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The articular aspect of the epiphysis is invested with a thick layer of hyaline cartilage, known as the articular cartilage, which would appear to be mainly nourished from the synovia. The external investment the periosteum is thick and vascular during the period of growth, but becomes thin and less vascular when the skeleton has attained maturity.
There really is no perceptible reason why the fiber should become weaker through fructification, which simply consists in the fact of the contents of the vascular cells changing into soluble matter, and gradually oozing away, the consequence of which is that the cells of the fiber are not replenished.
At present, whether the fault is in the cell proper, the conducting media, or the central center, we are unable to say. It is certainly not due to any vascular disturbances, as the skin shows no vascular changes. White spots on the nails are quite common, especially on young people. The mechanic cause is the presence of air between the lamellae of the affected parts, but their origin is unknown.
Through the lowest societies, as through the lowest creatures, the distribution of crude nutriment is by slow gurgitations and regurgitations. In creatures that have rude vascular systems, just as in societies that are beginning to have roads, there is no regular circulation along definite courses; but, instead, periodical changes of the currents now towards this point and now towards that.
So long as growth continues there intervenes between the shaft and each of the epiphyses a disc of actively growing cartilage the epiphysial cartilage; and at the junction of this cartilage with the shaft is a zone of young, vascular, spongy bone known as the metaphysis or epiphysial junction.
If, however, the tumor is very vascular, he prefers to leave the case to nature rather than expose the patient to the dangers of a bloody operation. The whole discussion of goiter is manifestly a paraphrase of the similar chapter of Roger, who also introduced into surgical practice the use of the seton.
In his excellent work, On the structure of the Heart in the Amphibia , Carl Rabl has shown how easily these curious cenogenetic facts can be explained by the secondary adaptation of the embryonic structure to the great extension of the food-yelk. The embryology of all the other parts of the vascular system also gives us abundant and valuable data for the purposes of phylogeny.
Again, two distinct organs, or the same organ under two very different forms, may simultaneously perform in the same individual the same function, and this is an extremely important means of transition: to give one instance there are fish with gills or branchiae that breathe the air dissolved in the water, at the same time that they breathe free air in their swim-bladders, this latter organ being divided by highly vascular partitions and having a ductus pneumaticus for the supply of air.
The hoof and vascular tissue in turn enclose the elastic apparatus and bony core. It is covered by a thin layer of horny tissue, the peripole, that coats over the wall and assists in preventing its drying out. On lifting the foot and examining its inferior surface, it is noticed that the wall at the heels is inflected under the foot and in a forward direction.
Dilatation of the cutaneous vascular areas leads to a larger flow of blood through the skin, and so tends to cool the body, and vice versa. Also the special nerves of perspiration can increase or lessen heat loss by promoting or diminishing the secretions of the skin.
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