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noticed in the blood cells of the chick, or endogenous transformation of its nuclei, as occurs in cartilage. 8. Change of form, special to each' tissue. 4. Chemical changes of the contents; and 5. The generation of nerve force, which is the most wonderful of all its powers.
These remarkable objects can be artificially generated, for it will be found that perfect cell-walls are produced when an intimate admixture of oily and albuminous matters is made.
The Cell theory supposes that all organic bodies, no matter how complex their construction, as likewise all their tissues, modified though they may be, have originated from a cell or cells. Of the tissues which are thus temporarily cellular may be mentioned, fibrous tissue, muscle fibrils, nerve tubes, blood-vessels; and of those that remain permanently so, the following are examples— lymph and blood cells, nerve cells, fat cells, pigment cells, gland cells, cartilage cells; and, amidst the products of disease, pus cells, cancer cells, and many others. Thus, both in Physiology and Pathology does the aphorism seem applicable, "Omnis cellula e cellula."
There are some, however, who advocate the "molecular theory," which supposes that cell formation is preceded by molecules, or minute granules, which therefore should be considered the histological starting point. Thus Prof. Bennett believes that the molecules of chyle and the intimate constituents of muscle must be regarded as antecedent to the existence of any cell.
There are, again, some matters in the human body which, from the non-discovery of any structure, are called amorphous or hyaline; for example, the cell-wall, basement membrane, myolemma, neurilemma, the intercellular substance of cartilage, the elastic laminae of the cornea, capsule of the crystalline lens, &c.
The illustrious Baron Holler regarded fibrous tissue as the simplest constituent of the human body, remarking that the fibre was to the physiologist what the line was to the geometrician; and many later histologists have adopted his view.
The general properties of the tissues may be here briefly discussed; and they readily arrange themselves as, 1. Physical; and 2. Vital.
1. Divisibility has always excited the wonder of those philosophers who study the phenomena of inorganic bodies, and the minuteness of many living organisms is no less astonishing. Thus, there are about eleven millions of exquisitely regular cells in the choroid pigmentary membrane, sixty-five and a-half millions of fat cells in the space of a cubic inch of adipose tissue, and one hundred and fifty millions of moving cilia throughout the air tubes of man. Activity is always proportional to minuteness of division.
Extensibility is a universal property of animal tissues; but its amount varies most widely, being, like the cognate property of pliancy, dependent on the amount of water each contains. The order of those of the dead body is as follows—muscle, vessels, nerve, hair, tendon, and bone.
Elasticity, by which the tissues re-act when a compressing or extending force is removed, is another striking physical endowment, especially conferred on the yellow fibrous tissue. Between the atoms which constitute living as well as inanimate bodies spaces must be conceived to exist, more or less considerable as the state of aggregation be the gaseous, liquid, or solid forms. To this porosity is due the osmose and dialysis, or interchange of liquids and gases which, as the following pages will show, constitutes the very fundamental nature of most physiological processes. In the human body there are likewise constantly taking place chemical changes, which will be presently referred to, and which are essential to the continuance of life. They are moreover so diverse, that the truths about them have been justly remarked to be more wonderful than the fabled transmigration of souls.
2. The Vital Properties of animal tissues are evidenced by molecular changes which follow the application of a stimulus. Such are contractility, the endowment of muscle, and neurility, which term conveniently groups the phenomena of common sensation, special sensation, and motive influence, with which both vesicular and tubular neurine are concerned, and the higher intellectual endowments of the nervous centres.
Between the physical forces, as Orove has demonstrated, and likewise between the vital, there exists mutual dependence or correlation; and it is the intimate union or sum of their phenomena we designate as "Life."
Before discussing the several functions, and the structural anatomy of the organs by which they are performed, I have deemed it advisable to notice briefly the characters which place man at the head of the organic world, and to consider the chemical features of the substances which constitute his body.
NATURAL HISTORY OF MAN.
Mail, zoologically considered, belongs to the class Vertebrata; order, Mammalia; sub-order, Hominidae; genus, Homo; and species, Sapiens; according to Linnaus, who gave as his characteristic the aphorism, "Nosce te ipsum." In the words of Owen, "man is the sole species of his genus (Homo)—the sole representative of his order (Bimana);" and it is equally unscientific and irreverent to group him, as Linnaus and some other naturalists have done, with Quadrumana, in the order Primates. His distinctive characters are thus enumerated by Blumenbach:—" Erect, two-handed, unarmed, rational, endowed with speech, a prominent chin, four incisor teeth, above and below; all the teeth equally approximated, the canine teeth of the same length as the others, the lower incisors erect."
The erect posture, an attitude peculiar to the human species, is preserved by the occipital condyles being placed on the base of the skull, as near the centre as can be allowed without the heavy, bony cranium and brain overbalancing the light, spongy bones of the face. The equilibrium is, however, maintained in some degree by muscular power; for if this is suspended, as when we nod in sleep, the head falls forward. The cranium then rests easily on the vertebral column, and all the great occipital and vertebral ridges are absent; the ligamentum nuchae is hardly developed. The face is placed rather below the cranium than in front of it, as in brutes— even the most anthropoid. This circumstance is best determined by what Camper termed "the facial angle," formed by drawing one line from auditory meatus to root of nose, and another touching the camper's Facial Angle, 85°. forehead and upper jaw, as here represented. The angle so included increases through mammalian skulls till it becomes 35° in the Troglodytes Gorilla, the most anthropoid of apes, according to Owen, who has shown that the much greater angle assigned to this animal by other writers depends on their having examined the skull before second dentition, after which period the jaws become more prognathous, or snouty, and the angle much less. Even in the adult gorilla's skull, the angle is not an index of cerebral proportions, as it is greatly exaggerated by a prominent supra-orbital ridge. From 70° in the Negro it increases to 80° or 85° in the European, and to 90° or 100° in the statues of Apollo and Medusa of Sisocles, which the Grecian artists thus endowed with sublimity and beauty. The difference between the quadrumana and man is vastly greater than between any human varieties. The angle as found in the Ethiopian and Caucasian races, and in the Chimpanzee, is here represented. Prof. Owen draws the lower line from occipital condyles along floor of nose, and thus includes a greater angle. The orbits are