Sir A. Cooper describes the vascular sup ply of the thymus as follows : —" With re spect to the arteries of this organ, they are principally derived from two sources. Each thoracic portion is supplied by a branch which is sent off by the internal mammary. It enters at the junction of the cervical with the thoracic part, generally on their outer side, but sometimes between the cervical portions, and, descending upon the middle of the gland, divides to supply the spirally disposed lobes. The other principal artery of the thymus is sometimes derived from the superior thyroi deal, at others from the inferior thyroideal artery, and, descending upon the lobes of the cervical portion, passes into them, and ulti mately anastomoses with the branch from the mammary artery. The thymic arteries may also arise from the trunk of the subcla vian, the vertebral, or the carotid artery, or even from the arch of the aorta. The ca pillary network in which the arteries termi nate, is stated by Mr. Simon to be of " the completest description. It is so arranged as to include each individual vesicle within a vascular capsule ; the capillaries are closely applied upon the transparent texture (limitary membrane) which bounds the cavities, and so exceedingly dense is their network that the meshes are of even less diameter than the vessels themselves. Every portion of the glandular substance is thus exposed in the completest manner, and at every point of its surface, to the penetration of the fluid ingre dients of the blood." "The vena thymicw" Sir A. Cooper states " have a different course to the arteries ; for although the internal mam mary and thyroideal veins receive small branches from the gland, yet the principal veins are those which end in the vena innoininata. A considerable vein springs from each thoracic portion, and passes from the posterior surface of this part of the thymus into the vena inno minata ; having received a branch from the cer vical portion, and vessels from the thoracic : it is found near the centre of the gland. A very small vein enters the thyroideal from the cer vical portion, and this vein anastomoses with that of the thoracic part." Respecting the absorbent vessels of the thymus very little seems to be known ; we may, however, fairly conclude from the analogy of other parts, that they commence by a network of minute vessels, which have no communication whatever with the glan dular cavities, and cannot, therefore, serve the purpose of excretory ducts as has been sup posed : the glands to which they proceed are those of the anterior mediastinum.
Mr. Simon describes the nervous supply of the thymus " as mainly derived from the plexus which surrounds the first part of the subclavian artery, and which has its chief origin from the inferior and middle cervical ganglia. A small twig detaches itself from this sympathetic plexus, just opposite the origin of the internal mammary artery, accom panies that vessel in its course, and, on arriving at the point where the thymic branch arises, sends filaments along it into the sub stance of the gland. A second source of supply is the cardiac branch of the pneumogas tric, which gives on each side a minute fila ment to the superior part of the gland." " In one instance I have seen a very minute fibril of the descenders noni emerge from the sub stance of the sterno-thyroid muscle, and reach the cellular investment of the thymus ; and I have sometimes seen delicate twigs of the phrenic also detached towards the gland ; but in each case the nerve has appeared to restrict its distribution to the surface and coverings of the organ, and has not accompanied any of its vessels." The exact arrangement of the nervous fibrils, both tubular and sympathetic, in the substance of the gland is yet unknown ; but it seems tolerably certain that they accompany the vessels, enlace them with their plexiform divisions, and terminate, in part at least, in a looping manner.
Early development. — The following quota tion from Professor Goodsir's paper in the Philos. Transactions, contains his views re specting the development of the thymus and two others of the ductless glands ; I am un able from my own observation to confirm or dispute the accuracy of his opinion, but can not do otherwise than refer to the labours of so distinguished a physiologist.
" That portion of the membrana intermedia which is separated from the rest of the mem brane, and included in the body of the embryo by the umbilical constriction, and which has not already been devoted to the formation of the heart, liver, pancreas, and external portion of the intestinal canal, is found massed along the trunks of the primitive venous system, the sides of the arches of the aorta, the terminal portion of that vessel, and the origins of the omphalo-mesenteric arteries. The portions of the membrana intermedia which are last in being converted into special organs, the Wolf fian bodies, are the parts which project one on each side of the aorta along the posterior part of the cardinal veins of Rathke, between the intestinal plates and visceral lamina. The portions of the membrana intermedia, which remain between the upper extremities of the Wolffian bodies and the heart and liver, and which surround the origins of the omphalo mesenteric arteries, do not become converted into organs of special structure, but retain during life the original constitution of the membrana intermedia of the blastoderma, and increase rapidly in the embryo constituting the supra-renal capsules. That portion of the membraua intermedia which is situated be tween those two aortic arches the extremities of which become the carotid and subclavian arteries, remains during life as the thyroid body. It receives its blood from the first and second aortic arches by two large trunks on each side, the superior and inferior thyroid arteries. That portion of the membrane which passes in two parts from near the base of the cranium back as far as the ductus Cuvieri and anterior portions of the veins of Ratlike, and which are united and concentrated in front of the heart, by passing from behind forwards in harmony with corresponding motions of the neighbouring part, becomes the thymus. The structure of these organs is identical with that of the blastoderma (?). Their probable func tion, namely, to prepare by the action of their nucleated cells, and to throw into the vascular system a matter necessary for the nutrition of the animal during the period of its active growth, a function which the observations and opinions of the majority of physiologists have assigned to them, is also essentially the same with that of the blastoderma." Development of Size.—Ilaving now examined the anatomy of the thymus, and traced its development with a view to the exact elucida tion of its structure, we have next to follow out the successive periods of its growth, in order to determine whether it be an organ having special relation to foetal or to extra uterine life. The former alternative was that to which the older anatomists, and even Sir Astley Cooper, inclined ; but the correctness of the latter seems now abundantly established. Meckel, Hewson, Cloquet, and Sir A. Cooper himself, all concur in stating that it continues to grow at least up to the end of the first year after birth ; and more recently the evidence accumulated by Mr. Simon from his own ob servations and those of Hangsted have quite set this important point at rest.