In all vertebrated animals the blood which supplies the liver with the matter that forms the bile, is veinous blood which has circulated in the intestines, and which, after reuniting in one trunk, called the vena porta, is spread again through the liver. All vertebrated animals have a particular secretion from two large glands attached to the sides of the backbone called the kidneys; this liquor, denominated urine, generally remains sometime in a reservoir called the bladder.

The sexes are always distinctly separated; but the mode of impregnation varies very much. The eggs of some reptiles, and of almost all fishes, are impregnated by the male after they are spawned.

We may perceive in the above characters, how far all vertebrated animals resemble each other: nevertheless they admit of four grand subdivisions or classes, characterised by the kind or strength of their motions, which also depend on the quantity of their respiration; since it is from the respiration that the muscular fibres derive their energy, and their irritability.

The quantity of respiration depends on two conditions: the first is the relative proportion of blood which is presented to the respiratory organs in a given time; the second, the relative proportion of oxygen which enters into the composition of the fluid in which the animal lives, whether water or air.

The quantity of blood which is acted on by respiration, depends on the structure and disposition of the organs of respir ation and circulation. The organs of circulation may be double, so that all the blood which is returned by the veins is obliged to circulate through the respiratory organs, before it is carried again to different parts by the arteries; or these organs may be simple, so that only a portion of the blood returned from the body to the heart is obliged to pass through the respiratory organs, and the rest circulates again through the body, without having been subjected to the effects of respiration. The latter is the case with reptiles; their quantity of respiration, and all the qualities that depend on it, vary according to the proportion of blood which enters the lungs at each pulsation. From these characters Cuvier forms the four subdivisions or classes of vertebrated animals, which are,

Class 1. Mammiferous Animals, which bring forth their young alive and suckle them, being provided with teats (Lat. mamma), whence the name is derived.

Class 2. Birds.

Class 3. Reptiles.

Class 4. Fish.

The general plan of the skeleton is the same in each class, though it admits of considerable modifications, as may be per

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ceived by the annexed figures. Fig. 30. a represents the human skeleton, man being placed at the head of the mammiferous class; b, the skeleton of a bird; c, that of a frog; and d, the skeleton of a fish. Man is preeminently gifted by his Creator with superior intellectual powers, he is distinguished also by his erect posture, which required a structure varying considerably from that of mammiferous quadrupeds: had we chosen the skeleton of any of the lower orders of this class, the form would have approached more closely to those of the other classes, but taking the extremes of the grand division of vertebrated animals, we still perceive the leading characters of the osteology to be the same. Namely, a skull containing the brain, supported by the vertebral column which contains the spinal cord, and to which the ribs are attached. With respect to the limbs, they admit of a great variety of form suited to the wants of the animal, and in the lowest order of the mammiferous class, which comprises dolphins and whales, we find only one pair of limbs, and in the latter, they are so concealed in the flesh, as not to be visible. The upper limbs of birds do not terminate in toes or claws. Some reptiles have only one pair of limbs, others, as serpents, have

none.

The distinctive characters of the four classes above enumerated founded not on the form of the skeleton, but on the circulation and respiration, are thus given by Cuvier.

MAMMIFEROUS ANIMALS have a double circulation, and the aërial respiration is simple, viz. it is effected by the lungs only.

BIRDS exceed mammiferous quadrupeds in the quantity of their respiration, for they have not only a double circulation, and an aërial respiration, but they respire also through other cavities besides the lungs, the air penetrating through the whole body, and bathing the branches of the aorta or great artery of the body, as well as those of the pulmonary artery.

FISHES have a double circulation, but their respiratory organs (the gills) are only formed to respire by the intervention of water, and their blood only receives the portion of oxygen dissolved or mixed in the water, so that the quantity of respiration is, perhaps, less than that of the next class, Reptiles.

REPTILES. The organs of circulation are simple, and only a portion of the blood brought back by the veins passes through the organs of respiration. Their quantity of respiration, and all the other qualities that depend on it, vary according to the proportion of blood which enters the lungs at each pulsation.

In mammiferous quadrupeds the quantity of respiration is less than that of birds; but it is greater than that of reptiles, on account of the structure of the respiratory organs; and exceeds that of fishes, on account of the different elements in which they live. Hence result the four kinds of movements, which the four classes of vertebrated animals are particularly destined to exert.

Mammiferous animals, in which the quantity of respiration is moderate, are generally formed to develope their strength in walking or running. Birds, which have a larger quantity of respiration, have the activity and strength of muscles necessary for flying. Reptiles, in which respiration is more feeble, are condemned to crawl; and many of them pass a part of their lives in a kind of torpor. Fishes require to be supported in an element nearly as heavy as themselves, in order to exert their proper motions in swimming.

All the circumstances of organisation proper to each of the four classes, and particularly those which regard their movements and exterior sensations, have a necessary relation with their essential characters; nevertheless, the class of mammiferous animals has particular characters belonging to their viviparous generation, the manner in which the foetus is nourished in the womb by means of the placenta, and the teats with which they suckle their young. On the contrary, the other three classes are oviparous; and if we contrast them together with the first class, we shall find certain resemblances which indicate in the three classes, birds, fishes, and reptiles, a special plan of organisation, comprised in the general plan of all vertebrated animals.

B.

ART. IV. Observations on a preternatural Growth of the Incisor Teeth, occasionally observed in certain of the Mammàlia rodéntia. By the Rev. LEONARD JENYNS, F.L.S.

INSTANCES of the wild rabbit have occasionally been met with in Cambridgeshire, in which the fore-teeth had grown to so great a length, as to be rendered wholly unfit for the purposes they are intended to serve.

This disease, as, in truth, it may be strictly called, has been often observed before, both in these, and, I believe, also in other animals of the order Rodéntia (les Rongeurs Cuv.), whose incisors are all constructed on the same, or nearly the same, plan. Nor is it for the novelty of the occurrence that the following remarks are offered: my present object is merely to draw attention to the circumstances of at least two cases, that were not attended by that accident which is usually supposed to produce the singularity in question.

It appears to be the statement of some authors, that, in order to occasion this anomaly, it is necessary that one pair of incisors, or one single incisor, be either broken or fallen out+; and that it is for want of the accustomed attrition against the teeth which are deficient, that the opposite pair grow to an unusual extent. This is so far true, that, I believe, in all cases, it is in immediate consequence of the cutting edge not being worn away, or at least to that degree that it is in healthy individuals from constant use, and to supply which loss these teeth are provided with the power of growth ‡, that this irregularity shows itself: but I would observe that the

* See Plott's Natural History of Staffordshire, p. 252. tab. 22. fig. 6.; also Morton's Natural History of Northamptonshire, p. 445.

+ Cuvier, speaking of the incisors of the Rodéntia, observes, "Leur forme prismatique fait qu'elles croissent de la racine à mesure qu'elles s'usent du tranchant, et cette disposition à croître est si forte, que si l'une d'elles se perd ou se casse, celle qui lui était opposée n'ayant plus rien qui la comminue, se développe au point de devenir monstrueuse." Règne Animal, tom. i. p. 187.

It is well known to comparative anatomists, that the incisors of the Rodéntia, like the tusks of the elephant and hippopotamus, are in a constant state of growth, and that they are furnished with roots which in length nearly equal the jaw itself, curving back underneath the grinders, and extending in some cases as far as the coronoid processes. In consequence of this singular provision of nature, so admirably adapted to the habits and economy of this tribe, there is a constant, yet gradual, advancement of the interior part of the tooth, to supply the place of the portion worn down in feeding, &c.; and, under ordinary circumstances, this increase is so nicely regulated, that the cutting edges of the two pairs of incisors always preserve the same relative situation with respect to each other. A good description and representation of this contrivance may be seen in Blake's Essay on Teeth. (Disputatio med. inaug. de Dentium Formatione, &c., p. 88—91., tab.3. fig. 9.)

ultimate cause of the evil may arise from other accidents besides those above mentioned. Thus, it may originate from too soft food; from a morbid and too rapid secretion of the osseous matter of the tooth, which is constantly being deposited at its root, or from some slight derangement of the under jaw; such as, for instance, a dislocation of one of its condyles, whereby the incisors of that jaw would be thrown out of their proper position, and their cutting edges could not be brought fairly into contact with those of the opposite pair. In either of these cases, the growth of the teeth will be over-proportioned to their abrasion by the acts of gnawing and feeding, and a preternatural elongation of that part which is above the gums will immediately take place. It is obvious that this diseased growth will be more or less rapid according to the degree of influence exerted by the predisposing cause, and the length of time it has operated. Perhaps, in the first stage of the malady, its progress may be very gradual, and not much interfere with the usual habits of the animal; but the teeth having once attained such a length that, under any circumstances, their edges cannot be brought to act upon each other, their growth must be much more rapid, and ultimately prove such an inconvenience, as must often terminate in the starvation of the sufferer.

It is to one of the above causes that I would attribute this singular accident in the two following instances, in neither of which was there any deficiency in the proper number of teeth, or appearance of their having

been broken.

In the first of these rabbits, which is preserved in the museum of the Cambridge Philosophical Society, and is a remarkably fine example of the anomaly in question, the lower incisors are the pair chiefly elongated (fig. 31.); and they are here so prodigiously developed, as to turn completely over the nose, measuring in length, from the surface of the gum to their cutting edges, no less than two inches and one eighth.* I am inclined to suggest, that,

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in this case, it was a too rapid secretion of the osseous matter * The usual length of this portion of the incisor in the wild rabbit is only three lines, or a quarter of an inch.