146 CLASSIFICATIONS OF THE TISSUES. These substances are then absorbed from the shriveled cell wall, which, in turn, finally disappears. Lastly, the protoplasm of the cell may be substituted by the salts of lime, and, undergoing this calcareous degeneration, cease to functionate as cells. The vast majority of cells die a natural death by fatty or calcareous degeneration. Recapitulation. The body of an animal or plant consists, as we have seen, of a single cell, or of an aggregation of such cells. The single-celled bodies are said to be simple, the manycelled, compound bodies. These structures are, however, in essence the same; the simple bodies being simply separated further from each other. One such body (cell) depends for its existence (sustenance) upon other bodies. In the compound body, the single cells are also in a sense individual, yet they are all mutually dependent. A compound body is like a colony of ants or a hive of bees; completely isolated individuals perish. The cells in a compound body are connected together by fluid, or more or less solid, intercellular substance. The intercellular substance is the product of cells. A compound body is made up of cells and intercellular substance. Classifications of the Tissues. Like cells, differentiated from the rest, and grouped together for a special purpose, constitute the organs or tissues of a compound body. The body thus comes to be made up of organs and tissues, each having a special and particular purpose to subserve for the benefit of other organs and tissues as well as of itself. The body is, hence, constructed upon a plan of a division of labor; just as society is composed of farmers, tradesmen, thinkers, etc. ANATOMICAL AND CHEMICAL CLASSIFICATION. 147 Anatomical Classification. If we regard the tissues of the body simply from an anatomical stand-point, we recognize:— 1. Tissues composed of simple cells, with fluid intercellular substance, as the blood, the lymph and chyle. 2. Tissues composed of simple cells, with a small. amount of solid intercellular substance, as the epithelium, nail, etc. transformed cells 3. Tissues composed of simple or cohering (in some cases), situated sometimes in homogeneous, sometimes in fibrous, and, as a rule, more or less solid intermediate substance, as cartilage, colloid tissue, adipose tissue, fibrous and elastic tissue, dentine and bone (connective tissue group). 4. Tissues composed of transformed, and, as a rule, non-cohering cells, with scanty, homogeneous and more or less solid, intermediate substance, as enamel, lens and muscle. 5. Tissues so mixed as to admit of no grouping under any of the above heads; as nerve tissue, gland tissue, vessels and hairs. This is the histological classification of Frey. Chemical Classification. If we regard the body from a purely chemical standpoint, we shall have to recognize its separation into the three great classes of proximate principles: 1. The inorganic principles; definite in their chemical composition, crystallizable, derived exclusively from without (forming to great extent the crust of the earth), subserving their special purpose in the body and then being voided from it, having undergone little or no change; as water, common salt, phosphate of lime, etc. 148 PHYSIOLOGICAL CLASSIFICATION. 2. The non-nitrogenized or hydro-carbonaceous principles; which, as their name implies, contain no nitrogen, but are made up of carbon in large quantity, and of hydrogen and oxygen; principles which do not belong to the crust of the earth, but are formed exclusively in the bodies of animals and plants, where they undergo such changes as to be entirely broken up and consumed as such; as starch, the sugars and the fats. 3. The nitrogenized, albumenoid or protein bodies; indefinite in their chemical composition, containing nitrogen and mineral matters in addition to the carbon, hydrogen and oxygen, varying in consistency according to the consistence of the organ in which they are found, hygroscopic (that is, having the power to absorb water and be restored to their original consistence after desiccation), coagulable, spontaneously or artificially, and undergoing fermentation and putrefaction, by serving as food to microscopic animals and plants, which split up their chemical combinations into new compounds (alcohol, carbonic acid gas, water and other products of decomposition), principles also formed exclusively in the vegetable or animal cell and undergoing entire change in the body. Examples of this class of proximate principles, which includes also the coloring matters and certain crystallized products of excretion, are albumen, fibrin, casein, myosin, pepsin, lecithin, hæmoglobulin, urea, etc. This is the chemical classification of Robin and Verdeil. Physiological Classification. If, however, we regard the construction of the body from a purely physiological stand-point, we shall have to separate the tissues into classes according to pre-eminence in especial properties, with all the rest of which each one is endowed in subordinate degree, as : PHYSIOLOGICAL CLASSIFICATION. 149 1. The tissues eminently mechanical; bone (including teeth), outside layers of epithelium (including hair and nails). 2. The tissues eminently contractile; the muscles. · 3. The tissues eminently irritable; the nerves (and nervecenters). 4. The tissues eminently secretory (including excretory); the digestive, genito-urinary, pulmonary, etc., epithelium. 5. The tissues eminently metabolic (elaborative or transformative); the gland cells. 6. The tissues eminently reproductive; the ovary and the testis. This is essentially the classification of Michael Foster. A study of these three methods of classification gives a clear survey of the construction and action of the various tissues in the body, whether simple or complex. These properties and actions in the simplest forms of protoplasm, like the amœba, are very few and very limited. Under favoring conditions and in many millions of years, aggregated masses of protoplasm build up the most complicated structures. At various periods in the history of our earth, "In days of yore, no matter where or when different animals have successively held the dominant place. At the present time the highest and most complicated structure is the body of man. But the principles of construction, the general properties of composition, remain everywhere the same. What is true of the monad, is true of man. The binary compounds of inorganic matter, carbonic acid gas (COO), and water (HHO), are raised in organic matter to ternary compounds, starch, etc. (CHO), and upon the question whether this rearrangement of elements happens naturally, i. e., chemically, or supernaturally, rests the most momentous problem of our day. 150 BONE AND ITS PROPERTIES. LECTURE VIII. BONE AND ITS PROPERTIES. CONTENTS. Anatomical Dignity of Bone-Relation of Bone to Nerve TissueThe Skeleton-The General Properties of Bones-The Histology of Bone-The Haversian Canals-The Lamella-The Bone Corpuscles and Lacunæ-The Canaliculi-The Chemistry of Bone-Difficulties Attending the Study of Osteology-Bones as Fucl-Gelatine as an Aliment-The Resistance and Resilience of Bone-Constancy of Chemical Composition-Rachitis and Ostco-Malacia-The Phosphate of Lime-The Preservation of Bone-Bone a Connective TissueThe Formation of Bone-The Periosteum-The Centre of Ossification -The Determination of Age-The Femoral Epiphyseal Centre-The Excavation of Bɔne-Air in Bone-The Marrow-Studies in Living Bone-Pone as a Symbol of the Body. The Anatomical Dignity of Bone. Bone has a structural dignity which is attained, with one exception, by no other hard or solid formation in the economy of nature. The existence of true bone in the body of an animal brings it to rank at once among the class of vertebrates. Wherever is bone, is a spinal column. The hard parts of all invertebrate animals are made of horn and shell, never of true bone. Even the teeth, which alone among other solid structures enjoy the same exceptional place, from the stand-point of comparative anatomy, could not be present at all, were it not for the fact that their roots are incrusted with a layer of bone, the cement, whose periosteum fusing with that of the alveolae of the bones of the jaws, secures their fixation in the body. Relation of Bone to Nerve Tissue. For the bone in the spinal column of the vertebrata stands in the most intimate relation to the great chain |