is not clear no attempt will be made to
describe its appearance under higher magnification than was employed in
the figure above. However, as will be noted below, there is probably
no great difference between the pyloric mucosa and that of the other
regions of the stomach.
[Illustration:
FIG. 46. The glands of the middle or fundic region of the stomach of
the hibernating animal, under high magnification; _A_, through duct;
_B_, through body of gland; _C_, through fundus of gland.]
Figure 46 shows portions of typical glands from the mucosa of the
middle region of the stomach, the posterior border of the large stomach
cavity; _A_ is a longitudinal section through two ducts where they
open to the surface; _B_ is a similar section through the body of a
gland below the region of the duct; _C_ is a transsection through the
bottom or fundus of a gland; all are drawn with a camera under the same
magnification.
As is seen in Figure 45, under low magnification, the duct is about
one third of the entire length of the gland. The lumen of the duct is
fairly wide, that of the body of the gland is reduced to a mere slit,
while that of the fundus is quite wide.
One, two, or possibly more, glands may open to the surface through one
duct, as is shown in Figure 46. There is nothing peculiar about the
epithelium of these glands. Near the opening of the duct the cells are
of a typical columnar character with finely granular cytoplasm, each
with a nucleus at its basal end.
In the deeper parts of the duct the cells become shorter until in the
body of the gland (Fig. 46, _B_) they are cuboidal in outline.
The bodies of the glands are so closely packed together that it is
difficult to pick out an individual tube that will show details clearly
enough to draw with a camera lucida. So far as could be observed all of
the cells of this region of the gland are alike.
The bottom or fundus of the gland, as seen in Figure 46, _C_, is
somewhat enlarged and has a wide lumen. The cells are of the same
general character as in the more distal parts of the gland except that
they are somewhat more columnar or pyramidal than in the body of the
gland. The nuclei of the body and fundus are usually somewhat larger
and more nearly spherical than in the columnar cells of the duct.
The feeding animals from which tissues were taken were considerably
smaller than the hibernating specimen, so that the stomach walls were
proportionately thinner; but, so far as could be discovered, there was
no difference in structure.
The relative thickness of the entire wall in each of the three regions
sectioned was about the same as described above.
As has been said, the mucosa on the pyloric or small region of the
stomach from the hibernating animal was so poorly fixed that its
structure could not be made out. In the feeding stage the mucosa of
this region was as well fixed as any of the other tissues and showed
that its structure is essentially like that shown in Figure 46, except
that the glands are proportionately not quite so long as in the fundic
and cardiac regions, and are somewhat more open--that is, they have
wider lumina; their lining cells are all of one kind and are unchanged
from what was seen in the hibernating condition.
_The Small Intestine._ Three regions of the small intestine will be
described: (1) an anterior, just caudad to the stomach; (2) a middle;
and (3) a posterior, one half inch cephalad to the rectum or large
intestine (Fig. 35).
As might be expected, the general structure of the wall of the
intestine is essentially the same in all three regions, the slight
differences noticeable being due mainly to variations in the thickness
of the various layers.
The middle and posterior regions have about the same diameter, while
the diameter of the anterior region is considerably greater, due
partly to the greater diameter of the lumen but mainly to the greater
thickness of the constituent layers, especially the mucosa. The mucosa
is also thrown into more numerous and complicated folds in the anterior
than in the middle and posterior regions; the complexity of the mucosa
seems to diminish as the intestine is followed caudad. In the anterior
region the mucosa may form at least one half of the entire thickness of
the wall, while in the posterior region it may form less than one third
of the thickness of the intestinal wall. The minute structure of the
intestinal epithelium will be described below.
The chief peculiarity of the intestinal wall is the apparent total
absence of a submucosa (Fig. 47). As will be described later, the
mucosal epithelium is laid upon the usual bed of fibrous and lymphatic
tissue, the tunica propria (Fig. 47, _tp_).
At the outer border of the tunica propria, and with no tissue
corresponding to a submucosa between it and the circular muscular
layer, is a thin and indistinct layer that has the appearance of a
longitudinal layer of muscle fibers; this should correspond to the
muscularis mucosa (Figs. 47, 48, 49, and 51, _mm_).
[Illustration:
FIG. 47. A transsection of the wall of the anterior region of the
small intestine of the hibernating animal, under low magnification;
_ln_, lymph node; _tp_, tunica propria; other letters as in Figure
42.]
The circular, _cm_, and longitudinal, _lm_, muscle layers are compact,
and are distinct from the other layers of the wall; the former is
approximately twice the thickness of the latter. The relative thickness
of all the layers in the three regions of the intestine may be seen by
comparing Figures 47, 48, and 49.
[Illustration:
FIG. 48. An outline of a transsection of the wall of the middle
region of the small intestine of the hibernating animal, under low
magnification; lettering as in Figure 42.]
The serosa, _s_, which is of about the same character in the three
regions under discussion, is a distinct and fairly dense layer of
connective tissue with numerous blood-vessels.
The general appearance of the mucous membrane as a whole is
sufficiently clear in the low-power drawing described above, so that
all that need be shown under a higher magnification is the epithelium
(Fig. 50). The upper part of this figure represents the lower end of
one of the intestinal glands cut longitudinally, below which is the end
of another gland in transverse section. Between the two sections is the
compact tunica propria of lymphatic tissue.
The section from which this particular figure was drawn was in the
anterior region, but the corresponding part of a section in either of
the other regions would have practically the same appearance.
The epithelium is of the stratified columnar type. The superficial
cells are very tall and narrow, with the nuclei generally at or near
the bases, though an occasional nucleus may be seen near the free end
of a cell. Below the tall columnar cells are four or five rows of
nuclei which represent smaller, irregular cells, though the cell walls
could not always be determined between the closely packed nuclei. No
goblet cells are to be seen at any place.
[Illustration:
FIG. 49. An outline of a transsection through the wall of the
posterior region of the small intestine of the hibernating animal,
under low magnification; lettering as in Figure 42.]
The relative diameters of the three regions of the small intestine in
the feeding condition are about the same as noted for the hibernating
stage; that is, the anterior region has the greatest diameter and the
other regions are smaller and have about the same average diameter.
The most marked difference between the intestine during hibernation and
feeding is in the relative thickness of the mucosa and muscular layers.
As described for the hibernating stage, so in the feeding stage,
the mucosa is relatively the thickest in the anterior regions and
diminishes in thickness caudad; but while, in the hibernating stage, it
forms, in the anterior region, as much as half of the entire thickness
of the wall, in the feeding condition it forms, in the same region, at
least two thirds of the entire wall and in the middle and posterior
regions more than half of the wall.
[Illustration:
FIG. 50. Part of the mucous membrane of the anterior region of the
small intestine of the hibernating animal, under high magnification.
The upper part of the figure shows a part of a gland cut
longitudinally, the lower part of the figure shows another gland cut
transversely; _e_, epithelium; _tp_, tunica propria.]
The feeding animals being the smaller, the diameter of the intestine
was considerably less than in the hibernating stage; but the actual
thickness of the mucosa was practically the same, so that the
difference in diameter was due to the difference in the thickness of
the muscular and fibrous layers. It is therefore probable that the
differences noted above are due rather to the differences in the size
of the animals from which the tissues were taken than to the different
conditions of hibernation and feeding. The point to be noticed is
that the increase in the diameter of the intestine is due almost if
not entirely to an increase in thickness of the connective tissue and
muscle layers.
No difference in the complexity of the folds of the mucosa of the two
stages can be noticed.
The thickness of the fibro-muscular part of the wall of the intestine
varies considerably on different sides of the same region, but it
consists of the same layers in about the same relative amounts.
[Illustration:
FIG. 51. An outline of a transsection of the wall of the middle
region of the small intestine of the feeding animal, under low
magnification; _m_, mucosa; other letters as in Figure 42.]
Figure 51 represents in outline the wall of the middle region of the
small intestine during feeding.
The epithelium is of the same thickness in the two stages, and the only
difference in its character that can be seen under a high magnification
is that, in the middle region at least, the nuclei are not crowded so
close together at the basal ends of the cells as in the hibernating
stage but are scattered more towards their free ends.
[Illustration:
FIG. 52. A transsection of the wall of the anterior region of the
rectum or large intestine of the hibernating animal, under low
magnification; _tp_, tunica propria; other letters as in Figure 42.]
Altogether, the differences in microscopic structure between the small
intestine of an alligator at the end of the hibernating period and at
the end of a period of regular feeding are very slight.
_The Large Intestine._ The planes of the two sections studied are shown
in Figure 35; a low-power drawing of the posterior region is shown in
Figure 52. The anterior and posterior regions of the large intestine
do not differ from each other sufficiently to make it worth while
to represent both by drawings. Had an entire section through either
region been drawn it would be seen that the wall is of very different
thickness in different places, as was noted in connection with the
small intestine; the posterior section was drawn where the wall was
thin.
It might be supposed that in the feeding season the fecal matter in the
posterior region of the rectum would stretch the walls sufficiently to
obliterate largely the prominent folds seen in Figure 52, but such does
not seem to be the case. The usual layers of the vertebrate intestine
are present.
[Illustration:
FIG. 53. The epithelium of the anterior region of the rectum of the
hibernating animal, under high magnification; _e_, epithelium; _tp_,
tunica propria.]
The epithelium, shown under high magnification in Figure 53, is of
the same character and thickness throughout, except that as the anal
aperture is approached the columnar epithelium changes into the
stratified variety. It consists of very tall and narrow columnar cells
apparently in one layer, though it is difficult to be sure of this.
With an occasional exception, near the top, all of the nuclei are
arranged in a fairly wide zone below the middle of the epithelium.
The nuclei are oval in shape and lie so close together that it is
difficult, as has been said, to be sure that the cell to which each
belongs extends throughout the entire thickness of the epithelium.
Beneath the epithelium (Fig. 52, _e_) is a dense tunica propria, _tp_,
underlaid, in turn, by the muscularis mucosa, _mm_, and a submucosa,
_sm_, of the usual character, which is thrown into marked folds.
The circular, _cm_, and longitudinal, _lm_, layers are of the usual
character except that they vary more in thickness, as noted above, and
in density than is usually the case.
The serosa, _s_, is comparatively thin and compact in both regions, and
varies somewhat in thickness at different places.
The large intestine of the feeding animal was sectioned in the same
regions as in the hibernating. As has been said, the feeding animals
used were much smaller than the hibernating, so that, as might be
expected, the diameter of the large intestine was much less in the
former than in the latter. Except for this difference in diameter there
was no noticeable difference between the two stages. In the case of the
small intestine, it will be remembered, the greater diameter of the
intestine of the larger animal was mainly due to the greater thickness
of the muscular and connective-tissue layers and not to any increase
in thickness of the mucous membrane. In the large intestine the mucosa
varies in thickness in the animals of different size as do the other
layers of the wall.
The glandular character of the lining of the large intestine seems to
indicate that this region of the intestine must have some digestive or
absorptive function and that it does not act merely as a receptacle
for fecal matter; this makes it all the more strange that there should
not be some change produced in its structure by five or six months of
feeding or of fasting.
_Summary._ The material used in this investigation was taken from
young animals at the end of a feeding period of about five months, and
towards the end of the hibernating period after fasting for four or
five months.
The regions of the enteron that were studied were as follows: the tip
and base of the tongue; the anterior and posterior regions of the roof
of the mouth; the anterior and posterior regions of the œsophagus; the
cardiac, fundic, and pyloric regions of the stomach; the anterior,
middle, and posterior regions of the small intestine; the anterior and
posterior regions of the large intestine. Since the work was started
at the end of the hibernating period, the tissues of that period were
studied and drawn first.
The only difference between the structure of the tip of the tongue
during hibernation and during the feeding season is that the scaly
epithelium with which it is covered is somewhat thicker and more
compact in the former than in the latter condition, though even this
difference may have been due to differences in the ages of the animals
used. The base of the tongue differs from the tip in having a thicker
epithelium and in having compound tubulo-alveolar glands. These glands
in the hibernating animal have many more alveoli than in the feeding
animal, though this, again, may have been due to the difference in age.
The lining of the roof of the mouth is essentially the same as that
of the tongue. The glands are found only in the posterior region. The
slight differences in the papillæ here found may easily be due to the
difference in age.
The œsophagus shows the usual layers for that region. Its epithelium
is partly ciliated in the anterior part. The muscularis mucosa is very
scant in the anterior region. The only difference between the two
stages is that in the feeding the muscularis mucosa in the anterior
region is much more strongly developed than in the hibernating stage;
and in the former the nuclei of the epithelium are not arranged in two
zones as in the latter.
The stomach has the usual layers, and has essentially the same
structure in the three regions studied, except that the wall in the
fundic region is much the thickest, due mainly to the great thickness
of the middle muscle layer. Only one kind of cell is found in the
gastric glands. No difference is to be noted between the hibernating
and feeding conditions.
The chief peculiarity of the small intestine is the apparent entire
absence of the submucosa. Goblet cells are also wanting. The greater
diameter of the anterior region is due both to the greater diameter
of the lumen and to the greater thickness of the walls. The middle
and posterior regions have about the same diameter, though the mucosa
becomes thinner and less complicated caudad. There is practically no
difference between the hibernating and feeding stages.
The anterior and posterior regions of the large intestine have
essentially the same structure. No difference can be seen between the
hibernating and feeding conditions.
CHAPTER VI
THE UROGENITAL ORGANS
Figure 54 represents the urogenital apparatus of a thirty-inch
female specimen of _Alligator mississippiensis_. Figure 55 shows the
corresponding organs of a male _A. lucius_; reproduced from Bronn.
The urogenital organs in the young animal are so similar in the two
sexes that one might easily be mistaken for the other; of course in
sexually mature animals, especially during the breeding season, this is
not the case.
The _kidneys_, Fig. 54, k, Fig. 55, a, are flattened, lobulated organs
lying against the dorsal body wall. The large anterior lobe of each
kidney is pointed at its anterior end and lies at some little distance
from its fellow; it is partially divided into secondary lobes and
is traversed on its ventral surface by branching blood-vessels. Its
antero-medial border is sometimes partially concealed, in a ventral
view, by the elongated gonad of that side. Caudad to the main lobe of
the kidney is a smaller, usually distinct, lobe in contact mesially
with its fellow of the opposite side.
A fairly wide _ureter_, Fig. 54, u, Fig. 55, d, extends from the
posterior end of each kidney to open (Fig. 54, u¹, Fig. 55, e) into
the anterior region of the cloaca, as described in connection with the
digestive system.
The _ovary_, Fig. 54, o, as noted above and as seen in Figs. 54 and 55,
in the young animal is of practically the same shape as the testis. The
ova at this stage are of microscopic size and are hence not visible
to the naked eye. The ovary, even at this stage, is more or less
distinctly marked off into lobules by a series of small grooves.
[Illustration: FIG. 54. FEMALE UROGENITAL SYSTEM.
_f_, oviduct; _f¹_, opening of oviduct; _k_, kidney; _m_, mesentery;
_o_, ovary; _u_, ureter; _u¹_, opening of ureter.]
The _oviduct_, Fig. 54, f, which at this stage is, of course, of small
diameter, extends across the ventral surface of its corresponding
kidney and opens, f¹, into the posterior part of the cloaca as has
already been described. Its peritoneal opening is some distance
cephalad to the head of the ovary. Its course from this opening is
straight until about the anterior end of the ovary; it then becomes
somewhat convoluted for a short distance, but gradually straightens
out, to pass to its posterior end as a nearly straight duct. The
anterior straight portion of the oviduct is connected with the head of
the ovary by a narrow band of mesentery.
Each _testis_, Fig. 55, b, like the ovary, lies along the ventro-mesial
border of its corresponding kidney and is connected with the posterior
region of the cloaca by a slender vas deferens, Fig. 55, c, f.
According to Rathke (in _C. acutus_) a small, slender epididymis lies
along the outer side of the posterior half of each testis.
_The Copulatory Organs._ The _penis_, Fig. 56, usually lies completely
hidden in the cloaca; with its glans projecting backwards it is
strongly arched; along the convex side of the arch, which is directed
towards the upper wall of the cloaca, runs a groove, which serves as a
penial urethra to conduct the semen.
According to Rathke there may be recognized in connection with the
penis two fibrous strands (resembling the corpora cavernosa of
mammals), a corpus cavernosus urethrea, and a covering derived from the
mucous membrane of the cloaca. The two fibrous strands arise from the
pubis as two thick plates that soon completely fuse together by their
adjacent sides to form the shaft, c, of the penis. These fused strands
taper gradually towards the glans, in which they end in a point. From
their mode of fusion there is left between them, on the side towards
the upper wall of the cloaca, a fairly deep furrow that extends to the
tip. According to Rathke these shafts are not of cavernous tissue, but
the tube is lined by a layer of this tissue.
[Illustration: FIG. 55. MALE UROGENITAL APPARATUS OF ALLIGATOR LUCIUS.
(After Bronn.)
_a_, kidney; _b_, testis; _c_, vas deferens; _d_, ureter; _e_,
opening of the ureter into the cloaca; _f_, opening of the vas
deferens into the cloaca; _g_, upper region of the cloaca; _h_,
hinder region of the cloaca; _i_, rectum.]
[Illustration: FIG. 56. MALE ORGAN OF ALLIGATOR LUCIUS, XI. (From
Bronn, after Rathke.)
_a_, the right crus penis; _b_, the mucous membrane of the cloaca
that covers the organ; _c_, shaft of the penis; _d_, base of glans;
_e_, point of glans; _f_, part of the ring muscle of the cloaca.]
The _glans_, e, consists of two parts between which, where they leave
the shaft of the penis, is a funnel-shaped hole, wider towards the free
end of the penis and divided into similar lateral halves by a fold
of skin. The glans is much shorter than the shaft of the penis. The
covering of the penis is much thinner than the mucous membrane of the
cloaca and is thinnest along the groove; it extends from the shaft over
the glans without forming a foreskin.
The base of the penis is attached to the pubis near its symphysis.
With this base the most anterior part of the strong ring-muscle of the
cloaca is closely attached by a fairly large mass of fibrous tissue.
Rathke fails to find any muscles that are concerned alone with the
copulatory organs.
In the copulation of the crocodile, according to Rathke, the penis
is erected, though how this is caused is difficult to say since the
corpora cavernosa consist only of fibrous tissue and the cavernous
tissue lining the groove is very thin. The penis can, therefore,
project only a short distance from the cloaca. The cavernous tissue
is capable of causing only a slight elongation of the shaft, but the
glans is considerably elongated by the strong influx of blood into that
structure. According to Voeltzkow (78) the penis in the Madagascar
crocodile is 20 cm. in length.
The clitoris of the sexually mature female crocodile is very much
smaller than the penis of a male of the same size, but, according to
Rathke, it varies greatly in size in different species. It is built on
exactly the same plan as the penis.
According to Bronn the clitoris as well as the penis projects from the
cloaca, out through the anus, in the embryo of the crocodile; this
was not observed by the present writer in the embryo of the Florida
alligator, _A. mississippiensis_.
CHAPTER VII
THE RESPIRATORY ORGANS
_The Larynx and Trachea._ In the Crocodilia the framework of the larynx
consists of three cartilages, of which two represent the arytenoids of
the Mammalia; the third represents the thyroid and cricoid of mammals.
The last is considerably larger than the first and is a broad closed
ring, differing in form in the different species. In spite of the fact
that some of them have a voice, the vocal cords, according to Bronn,
are wanting in the Crocodilia. According to Henle the vocal apparatus
is produced by the projection into the laryngeal cavity of the inner
border of the small arytenoid cartilages and by the infolding, under
these cartilages, of the mucous membrane of the larynx; this forms the
thick but fairly free folds that, when the glottis is narrowed, are
well adapted to produce the harsh tone of the animal.
The epiglottis is absent in the Crocodilia.
In many Crocodilia (_C. vulgaris_, for example) the trachea, Fig. 57,
tr, forms a loop which begins in some species before hatching, in other
species not until long after hatching. In the genus _Alligator_ the
trachea is straight.
More universal than this looped structure there is found another
peculiar structure in the crocodilian trachea. It is a short vertical
partition in the stem just before its division into the two branches.
This partition is partly membranous and possesses one or more
stiffening cartilaginous strands which are outgrowths of so many
cartilaginous rings of the trachea. The number of the stiffening fibers
varies in the different species.
[Illustration: FIG. 57. RESPIRATORY ORGANS.
_b_, bronchus; _e_, œsophagus; _g_, glottis; _l_, lung; _t_, tongue;
_tr_, trachea.]
The number of the tracheal rings varies not only in the different
species but also in different individuals of the same species. There
are between fifty and sixty in _A. mississippiensis_. According to
Rathke the number of rings in the individual animal almost certainly
does not increase with age. The number of rings is smallest in the
gavials and greatest in the crocodiles (genus _Crocodilus_). The number
of rings in the two divisions of the trachea does not increase with
age except, perhaps, in _C. acutus_ and _biporcatus_. The lateral bend
that the tracheal stem of so many Crocodilia exhibits is not due to the
greater number of rings because in some species (gavials) where the
bend is present the number of rings is smaller than in the Crocodilia
where the bend is absent.
According to Rathke and others most of the tracheal rings are closed,
but a varying, though at most small, number are open on the dorsal
side. These openings become wider as the larynx is approached. The
transverse muscle fibers which are found in the most anterior and
largest of these breaks in the tracheal rings were found, says Rathke,
in embryos after the middle period of incubation.
The cartilaginous rings of the bronchi, b, are also apparently open
for a time after their formation, but soon close. Not infrequently in
embryos and in young animals are found rings that are split like a
fork, with one or both branches fused with neighboring branches.
_The Lungs._ The lungs, Fig. 57, l, are more highly developed among
the Crocodilia than among any other Saurian or Hydrosaurian group. The
histological groundwork of the whole lung tissue is a connective tissue
of fine elastic fibers. In the lungs, on the canal that appears as the
elongation of the bronchus, cartilage appears, according to Rathke, as
bands lying one behind the other; some of these bands form complete,
others partial rings; some of the latter are forked. The hindermost
appear to be the broadest and most irregular. Their number is different
in different species and varies in different individuals of the same
species. They range in number, according to Rathke, from nine, in _A.
lucius_, to twenty-five, in _C. acutus_.[7]
[7] Miller (45c) says: “In the crocodile and alligator the bronchus
enters the lung near its center, and passes somewhat obliquely into
the lung until it reaches the junction of the lower middle third; here
it breaks up into eight to fifteen tubular passages. These tubular
passages are studded with a great many air-sacs. In these animals the
lung for the first time gives the structure as it is found in mammals.
There are many air-sacs which communicate with a common cavity, or
atrium, all of which in turn communicate with a single terminal
bronchus. A single lobule of the mammalian lung is simply enlarged
to form the lung of the crocodile; the lung of the former is only a
conglomerate of that of the latter.”
The arterial branch, carrying venous blood to the lungs, develops a
capillary network close to the alveolar walls, which leads away over
the low alveolar septa, while over the tops of the higher septa and
on the inner surface of the tube-like bronchial processes it forms a
wide-meshed network of capillaries that are apparently chiefly nutrient.
All the respiratory capillaries are attached by only one side to the
alveolar wall; the free side that projects into the air space of the
alveolus is covered by a continuous pavement epithelium.
While the respiratory surfaces are covered with an alveolar epithelium
of large polygonal cells, the free borders of all high septa and
ridges, as well as the inner surfaces of the bronchial processes, are
covered with ciliated cylindrical cells.
CHAPTER VIII
THE VASCULAR SYSTEM
The account given by Bronn in his _Thierreich_ is apparently the only
published description of the circulatory organs in the Crocodilia. This
account, even when translated, is not very satisfactory, especially
because it contains no diagrams of the circulation. It was, therefore,
deemed worth while to work out the circulation in the Florida alligator
in order that we might have not only a written description, but also a
series of more or less accurate diagrams of the veins and arteries.
A number of departures from the description of Bronn were found, some
of which are noted below.
Most of the work was done upon animals of about thirty inches length,
which were obtained alive from the Arkansas Alligator Farm at Hot
Springs, Ark.
The arteries were injected with a colored starch mass by inserting
a two-way cannula into the dorsal aorta. With the blood thus forced
into them from the arteries, the veins could, in most cases, be traced
without difficulty.
In the diagrams the outlines of the more important organs are
accurately shown by dotted lines, and the relative diameters of the
blood-vessels are shown as accurately as possible by the solid black
lines.
THE HEART
In the Crocodilia, as is well known, the heart is four-chambered and
has about the same general shape as in the higher vertebrates, Fig. 58.
The venous blood is emptied into a thin-walled sinus venosus on the
dorsal side of the heart by three large vessels and one small one. The
largest of these, the postcava, empties into the posterior side of the
sinus venosus and brings blood from the posterior regions of the body;
it is quite wide, but is exposed for a very short distance between the
liver and the heart. Two large hepatic veins empty into the postcava
so near the sinus venosus that they practically have openings into the
sinus, as is shown in a somewhat exaggerated way in Fig. 59. Near the
postcaval and hepatic openings is the distinct coronary vein, lying in
a slight depression between the right and left ventricles.
From the anterior regions of the body the blood is brought back through
two fairly wide but very thin-walled precaval veins which pass across
the dorsal surface of the heart to enter the sinus venosus.
[Illustration: FIG. 58. HEART OF A. LUCIUS. (Dorsal View.)
(From Bronn, after Fritsch.)
A_o_, _d_, _Ao_, _s_, right and left aortæ; _At_, _d_, _At_, _s_,
right and left atria; _Au_, _d_, _Au_, _s_, right and left auricles;
_Ca_, _pr_, primary carotid; _P_, _d_, _P_, _s_, right and left
pulmonary arteries; _Sc_, _d_, _Sc_, _s_, right and left subclavians;
_Sv_, sinus venosus; _V_, _cc_, coronary vein; _V_, _c_, _d_, _V_,
_c_, _i_, right _precava_, and inferior cava; _Ven_, _d_, _Ven_, _s_,
right and left ventricles; _V_, _h_, hepatic vein; _V_, _p_, _d_,
_V_, _p_, _s_, right and left pulmonary veins.]
The arterial blood is brought from the lungs by two wide, thin-walled
pulmonary veins, Fig. 58, V.p.s., V.p.d. They leave the lungs somewhat
caudad to their middle region, near the point of entrance of the
bronchii and the pulmonary arteries, pass mediad in a direction almost
at right angles to the long axis of the body, and enter the left
auricle at the same point.
Blood leaves the heart through five large vessels: (1) the pulmonary
artery, (2) the two aortic arches, (3) the right subclavian, (4) the
primary carotid.
The pulmonary leaves the small right ventricle as a single stem, which
soon branches into two arteries that pass cephalad and laterad to the
lungs, along with and close to the main bronchi. The other arteries
that carry blood into the systemic circulation are fused at their base
to form a sort of conus arteriosus which may be distended in injected
specimens until it is larger than the two ventricles together. When
opened this conus is found to contain two chambers that lead into the
left ventricle; the larger chamber gives origin to the right systemic
arch, the right subclavian, and the primary carotid; the smaller
chamber is the basal part of the left systemic arch.
The two systemic vessels, Fig. 58, Ao.s, Ao.d, pass, in the usual
manner, as two arches to the dorsal region, just posterior to the
ventricles, where they form the dorsal aorta in the manner to be
described in connection with the arterial system.
The further course of the primary carotid and of the right subclavian
will also be described in connection with the arterial system.
The auricles are very large in proportion to the ventricles, though
their relative sizes will, of course, vary with the amount of contained
blood.
[Illustration:
FIG. 59. The veins of the posterior region of the Florida alligator.
The postcaval system and its associated veins are shown in the main
figure; the hepatic portal system is shown in the smaller figure to
the left.--For lettering, see pages 224-25.]
THE VENOUS SYSTEM
_The Posterior Vena Cava and its Branches._ The _postcava_, Fig. 59,
pc, as noted above, is a wide, thin-walled vessel seen extending
across the short space between the anterior face of the right lobe of
the liver and the sinus venosus. As was also noted above, the hepatic
veins, vh,--at any rate that from the left lobe of the liver,--enter
the postcava so close to the heart that they may be considered to have
one or more distinct openings into the sinus venosus. Followed caudad,
the postcava may be traced through the large right lobe of the liver,
from which it receives several branches. Emerging from the posterior
border of the liver, it is seen to extend caudad, in the median
line, as a rather inconspicuous vessel that receives blood from the
reproductive organs and the kidneys that lie close on either side of it.
The _hepatic portal_ vein, h, has the usual distribution for that
vessel. Entering the liver in the neighborhood of the bile duct, it
receives first (_i.e._, nearest the liver) a small branch from the
pancreas, pv; near the pancreatic are one or two branches from the
stomach, g, and a branch from the spleen, sp. A short distance caudad
to these vessels are two or three mesenteric veins, m, leading from the
mesentery and small intestine. Caudad to the mesenterics, the portal
system may be seen as a vein of diminished caliber, i, leading from the
posterior part of the small intestine and from the large intestine.
The connection mentioned by Bronn between the rectal branch of the
portal vein and the caudal vein could not be demonstrated. After
entering the liver, the portal, of course, breaks up into capillaries,
and the blood thus distributed is re-collected by the capillaries of
the hepatic veins above mentioned.
The _internal epigastric_ veins, ep, are, perhaps, the most conspicuous
vessels of the postcaval system. When the ventral abdominal wall of the
animal is removed, they may be seen extending forward from the pelvic
region, on each side of the body, to enter the posterior edge of the
liver. The epigastric of the right side enters the large or right lobe
of the liver, where it breaks up into capillaries; the left epigastric
sends its main branch into the left lobe of the liver, but also sends a
branch over to enter the right lobe.
Following the epigastrics caudad, they are seen to receive vessels
from nearly all parts of the posterior region of the body. The left
epigastric, which extends across the ventral side of the stomach,
receives from that organ four or five branches, g¹; while the farther
removed right epigastric receives only one or two branches from the
stomach. Posterior to these gastric veins the epigastrics receive one
or more veins, b, from the body wall and skin. Posterior, again, to
the last-named veins, each epigastric receives, in the pelvic region,
a large vein, the _iliac_, il, which receives, in turn, a vein from
the pelvis, pl, and continues down the thigh and lower leg to the foot
as the _femoral_, f, the chief vein of the posterior appendage. After
receiving small branches from the muscles of the thigh, the femoral
receives near the knee a small branch from the posterior surface of
the lower leg, fb, and a larger one, t, that leads from the anterior
surface of the lower leg and foot.
The veins of the pes were so small, in the comparatively small
animals it was necessary to use, that their distribution could not be
determined with certainty, though they seemed to parallel very closely
their corresponding arteries to be described below.
A short distance caudad to the iliac veins, each epigastric receives
one or two fairly large branches from the pelvic region, called by
Bronn the _ischiadic_ veins, is. Caudad to the ischiadics and dorsal
to the cloaca, each epigastric is united with a short but wide _renal
portal_ or renal advehente vein, rp, leading to the posterior border of
its respective kidney and receiving, on the way, a short branch from
the pelvic region, shown just cephalad to the reference lines rt and rp.
Very close to its junction with the renal portals each epigastric gives
off a small branch which unites with its fellow of the opposite side to
form a median vein, rt, the _rectal_ leading from the posterior part
of the large intestine. A very short distance caudal to these last
veins, in the region just dorsal to the anal opening, the epigastrics
are formed by the division of the _caudal_ vein, cv, which, of course,
brings blood from the tail and is, on account of the large size of
that organ, of considerable caliber.
_The Anterior Venæ Cavæ and their Branches._ The entrance of the
precaval veins into the heart was mentioned above; their branches, in
order from the heart cephalad, will now be described. Since the two
precavæ are alike, it will be necessary to describe the branches of
only one side of the body. After leaving the heart, the precava may
be traced forward, for a short distance, at the side of the trachea
and œsophagus, as a wide, thin-walled trunk, Fig. 60, vca. The first
tributaries that it receives are the internal mammary and vertebral
veins, which join it at the base of the neck at almost the same place.
The _internal mammary_, Fig. 60, im, is a rather small vein, bringing
blood from the ventral wall of the thorax. It may be followed along
the inner surface of the ribs, near the sternum, in company with its
corresponding artery.
The _vertebral_ vein, Fig. 60, v, is also of small diameter and extends
to the dorsal body wall near the spinal column, from which region it
returns blood to the anterior vena cava; it is drawn too large in the
figure.
[Illustration: FIG. 60. The veins of the anterior region of the
Florida alligator. The veins of the left foreleg are shown at A.--For
lettering, see pages 224-25.]
Just cephalad to the vertebral and internal mammary, the _internal
jugular_, j, enters the precava. The internal jugular may be followed
directly forward, close to the side of the trachea and œsophagus,
from which it receives numerous branches. Near its point of entrance
to, or rather exit from, the skull, it anastomoses, by two or three
short branches, with the external jugular, ej, to be described later.
Its distribution in the cranial cavity could not be determined in the
available material. At the point of entry of the internal jugular the
precava passes laterad for a short distance and then divides into two
more or less equal branches, the above-mentioned external jugular, ej,
and the subclavian, s, of which the latter will first be described.
The _subclavian_, s, of course, returns blood from the regions of
the shoulder and arm. On reaching the body wall, where it might be
called the _axillary_, ax, it receives, on its posterior side, a large
_thoracic_ vein, t, which returns blood from the thorax, shoulder, and
skin. The thoracic receives a branch from the posterior surface of the
arm, which might be called the _postbrachial_, pb; this postbrachial
may be traced, as a rather small vessel, to the hand; at the elbow it
is connected, by one or more small branches, with the brachial.
Just distal to the thoracic the axillary vein receives two fairly large
vessels, the _subscapulars_, sc, that return blood from the shoulder
and upper arm. After receiving the subscapulars, the axillary may be
followed into the upper arm as the _brachial_, br. As has been said,
the brachial and postbrachial anastomose near the elbow, and in this
region the former receives a small vessel that extends parallel to it
from the manus.
In the forearm the brachial may be called the _radial_, Fig. 60, A, ra;
on the back of the manus the radial receives branches from the various
digits and from a rather complex plexus of vessels in the carpal region.
The _external jugular_, Fig. 60, ej, after separating from the
subclavian, may be traced cephalad, close beneath the skin, to the base
of the skull, where it is connected with the internal jugular by short
branches, as has already been noted. It receives several small branches
from the skin and muscles of the neck and shoulder regions. At the
region of its anastomosis with the internal jugular it receives a large
branch, the _muscular_, ms, from the massive muscle at the angle of the
jaw and from the skin of that region.
A short distance cephalad to the muscular the external jugular
receives, on its mesial side, two or three branches from the trachea,
larynx, and œsophagus, tr. Anterior to these vessels the external
jugular is formed by the union of two chief veins, the _lingual_,
l, from the ventro-lateral surface of the tongue, and the _inferior
dental_, id, from the mesial surface of the lower jaw. The connection
of the _superior dental_ (extending along the bases of the maxillary
teeth) with the jugular could not be determined with certainty, hence
that vessel is not shown in the figure. The same is true of the small
veins in the region of the cranium.
THE ARTERIAL SYSTEM
_The Abdominal Aorta and its Branches._ The right and left aortic
arches, Fig. 61, Aod, Aos, arising from the heart in the manner already
described, form a rather long loop and approach each other in the
middorsal line. Here they are united by a short, wide connective in
such a way that the left arch seems continued into the cœliac artery
and the right into the dorsal aorta proper. Each arch, anterior to the
connective, gives off two fairly large branches, oe, to the posterior
region of the œsophagus.
The _cœliac_ artery, Fig. 61, c, is the largest branch of the
abdominal aortic system. After giving off a couple of small branches,
oe, to the posterior region of the œsophagus, it gives off a large
_spleno-intestinal_ artery, si, to the spleen and small intestine.
The cœliac then breaks up into three arteries of about the same size:
the _gastro-hepatico-intestinal_, ghi, carrying blood to the stomach,
liver, and small intestine; the _pancreo-intestinal_, pi, leading to
the pancreas and small intestine; and the _gastric_, ga, to the greater
part of the stomach.
From the _dorsal aorta_ proper, da, which, as has been said, seems
to be the direct continuation of the right aortic arch, several
arteries are given off; these will be described as they occur in an
antero-posterior direction.
At about the point of union of the two aortic arches arises the most
anterior of seven or eight pairs of _lumbar_ arteries, lu 1-7; this
first lumbar artery is continued cephalad for some distance as a
longitudinal trunk that gives off several lateral branches to the walls
of the thoracic region. The other six or seven lumbars are distributed
to the dorsal body wall, and arise, at more or less regular intervals,
as far caudad as the sacrum, or even back of that point.
[Illustration:
FIG. 61. The arteries of the posterior region of the Florida
alligator.--For lettering, see pages 224-25.]
The first large branch of the aorta is the unpaired _mesenteric_
artery, m¹, which is given off in about the region of the fourth pair
of lumbars; it carries blood through the mesentery to the greater
part of the small intestine and also sends a small branch to the large
intestine.
Posterior to the mesenteric, the aorta gives off four or five pairs of
short arteries, the _urogenitals_, _u 1-4_, that lead to the nearby
reproductive organs and kidneys.
About the middle region of the kidneys, a short distance anterior to
the sacrum, is given off a pair of rather large arteries, called by
Bronn the _ischiadicæ_, is¹; each ischiadica, after giving off a couple
of small branches to the back, passes laterad and divides into three
main branches: (1¹) to the ventral body wall, (3¹) to the anterior
border and deeper region of the thigh, and (2¹) to the pelvis.
In the region of the sacrum is given off a pair of _iliac_ arteries,
il¹. Each iliac is of about the same diameter as the ischiadica and
gives off, soon after leaving the aorta, an artery, ab, that apparently
leads chiefly to the abdominal muscles. Distal to the origin of the
abdominal, the iliac gives off a small _pelvic_ artery, pa, which
leads, as the name would indicate, to the pelvis. The iliac then passes
into the thigh, where it gives off several large branches and may be
called the _sciatic_, sc. At the knee the sciatic gives off two rather
small branches: one, the _fibular_ artery, f¹, extends down along the
posterior side of the lower leg; the other is parallel to the first
and may be called the _tibial_ artery, tb, since it extends along the
anterior or tibial side of the shank. These two arteries give off
numerous branches to the muscles of the lower leg. After giving off the
fibular and tibial arteries, the sciatic passes, as a large vessel,
through the lower leg, to which it gives but few branches, and may here
be called the _crural_ artery, cr. At the tarsus it divides rather
suddenly and, perhaps, variably, into four chief branches, leading to
the toes.
[Illustration:
FIG. 62. The arteries of the anterior region of the Florida
alligator. The arteries of the left foreleg shown at A.--For
lettering, see pages 224-25.]
A short distance caudad to the origin of the iliacs the dorsal aorta
gives off a pair of small _pelvic_ arteries, pa¹, going to the
muscles of that region. Caudal to these pelvic arteries is given off
the unpaired _first hæmorrhoidal artery_, he¹, which divides into a
_rectal_, rt¹, and a _cloacal_, cl, branch.
Caudal to the first hæmorrhoidal arises the _second hæmorrhoidal_, he²;
also unpaired, leading to the cloaca.
Posterior to the second hæmorrhoidal, the aorta continues into the tail
as the large _caudal_ artery, ca.
_The Anterior Arteries._ The origin of the great arterial trunks--the
pulmonary, aortic arches, primary carotid, and right subclavian has
already been given and the distribution of the pulmonary arteries and
aortic arches has been described, so that it now remains to describe
the distribution of the right subclavian, Fig. 62, sc.d., and the
primary carotid, capr.
The _right subclavian_, sc.d., since it has an independent origin from
the heart, instead of arising as a branch of the primary carotid,
will be described first. After leaving the heart it passes cephalad
and laterad and gives off the following branches in order, beginning
at the heart: an _œsophageal_ artery, oe, a small, caudally directed
vessel carrying blood to the posterior region of the œsophagus. Close
to the œsophageal arises another small, caudally directed vessel, the
_pleural_ artery, plu, extending to the pleura and possibly to the
pericardium. From the same region as the preceding two arteries, but
extending cephalad along the trachea and œsophagus, arises the much
larger branch of the right subclavian, the right _collateralis colli_,
cc, whose course and distribution will be described later.
Close to the distal side of the collateralis colli arises the very
small _thyroid_ artery, th, leading to the oval thyroid gland that lies
against the ventral surface of the trachea a short distance anterior to
the heart.
A short distance distal to the thyroid artery the subclavian gives off
a fairly large artery, the _internal mammary_, im¹ (shown too large
in the figure), that passes to the inner surface of the ribs near the
sternum and lies parallel to the vein of the same name, described above.
A short distance distal to the internal mammary arises an artery of
about the same diameter, the _vertebral_, v¹; it passes dorsad and
caudad to the region of the thoracic vertebræ.
After giving off the five vessels just described, the subclavian artery
passes into the shoulder where it divides into three main branches: (a)
the _subscapular_, sc¹, going to the skin and muscles of the shoulder;
(b) the _thoracic_, t¹, carrying blood to the posterior muscles of
the shoulder and to the posterior region of the upper arm; (c) the
_brachial_, br¹, which is really the continuation of the subclavian and
is the chief artery of the anterior appendage.
After sending several branches to the upper arm the brachial divides,
in the region of the elbow, into two main vessels, the _radial_, ra¹,
and _ulnar_, ul¹, arteries, Fig. 62, A. The radial artery, in the
carpal region, divides in a complicated way into five main vessels that
extend into the digits. The ulnar artery gives off several branches to
the forearm, but apparently does not connect directly with the branches
to the digits.
The _primary carotid_, capr. After leaving the heart, this very large
vessel passes cephalad and laterad for some distance on the left side
of the body and then gives off, from its anterior side, the large left
subclavian artery, sc.s., to be described later. After giving off the
subclavian artery, it makes a short loop, still farther to the left,
and then turns sharply mediad to pass to the head in the median plane
directly dorsal to the œsophagus. Its distribution in the cervical and
cephalic region will be described later. The mate to the œsophageal
branch, oe (near heart), of the right subclavian which was mentioned
above is apparently sometimes given off from the primary carotid near
its base (as shown in Fig. 62) and sometimes as a branch of the left
pleural artery.
The left _subclavian_ artery, sc.s., although it has a different
origin, has the same branches as described in connection with the right
subclavian. The exact order in which the first of these (the thyroid,
th; the internal mammary, im¹; the collateralis colli, cc; the pleural,
plu; and the vertebral, v¹) are given off is, as might be expected,
subject to some variation.
The _collateralis colli_, cc (following Bronn’s nomenclature), whose
origin was noted above, will now be discussed; since the two are alike
only one need be described. After leaving the subclavian, it passes
cephalad, at the side of the trachea and œsophagus, in company with the
internal jugular vein, so that in this part of its course it would seem
to be the internal carotid artery. It gives numerous small twigs to the
trachea and œsophagus, oe. In the region of the posterior part of the
huge jaw muscle it is connected directly, x, with the adjacent branch,
cm (called by Bronn the common carotid) of the primary carotid, and
indirectly, x¹, with a complicated group of branches from the common
carotid. Cephalad to the connective x¹, which extends dorsad and is
hence foreshortened in the figure, the collateralis colli gives off a
small vessel, y (too large in Fig. 62), to the shoulder and skin; it
then sends a fairly large branch, jm, into the large jaw muscle, close
to which it now lies. Next a small branch, lg, is sent to the larynx.
Continuing cephalad and laterad (in Fig. 62 it is drawn farther to the
side than it actually lies) for a short distance farther, it divides
into three branches: (1) a short twig, mg, that goes to the musk
gland on the side of the mandible and to the skin of that region; (2)
a large branch, the _mandibular_, md, that enters the large foramen
on the mesial side of the mandible and extends in the cavity of that
bone throughout its entire length; (3) the _lingual_ artery, l¹,
which in turn divides, some distance cephalad, into two branches, one
extending along the lateral region, the other nearer the mid-ventral
surface of the tongue. It is seen, then, that the collateralis colli
arteries supply directly the lower side of the head--tongue, mandible,
etc.--though they may also send blood through the above-mentioned
connectives to the brain and dorsal regions of the skull.
The _primary carotid_, capr, as was noted above, makes a curve to the
left after leaving the heart and then passes back to the median plane,
where it may be seen lying against the ventral side of the neck muscles
and dorsal to the œsophagus; in this place it gives off a series
of unpaired _cervical_ arteries, Fig. 62, ce, each of which almost
immediately divides into an anterior and a posterior branch, that carry
blood to the cervical vertebræ. At the base of the skull, in the region
where it is united by the first connective, x, with the collateralis
colli, as described above, the primary carotid divides into two similar
branches, called by Bronn the _common carotids_, cm. The distribution
of these two vessels is symmetrical, so that only one need be
described. While the collateralis colli, as has been said, carry blood
chiefly to the tongue and lower jaw, the common carotids supply the
cranium and upper jaw.
Soon after its formation by the division of the primary carotid, the
common carotid is joined, as noted above, with the collateralis colli
of that side by the connective, x; since the common carotid and its
branches all lie dorsal to the collateralis colli and its branches,
the connectives x and x¹ extend in a more or less dorso-ventral
direction. The two common carotids, almost completely surrounded by
bone, in passing cephalad sweep first laterad, then mediad, so that
they together form almost a complete ellipse, as seen in Fig. 62; there
is, however, no apparent connection between them at the anterior region
where they lie so close together.
A short distance cephalad to the connective x the common carotid is
connected laterally, z, with a rather complicated plexus of vessels
lying at the base of the skull; it is through this plexus that the
common carotid is connected with the collateralis colli by the second
connective, x¹.
The short branch z quickly divides into three parts: (1) a small
anteriorly directed vessel which may be called the _internal carotid_,
ic, since it enters the skull through the most ventral of the three
foramina in the exoccipital, and probably supplies the brain, though
its further course could not be followed; (2) a somewhat larger
posteriorly directed artery, oc, going to the muscles at the occipital
region of the skull; (3) a short laterally directed stem, z¹. The
last-named branch, z¹, in turn, leads in three directions: (a) to
the collateralis colli artery through the connective x¹; (b) a short
anteriorly directed vessel, e, that passes into the skull, possibly to
the ear, through the large foramen that lies between the exoccipital
and quadrate bones; it gives off a small twig, q, to the muscles in the
region of the jaw articulation (quadrate); (c) the main stem of the
branch z continues laterad and cephalad as one of the chief arteries,
z², to the anterior region of the skull, giving off a fairly wide
branch, jm¹, to the large jaw muscle, and then two branches, o¹ and o²,
to the lateral surface of the eyeball and socket; it then anastomoses,
just cephalad and laterad to the eye, with the forward continuation,
cm¹, of the corresponding main stem, cm, of the common carotid, already
mentioned. The vessel cm¹, after almost meeting its fellow in the
middle line, passes cephalad and laterad across the ventral surface of
the eye to the union, above mentioned, with the lateral branch, z²;
at the posterior-mesial border of the eye it gives off a branch that
divides into two twigs, one, o³, for the posterior eye muscles, and
one, e¹ to the region of the ear and the top of the skull.
At the point of union of the branches cm¹ and z² a sort of simple
plexus may be formed from which two vessels, n, pass to the posterior
nasal region, and two vessels pass forward along the side of the upper
jaw. Of the latter two vessels one, which may be called the _inferior
dental_ of the maxilla, dm, is very small and extends along the maxilla
to its very tip, at the base of the teeth and ventral to the palatine
bone; the other, which is larger and may be called the _superior
dental_ of the maxilla, dm¹, extends cephalad along the mesial side of
the maxilla, dorsal to the palatine bone; it sends numerous twigs into
the maxillary bone among the roots of the teeth. After passing nearly
to the end of the snout, the superior dental, dm¹ suddenly forms a loop
towards the median line and passes as a straight branch, n¹, directly
caudad, near and parallel to the median plane. The branch n¹ extends
along the floor of the nasal cavity and, after giving off twigs to this
chamber, ends in a network of vessels, o⁴, on the anterior surface of
the eyeball and socket.
A pair of very small arteries, n², may be seen in the nasal chamber
between and parallel to the branches, n¹; they lie close to each
side of the nasal septum and supply the anterior nasal region. They
apparently arise, as shown by the broken lines, from the loop of the
superior dental artery, dm¹, though this could not be definitely
determined.
LETTERING FOR FIGURES 59-62
Aos., Aod., left and right aortic arches.
ab, abdominal artery.
ax, axillary vein.
b, veins from body wall.
br, brachial vein.
br¹, brachial artery.
c, cœliac artery.
ca, caudal artery.
capr, primary carotid.
cc, collateralis colli artery.
ce, cervical artery.
cl, cloacal artery.
cm, cm¹, common carotid artery.
cr, crural artery.
cv, caudal vein.
da, dorsal aorta.
dm, inferior dental artery of maxilla.
dm¹, superior dental artery of maxilla.
e, e¹, artery into skull, perhaps to ear.
ej, external jugular vein.
ep, internal epigastric vein.
f, femoral vein.
f¹, fibular artery.
fb, fibular vein.
g, gastric vein of portal.
g¹, gastric vein of epigastric.
ga, gastric artery.
ghi, gastro-hepatico-intestinal artery.
h, hepatic portal vein.
he¹, he², hæmorrhoidal arteries.
i, intestinal vein.
ic, internal carotid artery.
id, inferior dental vein.
il, iliac vein.
il¹, iliac artery.
im, internal mammary vein.
im¹, internal mammary artery.
is, ischiadic vein.
is¹, ischiadic artery.
j, internal jugular vein.
jm, jm¹, artery to jaw muscle.
l, lingual vein.
l¹, lingual artery.
lg, laryngeal artery.
lu, 1-7, lumber arteries (numbers on left side of figure).
m, mesenteric vein.
m¹, mesenteric artery.
md, mandibular artery.
mg, artery to musk gland.
ms, muscular vein.
n, artery to posterior nasal region.
n¹, artery to anterior and mid-nasal region.
n², artery to anterior nasal region.
o¹-o⁴, arteries to eye.
oc, artery to muscles at base of skull.
oe, œsophageal arteries.
pa, pelvic artery.
pa¹, second pelvic artery.
pb, post brachial vein.
pc, post cava.
pd, right pulmonary artery.
pi, pancreo-intestinal artery.
pl, pl¹, pelvic vein.
plu, pleural artery.
ps, left pulmonary artery.
q, artery to muscle at angle of jaw.
ra, radial vein.
ra¹, radial artery.
re, reproductive vein or artery.
rp, renal portal vein.
rt, rectal vein.
rt¹, rectal artery.
rv, renal vein.
s, subclavian vein.
sc, sciatic artery (Fig. 61).
sc, subscapular vein (Fig. 60).
sc¹, subscapular artery.
sc.d., sc.s., right and left subclavian arteries.
si, spleno-intestinal artery.
sp, splenic vein.
s.v., sinus venosus.
t, thoracic vein.
t¹, thoracic artery.
tb, tibial artery.
th, thyroid artery.
tr, tracheal vein.
u, 1-4, urogenital arteries (numbers on right side of figure).
ul¹, ulnar artery.
v, vertebral vein.
v¹, vertebral artery.
vca, anterior vena cava.
vh, hepatic vein.
vpd, vps, right and left pulmonary veins.
x, x¹, connectives between collateralis colli and carotid.
y, artery to shoulder and skin.
z, z¹, z², branches of common carotid.
1¹, 2¹, 3¹, branches of ischiadic artery.
CHAPTER IX
THE DEVELOPMENT OF THE ALLIGATOR
(_A. mississippiensis_)
INTRODUCTION
With the exception of S. F. Clarke’s well-known paper, to which
frequent reference will be made, practically no work has been done upon
the development of the American alligator. This is probably due to the
great difficulties experienced in obtaining the necessary embryological
material. Clarke, some twenty years ago, made three trips to the swamps
of Florida in quest of the desired material. The writer has also spent
parts of three summers in the Southern swamps--once in the Everglades,
once among the smaller swamps and lakes of central Florida, and once in
the Okefinokee Swamp. For the first of these expeditions he is indebted
to the Elizabeth Thompson Science Fund; but for the more successful
trip, when most of the material for this work was collected, he is
indebted to the Smithsonian Institution, from which a liberal grant of
money to defray the expenses of the expedition was received.
The writer also desires to express his appreciation of the numerous
courtesies that he has received from Dr. Samuel F. Clarke, especially
for the loan of several excellent series of sections, from which a
number of the earlier stages were drawn.
In preparing the material several kinds of fixation were employed,
but the ordinary corrosive sublimate-acetic mixture gave about the
most satisfactory results. Ten per cent. formalin, Parker’s mixture of
formalin and alcohol, etc., were also used. In all cases the embryos
were stained _in toto_ with borax carmine, and in most cases the
sections were also stained on the slide with Lyon’s blue. This double
stain gave excellent results. Transverse, sagittal, and horizontal
series of sections were made, the youngest embryos being cut into
sections five microns thick, the older stages ten microns or more in
thickness.
THE EGG
FIGURES 1, 1_a_ (PLATE VI.)
The egg (Fig. 1) is a perfect ellipse, the relative lengths of whose
axes vary considerably in the eggs of different nests and slightly in
the eggs of the same nest. Of more than four hundred eggs measured, the
longest was 85 mm.; the shortest 65 mm. Of the same eggs, the greatest
short diameter was 50 mm.; the least short diameter was 38 mm. The
average long diameter of these four hundred eggs was 73.74 mm.; the
average short diameter was 42.59 mm. The average variation in the long
axis of the eggs of any one nest was 11.32 mm., more than twice the
average variation in the short axis, which was 5.14 mm. No relation was
noticed between the size and the number of eggs in any one nest. Ten
eggs of average size weighed 812 grams--about 81 grams each.
Voeltzkow (78) states that the form of the egg of the Madagascar
crocodile is very variable. No two eggs in the same nest are exactly
alike, some being elliptical, some “egg-shaped,” and some “cylindrical
with rounded ends.” The average size is 68 mm. by 47 mm., shorter and
thicker than the average alligator egg.
When first laid, the eggs are pure white, and are quite slimy for a few
hours, but they generally become stained after a time by the damp and
decaying vegetation composing the nest in which they are closely packed.
The shell is thicker and of a coarser texture than that of the hen’s
egg. Being of a calcareous nature, it is easily dissolved in dilute
acids.
The shell membrane is in two not very distinct layers, the fibers of
which, according to S. F. Clarke, are spirally wound around the egg at
right angles to each other. No air-chamber, such as is found in the
hen’s egg, is found in any stage in the development.
In most--probably all normal--eggs a white band appears around the
lesser circumference a short time after being laid. This chalky band,
which is shown at about its maximum development in Fig. 1_a_, is
found, on removal of the shell, to be caused, not by a change in the
shell, but by the appearance of an area of chalky substance in the
shell membranes. Clarke thinks this change in the membrane is to aid
in the passage of gases to and from the developing embryo. Generally
this chalky area forms a distinct band entirely around the shorter
circumference of the egg, but sometimes extends only partly around it.
It varies in width from about 15 mm. to 35 mm., being narrowest at its
first appearance. Sometimes its borders are quite sharp and even (Fig.
1_a_); in other cases they are very irregular. If the embryo dies the
chalky band is likely to become spotted with dark areas.
The shell and shell membrane of the egg of the Madagascar crocodile are
essentially the same as those just described, except that the shell is
sometimes pierced by small pores that pass entirely through it. The
same chalky band surrounds the median zone of the egg (78).
The white of the egg is chiefly remarkable for its unusual density,
being so stiff that the entire egg may be emptied from the shell into
the hand and passed from one hand to the other without danger of
rupturing either the mass of albumen or the enclosed yolk. The albumen,
especially in the immediate neighborhood of the yolk, seems to consist
of a number of very thin concentric layers. It varies in color, in
different eggs, from a pale yellowish white, its usual color, to a very
decided green.
As might be expected, no chalazæ are present.
The yolk is a spherical mass, of a pale yellow color, lying in the
center of the white. Its diameter is so great that it lies very close
to the shell around the lesser circumference of the egg, so that it is
there covered by only a thin layer of white, and care must be taken in
removing the shell from this region in order not to rupture the yolk.
The yolk substance is quite fluid and is contained in a rather delicate
vitelline membrane.
The albumen and yolk of the crocodile’s egg, as described by Voeltzkow,
differ from those of the alligator only in the color of the albumen,
which in the crocodile is _normally_ light green (78).
As pointed out by Clarke, the position of the embryo upon the yolk is
subject to some variation. During the earliest stages it may occur
at the pole of the yolk nearest the side of the egg; later it may
generally be found toward the end of the egg; and still later it shifts
its position to the side of the egg. It is probable, as Clarke says,
that the position at the end of the egg secures better protection
by the greater amount of white, at that point, between the yolk and
the shell; while the later removal to the side of the egg, when the
vascular area and the allantois begin to function, secures a better
aëration of the blood of the embryo.
Around the embryo, during the stages that precede the formation of
the vascular area, is seen an irregular area of a lighter color and a
mottled appearance. This area is bounded by a distinct, narrow, white
line, and varies in size from perhaps a square centimeter to one third
the surface of the yolk.
During the earliest stages of development the embryo is very
transparent; so that, as there is no fixed place upon the yolk at which
it may be expected to occur, it is often very difficult to find. Owing
to this transparency, to the extreme delicacy of the embryo, and to the
character of the white, the removal of an early embryo from the egg of
the alligator is a difficult operation and is accomplished only after
some practice.
THE DEVELOPMENT OF THE EMBRYO
As the writer has pointed out elsewhere (59), the embryo of the
alligator is often of considerable size when the egg is laid. This
makes the obtaining of the earliest stages of development a difficult
matter; so that the writer, as has already been said, like S. F. Clarke
(17), made three trips to the South in quest of the desired material.
Voeltzkow (78) experienced the same difficulty in his work on the
crocodile, and made several trips to Africa before he succeeded in
obtaining all the desired stages of development.
To obtain the earliest stages, I watched the newly made nests until the
eggs were laid, and in this way a number of eggs were obtained within
a very few hours after they had been deposited, and all of these eggs
contained embryos of a more or less advanced stage of development.
Gravid females were then killed, and the eggs removed from the
oviducts. These eggs, although removed from a “cold-blooded” animal,
generally contained embryos of some size, and only one lot of eggs
thus obtained contained undeveloped embryos, which embryos refused to
develop further in spite of the most careful treatment. Voeltzkow (78)
found, in the same way, that the earlier stages of the crocodile were
extremely difficult to handle; so that, in order to obtain the earlier
stages, he was reduced to the rather cruel expedient of tying a gravid
female and periodically removing the eggs from the oviducts through a
slit cut in the body wall.
The older embryos are hardy and bear transportation well, so that it is
comparatively easy to obtain the later stages of development.
For the stages up to the formation of the first four or five somites, I
am indebted, as I have already said, to Professor Clarke, and, since I
have had opportunity to examine only the sections and not the surface
views of these stages, I shall quote directly Clarke’s paper in the
_Journal of Morphology_ (17) in description of these surface views.
STAGE I
FIGURES 2-2_f_ (PLATES VI., VII.)
The youngest embryo that we have for description is shown in Figures 2
and 2_a_. Of Figure 2 Clarke says:
“The limiting line between the opaque and pellucid areas is clearly
marked, and within the latter is a shield-shaped area connected by
the narrower region of the primitive streak with the area opaca. The
blastopore is already formed near the posterior end of the shield.
“A ventral view of another embryo of the same age (Fig. 2_a_), seen
from the ventral side, shows that the blastopore extends quite through
the blastoderm, in an oblique direction downwards and forwards, from
the dorsal to the ventral side. The thickened area of the primitive
streak is here very prominent. There is, too, the beginning of a curved
depression at the anterior end of the shield, the first formation of
the head-fold.”
Transverse sections of this stage are shown in Figures 2_b_-2_f_.
Figure 2_b_, through the anterior region of the blastoderm, shows a
sharply defined ectoderm (_ec_) which is composed of three or four
layers of cells in the median region, while it gradually thins
out laterally. Closely underlying this ectoderm is a thin sheet of
irregular cells, the entoderm (_en_).
Figure 2_c_ is about one fifth of the length of the blastoderm
posterior to the preceding and represents approximately the same
conditions, except that there is an irregular thickening of the
entoderm in the median region (_en_). This thickening apparently marks
the anterior limit of the mesoderm, to be discussed shortly.
Figure 2_d_ represents the condition of the blastoderm throughout
about one third of its length, posterior to the preceding section.
The somewhat regular folds in the ectoderm (_ec_) are probably not
medullary folds, but are such artificial folds as might easily be
produced in handling the delicate blastoderm. The thickening of the
entoderm, noticed in the preceding figure, is here more sharply
defined, and as we pass toward the blastopore becomes separated
somewhat from the entoderm proper as a middle layer or mesoderm (Fig.
2_e_, _mes_). It would thus seem, from a study of these sections, that
most of the mesoderm is derived from the entoderm. In fact, all of the
mesoderm in front of the blastopore seems to have this origin, for it
is not until the anterior edge of the blastopore is reached that there
is any connection between the ectoderm and entoderm (Fig. 2_e_).
Figure 2_e_ is a section through the region just mentioned, where,
medially, the ectoderm, mesoderm, and entoderm form a continuous mass
of cells. Laterally the mesoderm (_mes_) is a distinct layer of cells
of a fairly characteristic mesodermal type. The notochord is not yet
discernible, though a slight condensation of cells in the middle line
may indicate its position.
Figure 2_f_ is one of the four sections that were cut through the
blastopore (_blp_), which is a hole of considerable size that opens,
as the figure shows, entirely through the blastoderm. Along the walls
of the blastopore the ectoderm and entoderm are, of course, continuous
with each other and form a sharply defined boundary to the opening. As
we pass laterally from the blastopore the cells become less compact,
and are continued on each side as the mesodermal layer (_mes_). In this
series the sections posterior to the blastopore were somewhat torn, and
so were not drawn; but they probably did not differ materially from
those of the corresponding region of the immediately following stages,
which are shown in Figures 3_m_ and 6_i_ and will be described in their
proper order.
STAGE II
FIGURES 3-3_o_ (PLATE VII., VIII., IX.)
The next stage to be described is shown in surface views in Figures 3
and 3_a_. Of this stage Clarke says:
“The head-fold rapidly increases in depth and prominence, as shown in
Figure 3, which is a ventral view a few hours later [than the preceding
stage]. The time cannot be given exactly, as it is found that eggs of
the same nest are not equally advanced when laid, and differ in their
rate of development. The lighter curve in front of the head-fold is the
beginning of the anterior fold of the amnion. The notochord has been
rapidly forming, and now shows very distinctly on the ventral side,
when viewed by transmitted light. A dorsal view of the same embryo
(Fig. 3_a_) shows that the medullary or neural groove is appearing, and
that it ends abruptly anteriorly near the large transverse head-fold.
Posteriorly it terminates at the thickened area in front of the
blastopore, which still remains open.”
Figures 3_b_-_m_ are drawn from transsections of an embryo of about
this stage of development. For a short distance in front of the
beginning of the head-fold, there is a mass of cells of considerable
thickness between the ectoderm and entoderm. In Figure 3_b_ these cells
appear as an irregular thickening of the entoderm, while in Figure 3_c_
they form a continuous mass, uniting the upper and lower germ layers.
This condition is seen, though in a much less striking degree, in the
following stage of development. As to its significance the writer is
not prepared to decide.
Figure 3_d_ passes through the head-fold, which in this embryo
was probably not so far developed as it was in the embryo shown in
Figures 3 and 3_a_. Not having seen the embryo, however, before it was
sectioned, the writer cannot be certain of this point. The ectoderm and
entoderm are here of nearly the same thickness.
Figure 3_e_ is a short distance posterior to the preceding. It shows a
marked thickening of the ectoderm in the medial region (_ec_), which is
continuous posteriorly with the anterior ends of the medullary folds
that are just beginning to differentiate (Figs. 3_f_-_h_).
Figure 3_g_ passes through the anterior end of the medullary plate
or folds (_mf_), whichever they may be called. The ectoderm of the
folds is thickened and is considerably elevated above the rest of the
blastoderm. There is scarcely any sign, in this region, of a medullary
groove. The entoderm (_en_) is considerably thickened in the medial
region, this thickening being continuous posteriorly, as in the
preceding stage, with the mesoderm.
In Figure 3_h_, cut in a plane at some distance posterior to the
preceding, the medullary groove (_mg_) is well marked; its bordering
folds gradually thin out laterally to the thickness of the ordinary
ectoderm. The medial thickening of the entoderm is very marked, but it
has not in this region separated into a distinct mesoblastic layer.
Immediately under the medullary groove is a dense mass of cells
(_nt_), apparently the anterior end of the notochord in process of
formation.
Figure 3_i_, still farther toward the blastopore, shows the medullary
groove wider and shallower than in the more anterior sections. The
mesoderm (_mes_) is here a layer laterally distinct from the entoderm.
In the middle line it is still continuous with the entoderm, and at
this place it is the more dense mass of cells that may be recognized as
the notochord (_nt_). It is evidently difficult to decide whether this
group of cells (_nt_), which will later become a distinct body, the
notochord, is derived directly from the entoderm or from the mesoderm,
which is itself a derivative of the entoderm. There is here absolutely
no line of demarcation between the cells of the notochord and those of
the mesoderm and entoderm.
In Figure 3_j_ the ectoderm (_ec_) is nearly flat, scarcely a sign of
the medullary groove appearing. The mesoderm (_mes_) is here a distinct
layer, entirely separate from both notochord (_nt_) and entoderm
(_en_). The notochord is a clearly defined mass of cells, distinct,
as has been said, from the mesoderm, but still closely united with
the underlying entoderm, which is much thinner than the ectoderm.
This condition of the notochord, which is found throughout about one
third of the length of the embryo, would give the impression that the
notochord is of a distinctly entodermal origin.
In Figure 3_k_ there is no sign of the medullary groove, though the
ectoderm (_ec_) is still much thickened in the middle line. The section
passes, posterior to the notochord, through the anterior edge of the
ventral opening of the blastopore (_blp_). The mesoderm (_mes_) is here
again continuous with the entoderm, around the edge of the blastopore,
but is distinct from the ectoderm.
Figure 3_l_ represents the third section posterior to the preceding.
The blastopore, which passes upward and backward through the
blastoderm, is seen as an enclosed slit (_blp_). It is surrounded
by a distinct layer of compactly arranged cells continuous with the
thickened ectoderm (_ec_) above, with the thin entoderm (_en_) below,
and laterally with the gradually thinning and scattering mesoderm
(_mes_).
Figure 3_m_ is the next section posterior to the one just described.
It passes through the dorsal opening of the blastopore (_blp_), which
appears as a deep, narrow cleft with thick ectodermal borders. The
three germ layers are still continuous with each other, though the
connection of the entoderm with the other two is slight. The sections
posterior to this one will be described in the next stage, where they
have essentially the same structure and are better preserved.