CLASS XXIII.-PORIFERA.

Of all the beings usually known under the designation Zoophytes, the Sponges and their allies, constituting the class Porifera, appear to have the best claim to the title; since they present so complete an admixture of the characters of plants and animals, that it is difficult to say to which division of the organised world they properly belong. Like plants, they are fixed to one spot during the whole of their lives, subsequently, at least, to their first development; they seem to possess no sensibility, for they can be torn or wounded in any way without showing by their movements any indication of being affected by the injury; and they do not appear to have that power of executing voluntary motions which must be regarded as the distinguishing characteristic of animals. On the other hand, they present a structure which is not analogous to any thing found in plants, but is similar to that of beings undoubtedly belonging to the animal kingdom; with these beings they are connected by intermediate forms, presenting a regular gradation of increasing complexity of structure and variety of function; and there are certain movements, both in the adult and in the undeveloped Sponge, which are more analogous to those seen in higher animals than to any observed in plants. On the whole, however, the evidence for the animal character of the Sponges seems to preponderate; and they will be accordingly considered here. Still there is no doubt that, if they are included in the Animal Kingdom at all, the lowest place in the scale should be assigned to them. The common Sponge is a sufficiently characteristic form of the class, to serve as the foundation of a general

Section of Living Sponge. account of the structure which prevails in it. On looking at its exterior, we observe that it is covered by a number of closely set and minute orifices; and that larger openings are disposed at intervals among these. The former are termed pores, the latter vents. On cutting into the substance of the sponge, it is seen to consist of a sort of network of filaments, interlacing together in such a manner as to leave large channels and spaces of various forms, which communicate with each other. The large channels terminate in the vents; and, on tracing any one of them into the substance of the sponge, it is seen to divide and send off ramifying branches, which at last lose themselves in the spongy network that lies around them; and this communicates with the pores on the external surface.

The interlacing fibres, of which the walls of the large canals, and the spongy tissue between them, are alike composed, entirely consist (in the common sponge) of a sort of horny animal matter, as is shown by burning a small portion of them. But in other species we find spicula, or needle-like crystals, of silex or of carbonate of lime, disposed amongst these, giving to the structure much greater firmness, but depriving it, more or less completely, of that elasticity which is so useful in the common sponge.

The substance known as sponge is, however, but the skeleton of the being; when alive, this fibrous network is clothed, within and without, by a thin gelatinous substance, very like the white of an egg. This lines all the passages, and covers the exterior; but it drains away when the sponge is removed from the water. In this the peculiar vital endowments of the being appear to reside. These are manifested, not only by its slow but regular growth, but by a curious circulation of fluid, constantly taking place within the mass. When a sponge is examined in its living state, beneath the water, a constant current is seen to issue from the vents; being made evident by the disturbance of the water, and by the movement of particles floating in it. It may also be ascertained that a constant flow of water takes place inwards through the pores; for small solid particles upon their edges are occasionally seen to be sucked in. No intermission can be detected, during the life of the sponge, in these currents, which evidently have for their object to convey the nutritive matter contained in the water into the interior of the mass, and also to carry off the particles which are to be excreted, since thin films detached from the living tissue are seen to pass out from the vents along with the fluid ejected from them.

The relative position of the pores and vents differs much in the different kinds of sponge. Sometimes all the former are on one side, and the latter on the other. Not unfrequently the vents are placed on the summit of little conical prominences, which look like craters of a volcano; and the stream issuing from them, when seen under a microscope, may be likened to a miniature eruption. Sometimes the sponge assumes the form of a hollow cylinder, which hangs at one extremity from a rock; the pores are all upon the exterior surface, whilst the vents open into the interior cavity, and their united stream rushes out with considerable energy from the lower end of the cylinder.

Sponges may be multiplied, like plants, by artificial division, each portion becoming a new individual, but it does not appear that this is their natural mode of increase. They propagate by detaching little round gelatinous bodies, termed gemmules, from their living tissue; which in time develop the original form of the parent. These are produced in the network between the large canals, into which they find their way; when mature, they pass out of the vents in the current which sweeps through them, and by which they are conveyed to a distance. In these gemmules a peculiar motion, like that of animalcules, may be seen for some time; they swim hither and thither; at last they fix themselves, and begin to deposit the horny or earthy particles which are to form their skeleton; and the system of canals gradually shows itself in their substance. When once they have fixed themselves, they seem to lose all power of further movement, and remain during the rest of their lives attached to the same spot.

Some kinds of sponges are found on almost all shores; and some frequent deep water, whence they can only be obtained by dredging. It is in this manner that the common sponge is procured from the Mediterranean, the Grecian archipelago, and the other localities it frequents. Sponges are not confined to the sea, however; for there is a species which inhabits fresh water.

With the notice of this group we appropriately close the subject of Zoology.

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NUMBER 35.

PRICE lad.

THE external appearance of the human body is necessarily familiar to all. Its internal structure, however, and the manner in which its different functions are performed, are not understood as they ought to be by the generality of people. The more fully that we comprehend the structure of our frame, the more attentive shall we be to its preservation in a state of health, and the more capable of accomplishing that all-important object. The present treatise will therefore be devoted to a description of the human body, in language as popular as the subject will permit, and as concise, at the same time, as possible.

Zoological science places the human being in the class of Mammalia, or suck-giving animals, and in the order of Bimana, comprising the two-handed creatures of that class. An erect posture is the peculiar characteristic of man, and it is one which gives to his aspect that dignity becoming his high place in creation. By the adaptation of an erect structure, also, his hands are left disengaged, and ready for the numerous operations to which he is inclined by his judgment or urged by his wants. His general stature is between five and six feet. A combination of hard and soft parts forms the material of his frame, the soft portions being arranged, generally speaking, upon and around the more solid parts of the structure. These latter parts consist of a beautiful framework of bones, termed the skeleton, which naturally occupies the first place in our description. Muscles and tendons, which are the organs of locomotion; the brain and nervous system, or organs of sense, feeling, and intellect; the lungs, for respiring the air essential to the maintenance of the principle of life; the stomach and digestive organs, for the supply of nourishment; the heart, blood-vessels, and absorbents, for the circulation of vital fluids through the body;-these and other important parts will fall to be described after the solid framework on which they rest has received its due share of our attention.

The skeleton comprehends three main divisions, the head, trunk, and extremities, which consist, in all, of 254 bones, joined together in a manner combining great strength with ease and freedom of motion. The whole of the bones are composed of nearly the same materials, namely, earthy matter, chiefly lime, and gelatine or animal glue. The lime gives them hardness and solidity, while the animal matter cements or binds them together, and renders them not easily broken. They differ to some extent in solidity and weight. A middlesized adult skeleton, weighed all together, ranges between 160 and 200 ounces, or from 10 to 13 pounds avoirdupois. The heaviest bones in the body, in proportion to their size, are the bones of the skull, extremities, and pelvis, or under part of the trunk. Their surface is for the most part smooth, and the interior, beneath a cake or coating of more condensed substance, is porous and spongy. The bones of the extremities are hollow, like pipes, by which arrangement they are rendered at

once light and strong. The marrow is contained in the internal cavity. Compact as they are, the bones are nevertheless pervaded by blood-vessels, which, indeed, are essential to their vitality.

The crown or summit of the osseous fabric is occupied by the cranium or skull, which is composed of eight bones, the frontal, the occipital, and the two parietal, constituting the greater part of the outward skull, before, behind, and laterally. The two temporal or temple bones, on the under part of each side, and the sphenoid and ethmoid bones, placed at the base of the skull internally, are the remaining bones of the head. The union of these bones is remarkably firm and strong in the adult being. In some cases, the osseous plates are joined by serrated or ragged edges, like the teeth of a saw. In other instances, they overlap each other, like the ridge of a house; the arrangement, in each case, being precisely the one best fitted to ensure strength and stability in the particular part. Altogether, an arch of the most powerful kind is formed, for the safe protection of the important organ within, the brain. The bones of the face, situated below and before the cranium, are numerous. Among the facial bones are reckoned the two upper maxillary or jaw bones; two malar or cheek bones; two nasal or nose bones; two small bones, attached to the nose internally, called the turbinated bones; two palate bones; the two lacrymal bones, situated in the orbit; the vomer, or ploughshare bone, forming a part of the basis of the nose; and the single lower jawbone, or maxillary bone of the lower jaw.

The bones of the skull and face rest upon the top of the spine or backbone, which consists of twenty-four separate pieces, called vertebre, firmly and curiously jointed the one into the other. The column of the spine is curved in several places, the most prominent being a curve forwards near the middle of the back. Seven of the vertebræ are called cervical, twelve dorsal, and five lumbar, from being situated respectively in the neck, back, and loins. Each vertebra has various projections and depressions, to admit of 'a firm union with those adjoining it; and, by the junction of the whole, a long hollow or canal is made, for the reception of the spinal marrow. In the annexed figure, the upper part of the vertebral column is marked á, and the lower a. The second of the vertebræ of the neck sends upwards a projecting pinion or tooth, which is received into a corresponding depression in the one above, thus forming the pivot upon which the head turns. The bones of the spine rest upon the pelvis, s s,

* A ligamentous band keeps the tooth-like projection of the second vertebra in the depression of the first. Nothing can better show how completely our life, during every instant of its duration, depends on the maintenance of every single part, however minute, in order. If this ligament, scarcely thicker than strong paper, were to give way, instant death, from pressure on the spinal marrow, would ensue. A porter, carrying a burden, dropped down dead. On examination, no cause of death could be

giving to it firmness and stability. The spine rests on it by means of the sacrum, w, a series of five imperfect vertebræ, consolidated into one piece in advanced life, which sink like a wedge between the pelvic bones of each side. The sacrum terminates in a loose osseous peak, called the os coccygis. The strong, hollowed, cuplike bones of the pelvis, are marked by large round depressions on the outer and under surface of each, which form sockets for the two upper bones of the leg. At the top of the spine, immediately below the vertebræ of the neck, are situated on each side the collarbones or clavicles, y y, which are long and narrow in shape, and pass in a semicircle or arch from the front of the chest backwards, or, in other words, from the sternum, x (breast-bone) to the top of the shoulders. On the back of the ribs, at each side, lie the shoulder-blades or scapula, which are thin flat bones, of a triangular shape. They rest loosely on the back, having scarcely any attachment except by muscles to any of the neighbouring bones. By this means they have a free and easy motion, and also communicate the same property in part to the arms, the upper bone of which is attached, on each side, to the scapula. A very small cavity in the latter bone admits the round ball-like head of the humerus, giving to it the most unconfined play of movement, whether of a rotatory kind, upwards, downwards, or sideways. Nothing can be more beautiful than the whole arrangements for permitting the arm to perform the multifarious motions which man requires from it. The humerus, a single bone in each arm, bb, cylindrically shaped, is united at the elbow-joint, cc, to the two bones of the fore-arm, termed the radius, dd, and ulna, ee. One of these, the ulna, is attached to the humerus by a hinge-joint (like that of a common door), while the found, till this apparently petty ligament was noticed to have given way, producing instantaneously the fatal result.

| radius is connected to the same bone by a round button-like head, which, being slightly concave, receives a projecting knob of the humerus, and admits of rotatory movements being performed by the lower part of the arm. These peculiarities of structure are essential to the free use of the hand. At the wrist, the position of the radius and ulna is in some measure reversed, the radius forming with the carpal bones, ff, a joint like that of a door-hinge, while the ulna is in a measure left loose. The carpal or wrist bones are eight in number. They are of small size, and lie in two rows, being jointed together in a manner that combines great strength with a certain degree of mobility. In the direction of the points of the fingers, they are united with the metacarpal bones, forming the palm of the hand, and to which the phalanges, gg, or finger bones, are attached. Each finger has three bones in it; the thumb has only two.

As has been said, the bones of the pelvis, on each side, are marked by deep cup-like concavities, which receive the heads of the thigh-bones, hh, ii, (femur) the upper bones of the lower extremities. As was required by the different nature of the purpose to be served, the ball-and-socket joint of the leg is much stronger than that of the arm, and permits of much less freedom of motion. The femur or thigh-bone is a rounded cylindrical bone, terminating at the knee in a connexion with the tibia, m, the principal bone of the inferior part of the lower extremity. The knee-joint is a hinge one, but permits of a slight rotatory motion when the leg is bent. The tibia has a smaller bone, the fibula, nn, placed by its side, and over the knee-joint is situated a small bone called the patella, ll, or knee-pan, to which the principal muscles that move the joint are attached, and which serves to protect the parts against injury. The tibia and fibula form a union at the ankle, oo, with the bones of the tarsus, which are seven in number, and constitute the heel or back part of the foot. These again are united to the metatarsal bones, pp, forming the body of the foot, and five in number. To these again are joined the phalanges of the foot, fourteen in all, two being attached to the great toe, and three to each of the others.

The coste or ribs, rr, proceed from the vertebræ or backbones, and are twelve in number on each side. They bend round in a circular manner from their point of union behind, and seven of them, called the true ribs, are joined directly by gristle or cartilage to the breast-bone, while the remaining five terminate anteriorly in a common cartilage, which unites with the sternum below. Altogether, the ribs form a large hollow space for the reception of the lungs, heart, and other organs, and protect them from injury. The ribs move in an easy joint formed with the back-bone, and, with the intercostal muscles, contract and expand to suit the motions of the lungs.

These are the principal bones forming the skeleton of the human being. All animals have not this osseous framework; it is only found in a certain number of classes, including man, quadrupeds, birds, reptiles, and some fishes, all of which, from the principal feature in their structure, are called vertebrated animals. Some of the other tribes of beings have their framework, corresponding in purpose to bones, on the outside of the body, in the form of a coat of mail. This is the case with the shellfish, as the lobster, and with many insects that have a hard external covering, as beetles.

THE MUSCLES.

The soft fleshy substance of the body, which gives plumpness and form to the whole, is the muscular part, or muscles. These are the instruments of motion. And when we consider the various positions which the body and its members assume, the agility and quickness with which the most intricate movements are made, the ceaseless play of the heart, the heaving of the lungs, and the singular rapidity of articulation and speech, we need not be surprised that these muscles, upon which all such movements depend, should be many in num