A, afferent nerve;

volitional action. 1, sensitive surface. 2, motor structure. C, efferent nerve; B, nerve cell (for simple reflex action). D, afferent nerve; F efferent nerve; E, nerve cell (for volitional action). (p. 238)

The Structure of Bone and Muscle, and the Modus Operandi of Reflex Action.

of nervous matter, the spinal cord, which it encloses and protects. Each vertebra corresponds to, and is developed in connection with, one of the primitive ganglia of the nervous system. Above it and below it emerge the spinal nerves. So that a vertebra has been defined to be "a bone or axial segment of the skeleton included between two pairs of nerves." The subsequent growth of the column, however, faster than the cord, disturbs this topical relation so that the cord which at the third month of intra-uterine life reaches as low as the end of the sacrum, if not to the coccyx, stops short in the adult at the second lumbar vertebra. The spinal nerves then come to pass down from the cord and out through the intervertebral foramina with such a degree of obliquity, increasing from the first to the last pair, that it would seem as though the cord had been drawn upwards, bodily, within the spinal canal.

The Skeleton.

The bones of various shape and size, jointed together in various ways, constitute the skeleton or scaffolding, whose disposition determines the size, shape, posture, in short, the configuration and habitus of the animal:-Ossa autem corpori humani formam, rectitudinem et firmitatem conciliant (Galen).

The word skeleton is often said to be derived from oxéλλ, to dry up, in the sense in which this term was used by Herodotus who speaks of the sole aridum et exsiccatum cadaver, which the Egyptians exhibited at their festivities, with the greeting, edite et bibite, post-mortem tales eritis! Skeleton is more probably derived from oxos, thigh bone, which, as the largest bone in the body, had its name used as a designation for the whole structure (Hyrtl).

In the crustacea, molluscs and in insects, the skeleton,

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GENERAL PROPERTIES OF BONES.

of shell or horn, is on the exterior of the body. The first example of internal skeleton is met in the cephalopodous molluscs in which certain cartilaginous plates are inclosed in the body of the animal protecting certain parts of the nervous system.

General Properties of Bones.

The elongated, tubular, bones of the extremities serve as levers for the muscles, to the action of which, by their restriction of movement, they lend precision and effect. Hence the bones are often mentioned as the passive organs of locomotion. Spread out in surfaces, more or less flat, or stretched out in rods, more or less flexible, they form cavities, the skull; or basins, the pelvis; or cages, the thorax, for the protection and support of the softer organs. Broken up, as it were, into smaller fragments, with irregular surfaces and spongier structure, often with intercalated cartilaginous pads, they act as bumpers or cushions to break the force of shocks, as at the wrist and ankle and in the spinal column. Contracted in their shafts, or alternately convex and concave in their course, as in the long bones and vertebral column, they give space for the origin and lodgment of muscles, and support to the heavier organs, and expanded at their ends, they widen the surface of articulation, afford room for the insertion of ligaments and tendons, and furnish fulcra for the action of the muscles, by distancing the point of insertion from the centre of motion. Increased in weight and size near the trunk, they give momentum and range to the movements of the body, and abbreviated in all their dimensions at their distal extremities, they lend celerity and accuracy to its various actions. They are thickened into massive bars and plates as in the pubes and ilium, thinned down to papyraceous sheets, or rolled up in delicate scrolls as in the nasal foss, projected in vast promontories

THE HAVERSIAN CANALS.

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in the various tuberosities, or reduced to insignificant fragments as in the bones of the ear, all for purposes familiar to every student of anatomy.

The Histology of Bone.

The microscopic appearance of bone is characteristic. For bone belongs, histologically, to the group of the connective tissues and hence consists, primarily, of a network of stellate ramifying cells (bone cells, bone corpuscles, corpuscles of Purkinje, etc.). In the subsequent process of ossification these cells desiccate and leave spaces, lacunæ, filled with serum or air (carbonic acid gas). The hardness of bone is due to the deposit of various mineral salts, principally of lime, in the intercellular spaces. Any doubt as to the nature of a substance is at once resolved under the microscope, so far as bone is concerned, at a single glance. The distinguishing characteristics of bone are four, viz., First,

The Haversian Canals.

These canals are the conduit tubes for the blood vessels, the main trunks of which penetrate the surface of the bones at the so-called nutritious foramina. Reaching the interior of the bone, the canals ramify throughout its substance, varying in size from of an inch (0.1128-0.0149 mm.) in diameter, so completely supplying all parts of it, that no osseous tissue is removed from its nutrient blood at a greater distance than the of an inch. The abundance of the blood supply to bone imparts to it a pinkish hue, though the fundamental color of bloodless bone is a pale gray, except in some fishes where it is green. Bone, like all other vascular structures, is liable to inflammations, and wounded bone bleeds. In cross sections of bone, the blood vessel canals, called also the