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"bi-lateral symmetry" extends to the arrangement of those internal parts which are connected with the functions of animal life; namely, the nervous system, the organs of sense, and the muscular apparatus. But it does not always extend to the organs of nutrition, which are unequally disposed on the two sides: thus, in Man, the heart and stomach are on the left side, and the liver on the right, while the lungs are much larger on the right side than on the left. But in many of the lower Vertebrata, there is an almost perfect symmetry in the disposition of these organs, as there is also in the early embryo of those in which this symmetry is subsequently departed from; so that it may be truly said that this symmetry is characteristic of the Vertebrate type, although for special purposes it is frequently superseded.

Fig. 25.-SKELETON OF THE OSTRICH. 71. In all Vertebrated animals, the skeleton is chiefly internal (fig. 25); and consists of bones, which are capable of

growing, and of being reproduced after injury, like any other part of the living tissue; being copiously supplied with bloodvessels, which penetrate into their interior. These bones give support, and afford points of attachment, to the soft parts, in the limbs (where they exist) as well as in the trunk ; but the former are not unfrequently wanting, as in Serpents: and we must look

in the trunk, therefore, for that peculiar arrangement which is characteristic of this division of the Animal Kingdom. The back-bone, as it is commonly termed, is found in all Vertebrated animals; though in a few among them (the lowest Fishes) it is very imperfect (§ 53). It consists of several pieces jointed together, so as to possess great flexibility; whilst they are so firmly connected by ligaments, that they cannot easily be torn asunder or displaced. The number of these pieces varies considerably; in Man there are only 33; in some long-tailed Mammals there are more than 70; but in many Serpents there are several hundred. Each of them is termed a vertebra; and the whole structure, composed of the Fig. 26.-VERTE-united vertebræ, is termed the vertebral column BRAL COLUMN. (fig. 26). The ordinary character of the vertebræ is, that each is perforated by an aperture, which, united to the corresponding apertures of those above and below it, forms a continuous canal; and in this canal, one of the most important parts of the nervous system, the spinal

VERTEBRA.

C

cord (commonly but erroneously termed the spinal marrow), is contained. The solid portion of the vertebra (fig. 27, a) is termed its body; and the projections, b and c, are termed its processes, the former spinous, the latter transverse. The row of spinous processes forms the ridge which we Fig. 27.-SINGLE feel passing down the back; it is seen on the right-hand side of fig. 26. To the transverse processes the ribs are attached. The vertebral column is expanded (as it were) at its upper extremity, to form the skull; in the large cavity which it contains, the brain is lodged; and its bones are so arranged as to give protection to the organs of sense also. At the opposite extremity we see it contracted into the tail; which is composed of a series of vertebræ

NERVOUS SYSTEM OF VERTEBRATA.

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resembling those of the back, but simpler in their form, and not possessing a cavity for the spinal cord. We commonly find that in those animals in which the skull is very large, the tail is short; and that where the tail is very long or powerful, the head is small. Thus in man and in the apes, the head is large, and there is no external appearance of a tail; but there are some very imperfect vertebræ at the lower end of the spinal column, which constitute the rudiment of it. In the long-tailed monkeys and in the kangaroo (whose tail is like a third hindleg), the head is comparatively small. But this rule does not hold good universally.

72. The Nervous system of Vertebrated animals consists of a Brain and Spinal Cord (fig. 28), which are lodged within the skull and vertebral column; and of nervous trunks proceeding from these, which are distributed to all parts of the body. The Brain is not (as commonly reputed) a single organ, but is composed of a number of ganglionic masses, differing considerably in their functions. Thus each of the nerves of special sense (smell, sight, hearing, and taste) has its own proper centre; and there is another of considerable size, which seems to perform the same office in regard to common sensation. These are found in Vertebrata generally; and their proportionate size corresponds with the relative development and activity of the several organs of sense with which they are connected. The

CORD OF MAN.

bulk of the brain of Man, however, is Fig. 28.-BRAIN AND SPINAL made up by two large masses of nervous matter, which are known as the Cerebral Hemispheres; these, as will be shown hereafter (chap. x.), are so small in the brains of Fishes as to be scarcely distinguishable; and their relative size

88

NERVOUS SYSTEM OF VERTEBRATA.

and complexity of structure increase as we ascend the scale, in pretty close accordance with the increase of the intelligence or reasoning faculty. There is also another large nervous mass, called the Cerebellum; the function of which seems to consist in the regulation of the more complex movements. The Spinal Cord is made up of a longitudinal succession of independent centres, of which one corresponds with each of the vertebral segments of the body.

73. The distinguishing feature of the Nervous system in Vertebrata is, that its several centres are thus united into one large mass, instead of forming a number of separate small masses or ganglia, as we shall find that they do in the lower classes of animals and that it is inclosed in the bony casing which has been described as peculiarly destined for its protection, instead of being enveloped with all the other organs in a hard covering, as in the Lobster, or of being entirely destitute of protection, as in the Slug. That it should receive this peculiar protection is quite necessary, in consequence of the much higher development which it attains, and the much greater importance which it possesses, in this division of the animal kingdom, than in any other. In all but the very lowest Vertebrata, all five kinds of sensation exist;—namely, sight, hearing, smell, taste, and touch. We find in this group more intelligence than in any other; that is to say, the animals composing it act more with a designed adaptation of means to ends; instead of being impelled by a blind instinct to perform actions of whose objects they are not aware. And we find, by observing and comparing the structure and actions of the dif ferent groups, that the intelligence gains upon the instinct, as we ascend from the lowest Fishes towards Man, in whom the intelligence is at its highest; whilst we observe a similar increase in the proportion which the brain bears to the rest of the nervous system. Hence we conclude, that the brain is the organ of intelligence, or of the reasoning faculties.

74. The general arrangement of the other organs in Vertebrated animals, is shown in fig. 29. At m is seen the mouth, forming the entrance to the digestive cavity, of which the termination is at the opposite extremity of the body; i, i, is the intestinal canal, and 7, the liver: these organs occupy the part of the body which is called the abdomen or belly. The mouth also opens, however, into the windpipe, or trachea, t,

GENERAL STRUCTURE OF VERTEBRATA.

89

which conducts air into the lungs, p; these organs, with the heart, h, are contained in the portion. of the trunk called the

Fig. 29.-DIAGRAM, SHOWING THE POSITION OF THE PRINCIPAL ORGANS IN VERTEBRATA.

thorax, or chest. At b is seen the position of the brain; and at s that of the spinal cord.

75. The foregoing characters apply, with greater or less modification as to details, to the classes of Mammals (commonly termed Quadrupeds), Birds, Reptiles, and Fishes; and these further agree in the following points, all of which, therefore, enter into our idea of a Vertebrated animal. The number of limbs or members never exceeds four; and of these, two, or even all four, may be absent. In all the classes just named, four is the general number; and the absence of two or more is the exception. Thus in Mammals, we find all four present in every tribe save that of Whales, which want the hinder pair; though the upper or anterior pair may take the form of arms, wings, legs, or fins, according to the element which the animal is formed to inhabit. In Birds we find the posterior pair invariably present in the form of legs; whilst the anterior pair, though almost always developed into wings, is absent in a few instances. In Reptiles we find considerable variety; all four members are present in the Turtle tribe, and in most Lizards, as well as in the Frog tribe; but they are entirely absent in the whole tribe of Serpents; and there are Lizards which have only one pair. And in Fishes, we usually find two pairs, constituting the pectoral and ventral fins; but one or both pairs are sometimes absent, as in the Eel, Lamprey, &c. We have further to remark, in regard to the general characters of Vertebrated animals, that,