1091 



THYMUS GLAND. 



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 venae thymicaa" 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 innominata. 

 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 tliymic 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 descendens 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 Rathke, and 



