Natural History Review/Series 2/Volume 1/Number 1/Notes on the Anatomy of the Alimentary System of the Axolotl (Siredon Mexicanum)
The earlier investigators of the anatomy of the axolotl appear to have regarded it as a larval form. This, some of them, as Rusconi, did, judging merely from its external appearance; others, as Cuvier, even after a somewhat minute investigation into its anatomy.
Hunter, it is true, was convinced that they were adult forms, and merited but little the censures passed upon him by Rusconi, who, from constantly studying the salamanders and their metamorphosis, dogmatically refused to believe in the existence of a persistent larval form among the Amphibia. I think, however, it is more than probable that Cuvier's memoir on "Doubtful Reptiles," published in Humboldt's Recueil d'Observations de Zoologie,[1] was seen by Rusconi, or at least heard of by him, as it was read before the French Institute, as early as January, 1807; and this great anatomist insists so strongly on axolotl being a larval condition of some salamander, and saw so many things in its anatomy that he says strengthened him in this opinion, that it is really no wonder the Italian salamander-observer, feeling himself so strongly supported, indulged in a rather contemptuous laugh at our great English anatomist. Cuvier's account of the visceral anatomy is so short, that we venture to subjoin it here; it will be found at page 109 of the work referred to, and is illustrated with several plates. With the greatest deference to the memory of one of the greatest of modern anatomists, and the author of the "Memoirs of the Mollusca," a work which exhibits a wonderful skill in minute dissection, yet I have never met with an anatomical description which seems so decidedly written to prove a foregone conclusion. Cuvier thought axolotl a larval form; through the kindness of Humboldt, he was given specimens, from the anatomy of which much was to be proven; and yet we read such statements as that the "spleen is very small, and in the middle of the mesentery;" that the "oviducts were so very delicate, that one could perceive them with difficulty;" with what justice these facts, tending to prove an immaturity of condition, are stated, will be seen a little further on:—
"In axolotl," writes Cuvier, "the œsophagus is short, plicated longitudinally, and is continuous with the stomach; this latter is large, membranous; the forepart is a little plumpish, but towards the pyloric orifice, it is much contracted. I found it full in the two specimens (examined) of small fresh-water Crustacea, strongly resembling our native ones. The animals had swallowed these without masticating them; and their legs were found undigested down as far as the rectum.
"The intestinal tract is tolerably large; more especially the portion nearest the liver, and tolerably long; it consists of two principal loops, and is furnished neither with a cœcum nor internal valve of any kind.
"The liver is rectangular, and without any deep lobes. I could not detect the presence of a gall-bladder.
"The spleen is very small, and is placed in the middle of the mesentery; this latter is as we find it in the ordinary salamanders. Indeed, all the intestines are just those of a salamander.
"The ovaries are very small, flabby, and contained hardly a trace of ovæ. They occupied the same place, and are furnished with the same greasy appendages that are found in the common salamanders. Again, the oviducts are so delicate (si frêles) that one can scarce perceive them.
"From all these marks of immaturity, and that intimate resemblance which all the viscera bear to those of the salamander and their larva, I conclude that the Mexican axolotl is but the larva of some huge salamander, perhaps the same that is alluded to by Michaux."
Before proceeding to treat of the alimentary canal, I may take this opportunity to refer to a paper by Sir Everard Home, in the Philosophical Tranactions for 1824, "On the Generative Organs of the Mexican Proteus." The paper itself, so far as anatomy is concerned, contains little that is noteworthy. The specimens dissected were discovered by Bullock, in a lake three miles above Mexico—this lake being some 8000 feet above the sea level, and of 60° of temperature. Those taken at Lesenco are brought by the peasantry to the Mexican markets in thousands, in strings of from sixty to seventy each. This paper, however, is illustrated, and the artist has done his work, and done it well; and his master has given names to the different parts figured. From a careful comparison of the plates representing the organs of generation in the male and female axolotl, with my own preparations, I am prepared to acknowledge the very general correctness of these fine drawings. The organs in the female, in an immature state, are likewise figured; and the ovaries are neither so small, even in an unimpregnated condition; nor the oviducts so delicate as to afford any difficulty in seeing them, to an ordinary investigator; even the kidneys and urinary bladder (?) are quite perceptible to the most careless observer.
On opening the walls of the abdomen, from the junction of the pectoral muscles to the curious cloacal aperture, and turning back the muscles, the following viscera are seen (vide Pl. II., fig. 1),—First, the large and well-marked liver, slightly divided into two lobes by the entrance of the suspensory ligament; next the convolutions of the intestines, ending in the strongly-marked straight rectum; on each side of which we find two glandular bodies—the supposed Cowper's glands of Sir Everard Home; above these, and below the coils of intestine, the apices of the kidneys are to be seen. If we now remove the left lobe of the liver, we will discover the stomachal portion of the alimentary canal of an elongated shape—the œsophageal portion, as Cuvier says, a little plumpish and enlarged, and the pyloric end much contracted. But we also have no difficulty in finding a glandular organ, closely attached to the middle-third of the stomach, and tied down to it by a mesenteric attachment (vide Eig. 2), which is the spleen—said by Cuvier to be placed in the midst of the mesentery, and to be very small. In Fig. 2 it is represented of the natural size. At a short distance below the junction of this gland with the stomach, the intestine contracts, and twists upon itself. There is no true pyloric valve, but this turn in the intestine to all intents and purposes acts as one. The intestine next proceeds towards the liver. This organ is large, its upper surface concave, its lower convex; it is divided by the suspensory ligament, which attaches it to the walls of the abdomen, into two lobes; in the adult male it is of a dark brownish colour, mottled; it overlaps the stomach and portions of the small intestine. The right lobe is the larger, and is slightly notched on its outer free margin, to receive the well-developed gallbladder, which, though not mentioned in the text by Cuvier, is very imperfectly figured in one of the Plates;[2] when inflated, it is pyriform; in the empty condition, it assumes the outline of the liver (see Plate II, Fig. 3). The biliary ducts open into the intestine, just where it is connected with the liver, by a common duct.
The small intestines make two principal convolutions, and are kept in their place by a well-developed mesentery. I could detect no trace of a pancreas. As the intestine approaches the rectum, it becomes excessively narrow, and at last ends almost by intussusception (Fig. 5); the wide and capacious rectum ends in the cloaca. Before examining the internal structure of these parts, it may be well to compare them with similar organs occurring in the salamanders. For this purpose I have selected Triton cristatus and Salamander maculosa. So far as the œsophagus and stomach are concerned, the relative size and proportions are nearly similar. The stomach is more pear-shaped in S. maculosa. In Triton, the spleen is a small, flat, oval gland, attached to the right side of the stomach by a loose fold of mesentery, but by no means closely so; in Salamander,[3] it is a long, narrow, ribbon-like body, closely attached to the right side of the stomach. In Triton, the liver is small, but divided into lobes; the gall-bladder is well developed. In Salamander, the liver is rather small in proportion, not much divided, and the gallbladder is also small. In both Triton and Salamander, the small intestine is very well developed, and in both does it contract as it approaches the rectum, which here, as in Siredon, is much wider than the rest of the intestinal canal.
In the axolotl the œsophagus is short, the mucous surface is longitudinally and finely striated, the external muscular fibres are circular, and act as a sphincter; in the stomach, the mucous membrane is continuous with that of the œsophagus, but here it is thrown into deep folds. The fine striæ of the œsophageal portion are continued; and at what may be considered as the cardiac orifice, these folds of the mucous surface are brought into such close apposition, their dimensions at this spot, too, are so greatly increased, forming four or five little protuberances; as to take the place of a valve and effectually prevent any regurgitation into the mouth. Though there are a few file-like teeth in the upper jaw, yet they serve more for organs of prehension, and cannot be of much use in mastication; and, undoutedly, the process of comminuting the food is mostly accomplished in the stomach.
In this organ, as I have said, the mucous membrane which lines the whole alimentary tract is thrown into a series of very deep folds, which appear to be a continuation of the longtitudinal mucous folds of the œsophagus; they wind to and fro in such a manner, bending backwards and forwards, and interlacing with each other, as strongly to resemble the appearance presented by the gizzard of a fowl (see Fig. 4, Pl. II.); and when acting under the control of the muscular coats of the stomach, must form a very effective triturating apparatus. Four or five of these folds enter into the intestine, and here, for about a quarter of an inch, they become but very slightly elevated; as they approach that peculiar semiflexion in the intestine referred to above, they increase in number, and also in depth (Fig. 4), and, from their very close and compact appearance, I am led to suspect that this portion of the intestine, between the pyloric orifice of the true stomach and the orifice of the biliary ducts, is more than an ordinary duodenum, and acts somewhat as a secondary stomachal cavity. This idea is strengthened by the additional fact, that the true stomach is lined with a series of minute pores, thickly scattered over the mucous surface, and covering both the raised folds of the mucous membrane and the intestines between them. These small pit-like indentations are minute glandular bodies, secreting the gastric juice; they commence just below the cardiac orifice of the stomach, and are continuous to the entrance of the biliary ducts. While every part of this portion of the intestine is supplied with these crypts, of course they are most numerous when the mucous membrane is thrown into a series of folds; this occurs in the secondary stomachal portion alluded to, which, in every anatomical particular, is a miniature of the larger one.
The mucous surface of the small intestine becomes much smoother after it has received the contents of the liver. But in no one spot throughout its length do we find it absolutely smooth; it is always arranged, more or less, in a series of delicate, longitudinal folds; and, as we approach the rectal portion, these folds assume a slightly twisted appearance, but not at all distinct enough to be alluded to as a spiral valve. When the small intestine joins the large rectal cavity, the gut, as above-said, contracts very much, and the mucous membrane is packed up into two or three little eminences, which act the part of a valve. In the rectal portion, the lining membrane is thin, and very smooth. In this, as well as in its large diameter, in comparison with the small intestine, it remarkably resembles the same parts in the Tritons and Salamanders.
From a survey of the details thus glanced at, it will be seen that there is nothing in the alimentary canal of the axolotl to predicate of it that it is a larval form; though it may resemble the same parts in an adult Salamander and Triton, yet it differs from these more than was at first thought, and more than one would imagine from the only account that I have found attainable, namely, that of Baron Cuvier. The osseous system has been too well described by Cuvier, and the reproductive by the paper and illustrations of Sir Everard Home, to need further allusion to at my hand.
At the conclusion of my observations on the alimentary canal of the axolotl, I received a copy of Professor Luigi Calori's paper, entitled "Sulla Anatomia dell' Axolotl," published in the memoirs of the Academy of Science, Bologna.[4] This elaborate paper leaves but little to be said in addition on the anatomy of this animal; and yet, perhaps, the publications of this Academy—highly valuable though they be—maybe as little accessible in Britain as those of our own Royal Society appear to be in Italy. Were the latter otherwise, Dr. Calori would have known of the paper of Sir Everard Home, referred to by us, "On the Reproductive Organs of Axolotl," and not have claimed priority for his discovery and very careful details of these same structures, so unaccountably passed over by Cuvier. This thought, and the fact that, though Dr. Calori's paper is illustrated with five plates, not one represents the viscera in situ—nor is there a correct representation of the spleen, or of the gall-bladder—makes us not hesitate to give the result, as detailed in the previous paper, of the dissections of two fine specimens, male and female, of the axolotl (for which we are indebted to the kindness of Dr. Dickie, late Professor of Natural History, Queen's College, Belfast, now of Botany, at Aberdeen); while, in justice to the important paper in the Bologna memoirs, we now append a brief abstract of the portions that more especially treat of the alimentary tract. The osseous and blood system are equally painstakingly investigated; but for these we will refer to the work itself:—
"The interior of the stomach is lined with a dry scaly epidermoidal-like stratum, which is also met with in the pharynx and œsophagus,—a net-like structure, with wide meshes, occurs, seemingly depending from the blood-vessels. In these meshes are a large number of small, regularly disposed cells, probably a smaller vascular net-work, not easily perceived without the assistance of high powers. Within the meshes of this smaller net, the (mucous) glands of the stomach open,—they are very numerous, tubular, and quite microscopic.[5]
"The muscular coating of the stomach is rather thin, composed of longitudinal fibres, and is continuous with the muscular coats of the oesophagus, and likewise of the intestinal tract. The transverse fibres are more abundant towards the pyloric orifice, and here the coats of the intestine are somewhat thicker; there is no lack of an intermediate cellular membrane between the muscular and mucous coats, but it is very thin.
"The serous investment is very thin, and, having enveloped both sides of the stomach, it is prolonged into two folds: one, the right and lower, going to the liver, forms the gastro-hepatic ligament: the other, the left and higher, goes to the spleen, not placed as Cuvier thinks, in the centre of the mesentery, but against the left side of the stomach.
"The œsophageal orifice is larger than the pyloric one; this latter is very narrow, is slightly twisted, and a series of folds in the lining membrane almost completely closes it. These folds are prolonged into the first portion of the small intestine. These folds in the pyloric orifice take the place of a valve; it is externally marked by a constriction, inferior to which the duodenum commences; this is at first very narrow, and descends in the same general direction as the stomach, it next crosses from left to right, twisting slightly; it then enlarges, and reaches the liver, with which it becomes united by a fold of the peritoneum; receiving the contents of the biliary ducts, it bends downwards towards the right side,—is prolonged into the small intestine, which, preforming some convolutions, ends in the large rectal portion almost by intussusception. The rectum is twice as wide as the small intestine, but short and straight, ending in the cloaca, open externally through a longitudinal fissure with swollen lips.[6]
"The intestines are, for the most part, membranous; the first portion of the duodenum has, however, very thick coats, and internally has longitudinal plaiting, which occupies almost all its cavity; in this it repeats the arrangement of the mucous membrane of the œsophagus. This disposition of the fibres leads me to suppose that this part of the intestine has the power of enlarging itself; between these folds there appears a glandulous net-like structure, without doubt of great importance in digestion; this is also observed in the Salamanders.[7]
"In the small intestine, we find a few longitudinal plaits: but these are scarcely visible, and disappear as the intestine approaches the rectum; there is, also, a very minute net-like vascular structure, visible to the microscope. Meckel says, there is some villosity present in the small intestine of Salamanders; but in Axolotl, at least, I could find nothing of the kind. No valve intervenes between the small intestine and rectum,—this latter does not exhibit any longitudinal foldings, unless indeed in its lower extremity, close to the external orifice of the cloaca, where many are seen rising like crests; there are here, besides, four little bodies, like carunculæ. In this cloaca open the urinary and genital organs, and the bladder,[8] not much developed; however, the small intestine, equally with the rectum, is furnished with a fine stratum of longitudinal and transverse fibres, but very delicate and microscopic, and is attached to the vertebral column by a thin band of mesentery.
"Salivary glands are totally wanting; there also seems to be a total absence of pancreas, but this gland is wanting in other Saurobatrachii—in Hypochthon, for example; when existing, it is usually very small, and quite rudimentary. Cuvier does not refer to its existence. The spleen is found; its position I have already alluded to; it is four times as broad as it is thick: it ends in an obtuse point at both extremities. I found it full of rather conspicuous corpuscles, and of black pigment.
"The liver is rectangular, of a yellowish-red colour, with many black spots; its under surface is convex, its upper, concave: the first is connected by the falciform ligament to about the anterior half of the middle line of the lower soft coats of the abdomen, and is divided by it into two unequal portions—these form the two lobes of the liver, the right being the larger. On the under surface, the same division is made, here in a notch extending inwards through about four-fifths of the substance of the liver, enters the umbilical vein, also the biliary ducts and the blood-vessels of the portal system.
"The thin border of the right lobe is notched to receive the small biliary bladder, which, being inflated with air, rises up from the posterior hepatic border, of which it is free. Cuvier says, in the text of his paper, that he was not able to see it; but in the plate accompanying it, it is delineated, though confusedly and incorrectly. It is pyriform, but there is nothing remarkable in its structure; it empties itself, with the bile ducts, united to it, into the duodenum, not far from where the latter is attached to the liver."
Such are the more salient points of Dr. Calori's paper, a translation of which I have thought it better to lay before the reader. There remains no longer any necessity to discuss the question whether this amphibia is a larval form; but still there is much to be done in reference to its organs of respiration in its early life. From finding the lungs in the young axolotl in a complete state of acatylectesis, while the tissue is be; utifully developed in those of adult form, I am led to believe that branchial respiration is that of youug life, while the older animal becomes as equally dependent for respiration on its lungs.
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SIREDON MEXICANUM.
- ↑ "Recueil d'Observations de Zoologie et d'Anatomie comparée faites dans l'Oceán Atlantique, dans l'intérieur du Nouveau Continent et dans la Mer du Sud, pendant 1799, 1803." Par Al. de Humboldt et A. Bonpland. 1er volume. 1811.
- ↑ Loc. cit., Plate 12, Fig. 4.
- ↑ Here I would observe, that I cannot agree with Schneider, who, in his Natural History of Amphibia, has united the aquatic (Triton) with the land (Salamander) Salamanders; although in both genera the ovæ are impregnated before being laid, yet in the one (Triton), we have the eggs deposited on aquatic plants; the young Tritons, when hatched, retaining their branchiæ for a longer or shorter length of time; in the other (Salamander) we have the oviducts large and capacious, the ovæ are hatched in them, making their exit into the world almost miniatures of their parent. Surely such embryological distinctions point to at least a difference in the general—in the ordinary acceptation of this word—of these creatures.
- ↑ Memoire dell' Accademia delle Scienze dell' Instituto di Bologna. Tomo iii. 1851, pp. 269–361.
- ↑ It will be seen by reference to our remarks on the internal structure of the stomach, that Dr. Calori has not rightly comprehended this portion, but his specimen was a very young one: contrast his fig. 9, plate XXIII., with ours, fig. 4. plate II. The general outline of this viscus is likewise very much exagerated in fig. 8a, x., Plate XXIII., of Calori.
- ↑ The external appearance of this fissure differs much in the two sexes—in the male it is, as described by Calori, with swollen and corrugated lips: in the female the orifice is simple, as in many of the Amphibia.
- ↑ We think many portions of the intestinal tract act the part of a kind of secondary stomach. In the Salamanders, as in Triton, we have found, even below the liver portion of the duodenum, widenings of the intestine, to all appearance having the function of second stomachs; perhaps this portion might be regarded as an additional stomach, regarding as duodenum that portion only where it is continuous from it to the liver.
- ↑ In the plate the bladder is represented so as to lead one to suppose it is on the right side; its right position is, as figured by Home, towards the left; its shape varies somewhat in different specimens.