a. Yellow elastic fibers.
b. White inelastic fibers.
c. Nuclei.
d. Fiber, with nucleus.
The inner layer of the middle coat, or muscular coat, as it may be justly termed, forms, it will be seen, the greatest part of the thickness of the wall of certain arteries, and in some instances, as in the anterior tibial artery, constitutes nearly the entire thickness of the vessel. The internal coat in all is frequently seen puckered in a longitudinal direction.
175. The arteries are supplied with blood by vessels of small size, which do not come off immediately from the part of the artery they are destined to supply, but principally from neighboring vessels. They are called vasa vasorum. They are arranged precisely in the same manner as those of the areolar tissue. A few of these vessels penetrate as far as the middle or muscular coat, but do not reach the inner, which has no vessels, proximity to the circulating fluid being apparently sufficient for its nutrition.
Arteries are supplied with nervous influence by branches from the sympathetic system running in their walls, and through their connection by ganglions with the organs they supply with blood.
176. The cells, nuclei, and nucleoli alluded to are supposed to be thus produced. In a shapeless, consistent, sometimes almost gelatinous mass, to which the name of cyto-blastema or formative substance has been given, containing the materials requisite for the production of cells, small, round grains or nucleoli are perceived in the act of formation. Around these grains a layer of granular matter is deposited, which continually increases in thickness, and constitutes the kernel or nucleus. This is oval shaped or round, almost always opaque, has a granular surface, and is considered to be a vesicle, a little cell itself. From the surface of this kernel a small, very thin transparent vesicle is raised, appearing as a segment of a sphere, which soon expands, and becomes so large, when full grown, that the kernel lies as a minute corpuscle upon its interior wall; the material for its formation being supplied by the cyto-blastema, it is converted into a vesicle by the kernel which is first formed, its embryo existing in the formative substance.
The first trace of organization is the production of a small, perceptible body, or nucleolus, which deposits on the surface a granular substance from the cyto-blastema, to give rise to a little producing organ, the kernel or nucleus. This further transforms the surrounding cyto-blastema into a granular surface, from which the vesicle is formed, raised, expanded, and filled with a liquid, in which vesicle thus enlarged the kernel remains inclosed and adhering to a certain spot of its wall.
If two nucleoli lie close to one another, they coalesce and become one solid mass, capable of producing one cell only, containing one kernel and two nucleoli. This view is that of Schleiden and Schwann, supported by Mülder, but not entirely approved by Henle; inasmuch as no kernel can be perceived in the cells of many cellular systems while in the act of formation. In the elementary parts of animals which have long since lost their cellular form, the remnants of kernels are frequently found, as has been demonstrated in the preceding diagrams. The manner, however, in which the elementary first-seen granules are formed in the cyto-blastema, science has not yet been able to discover. The chemists have proved that all elementary organic substances consist of carbon, hydrogen, oxygen, and nitrogen, susceptible of endless modifications of their respective forces, under which an organic molecule or ovum is produced, and after that, under certain circumstances, an animal such as man.
177. When the current of blood through the main trunk of the arteries of an extremity is cut off, the circulation is carried on by the collateral branches. This collateral circulation is more perfect, more active in young persons during the increase or growth of the body, than it is either at maturity or in the decline of life. The important point is not, however, alone referable to the time of life at which the continuity and permeability of the main trunk cease to exist, but to the nature of the disease or injury which has given rise to it.
When an aneurismal limb has been injected, on which an operation has not been performed, the collateral vessels have all been found larger and more fully shown than on the opposite side, although not to the same extent as in cases of a similar nature in which the operation has been done.