590 DEVELOPMENT OF CIRCULATING APPARATUS. arterial trunk that springs from the ventricle, divides into a set of arches on each side (fig. 326), which closely resemble the branchial arches of Fishes and Tadpoles. Although no gills are present, yet there is a series of clefts on each side of the neck, passing through to the pharynx, which are analogous to the branchial apertures of the cartilaginous Fishes (§ 317). After a time, however, the auricle and ventricle of the heart are each divided by a vertical partition, so that four cavities are formed, out of the two which previously existed; and at the same period, the arrangement of the vessels undergoes a change, by the division of some trunks, and the obliteration of others, so that they gradually assume the distribution which is characteristic of warm-blooded animals (§ 281). But even up to the time of the birth of the Mammalia, there is a communication between the two sides of the heart, and between the pulmonary and systemic vessels, which is closely analogous to that which permanently exists in the Crocodile (§ 283). 763. Again, the space within the head of the embryo, into which the vertebral canal widens-out (§ 757), is occupied in the first instance by a succession of vesicles or bags, arranged in a linear series (fig. 323, d, e, ƒ); each of which is the rudiment of one of those principal ganglionic masses, that collectively make-up the brain of the Fish (§ 453), in which they present a very similar aspect (fig. 192). As in many Fishes, too, the Cerebrum is inferior in size to the Optic ganglia, and only comes to surpass and finally (as it were) to overpower them (§§ 455, 456) in the later periods of embryonic development. 764. The true representation of these and similar facts is not, as was maintained when they were first brought into view, that the several organs of the higher animals go through a series of forms which remain permanent in the lower; but that the development of all animals formed upon the same general plan commences in the same manner, their special differences manifesting themselves as development proceeds, Thus, as we have seen, the foundation of the Vertebral column is laid in all Vertebrata in precisely the same method (§ 757); in some of the lowest Fishes, the evolution of this structure is checked at so early a period that it never advances beyond the embryonic type (§ 53); but the fully-formed spine has a VON BAER'S LAW OF DEVELOPMENT. 591 characteristically-different structure in each of the classes of Vertebrata, which is not presented at any period in the history of the others. So, the evolution of the Circulating apparatus commences in all Vertebrata upon the same original plan; and from this plan there is but little departure in the Fish; but the circulating apparatus of the early Human embryo, however like that of the adult Fish, differs from it in this essential particular, the absence of gill-tufts receiving capillary vessels from the branchial arches (§ 286). The like is true in regard to the Nervous centres; for although the earliest condition of the Human brain very closely resembles that of the brain of the foetal Fish, it never bears any exact analogy to that of the adult Fish. 765. Hence the principal facts of Organic Development admit of being stated in this general formula, which we owe to the sagacity of Von Baer,-that the more special forms of structure arise progressively out of the more general,—a principle than which there is none more comprehensive or more important in the whole range of Physiological Science. 766. THE Unity of Plan which is visible through the whole Animal Kingdom, is nowhere more remarkable than in the function of which an outline has now been given. We have seen that, however apparently different, the essential character of the Reproductive process is the same in the highest Animal as in the lowest. It has been shown that the development of the highly-organized body of Man,-though it is to serve as the instrument of those exalted faculties, by the right employment of which he is made "but a little lower than the Angels," commences from the same starting-point with that of the meanest creature living: for even Man, in all the pride of his philosophy, and all the splendour of his luxury, was once but a single cell, undistinguishable, by all human means of observation, from that which constitutes the entire fabric of the simplest Protozoon. And when the Physiologist is inclined to dwell unduly upon his capacity for penetrating the secrets of Nature, it may be salutary for him to reflect that,- -even when he has attained the furthest limits of his Science, by advancing to those general principles which tend to place it on the elevation which others have already reached,—he yet knows nothing of those wondrous operations, which are the essential parts of every one of those complicated functions by which the life of the body is sustained. Why one cell should absorb,-why another, that seems exactly to resemble it, should assimilate,-why a third should secrete,-why a fourth should prepare the reproductive germs,-and why, of the two germs that seem exactly similar, one should be developed into the simplest Zoophyte, and another into the complex fabric of Man, are questions that Physiology is not likely ever to answer. All our science is but the investigation of the mode or plan on which the Creator acts; the Power which operates is Infinite, and therefore inscrutable to our limited comprehension. But when Man shall have passed through this embryo state, and shall have undergone that metamorphosis by which everything whose purpose was temporary shall be thrown aside, and his permanent or immortal essence shall alone remain, then, we are encouraged to believe, his finite mind shall be raised more nearly to the character of the Infinite, all his highest aspirations shall be gratified, and neverending sources of delightful contemplation shall be continually opening to his view. The Philosopher who has attained the highest summit of mortal wisdom, is he who, if he use his mind aright, has the clearest perception of the limits of human knowledge, and the most earnest desires for the lifting of the veil that separates him from the Unseen. He, then, has the strongest motives for that humility of spirit and purity of heart, without which, we are assured, none shall see God. INDEX. A. N.B.-The numbers refer to the paragraphs. ABERRATION, chromatic, 548, 549; ABSORPTION, nutritive, 171; in Verte- Abstinence, power of, 140, 141, 152. Acephalous Mollusca, 111, 113. Aeration of blood, 253, 303. Agamic Reproduction, of Insects, 746, bladder of Fishes, 324. tubes and sacs of Insects, 320-322. Albumen, chemical composition of, 13, Albuminous principles of food, 153; des- Aliment, definition of, 132 (see Food). Amphibia, 85, 87; circulation in, 287- Anabas, respiratory organs of, 318. Anemone, Sea, 126, 127; reproduction Animals, distinctive characters of, 6- Animalcules, structure of, 13, 136, 137; Annular ligament, of wrist, 641; of Anobium, sound of, 677. Antennæ of Insects, uses of, 498. Aorta, 258; valves of, 273. Aphides, agamic reproduction of, 727, Aplysia, nervous system of, 438. ARACHNIDA, general characters of, 106; vasculosa, 758. Areas, comparative, of arteries, 247. Areolar tissue, structure and properties Argonaut, reproduction of, 753. Arm, bones and muscles of, 638-640. °° Articulation of bones, different modes Arytenoid cartilages, 680, 681. Ascidia, 114; nervous system of, 435. Attention, effect of, on sense of touch, Auricles of heart, 257; action of, 270. Azotized principles of food, 153, 154; Azotized compounds, excretion of, 346 B. Balancing of body, 480, 481. Balanus, 102; development of, 749. Barnacle, 102; development of, 749. Bat, wings of, 669; ear of, 515; hyber- 494. Batrachia, 86, 87; reparative power of, Beat of heart, 269, 271. Beaumont, Dr., his experiments upon Bile, chemical characters of, 364; use of, Bilious complaints, 350. Bladder, gall, 362; urinary, 362. Blood of Invertebrata, character of, 235. red, 35, 229-234; ac- Blood-vessels, 244; formation of in new Bone, structure of, 49, 50; chemical Brain of Vertebrata, 72; not the only Bronchial tubes, 326, 328. Bryozoa, 115; (see Polyzoa). Buds, reproduction by (see Gemmation). Bulk required in food, 205. C. Caddice-worms, 701. Camel, stomach of, 198; skeleton of, 644. Cancelli of bone, 49; formation of, 52. Capillary vessels, 251; movement of Carbon, modes of excretion of, 345–351, |