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Updated: June 4, 2025
Even in this modified chordaea-theory the six fundamental organs of the chordula would retain their great value. The medullary tube would be originally a chemical sensory organ, a dorsal olfactory tube, taking in respiratory-water and food by the neuroporus in front and conveying them by the neurenteric canal into the primitive gut.
They consist of a rod, or central core, continuous throughout the whole length of the nerve, called the axis cylinder. This core is surrounded by the white substance of Schwann, or medullary sheath, which gives the nerve its characteristic ivory-white appearance. The whole is enclosed in a thin, delicate sheath, known as neurilemma.
The excavations in the Montesquieu-Avantes Cave, about six miles from Saint-Girons, have brought to light a hearth covered over with a layer of stalagmite; numerous fragments of human bones, crania, femora, tibiae, humeri, and radii were found in this layer, and in that of the subjacent clay. In many cases the medullary orifice had been enlarged to make it easier to get out the marrow.
Central or medullary tumours as they increase in size replace the surrounding bone, and simultaneously new bone is formed on the surface; as this is in its turn absorbed, further bone is formed beneath the periosteum, so that in time the bone is increased in girth, and is said to be "expanded" by the growth in its interior.
MECHANICS is one of the sciences by which an entrance is made into things rational, which are the ground of rational wisdom. 163. MEATS, There are in heaven, as in the world, both meats and drinks, 6. See Food. MEDIUMS are conducive to what is first in itself, 98. MEDIUM, the, of conjunction of the Lord with man, is the Word, 128. MEDULLARY substance of the brain, 315.
Their number varies; it is small when the mother, disturbed in her operations, has flown away to continue her work elsewhere; but they number thirty or forty, more or less, when they contain the whole of her eggs. Each one of the perforations is the entrance to an oblique tunnel, which is bored in the medullary sheath of the twig.
Man's ancestors are skull-less Vertebrates, like the Amphioxus. The body is a series of metamera, as several of the primitive segments are developed. The head contains in the ventral half the branchial gut, the trunk the hepatic gut. The medullary tube is still simple. No skull, jaws, or limbs. The number of metamera increases.
When the new bone is laid down in the Haversian canals, cancellous spaces and medulla, the bone becomes denser and heavier, and is said to be sclerosed; in extreme instances this may result in obliteration of the medullary canal.
One of these, especially constant and large in many bones, is called the nutrient foramen, and transmits an artery to nourish the bone. At the ends of a long bone, where it expands, there is no medullary canal, and the bony tissue is spongy, with only a thin layer of dense bone around it. In flat bones we find two layers or plates of compact tissue at the surface, and a spongy tissue between.
In Figs. 11 and 12, which show a few tracheæ and medullary rays of the ligneous bands of the same plant, we observe the same phenomenon. We might cite a large number of analogous examples, but shall be content to give the following: Figs. 13 and 15 represent radial and tangential sections of the bark of Syringodendron pes-capræ.
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