STRUCTURE OF THE THYMUS GLAND. 



925 



He is of opinion that the central cavity described and figured by Cooper is 

 preternaturally enlarged, owing to over- distension ; but that, nevertheless, all 

 the parts of each lateral mass of the thymus are connected with a single 



Fig. 648. TRANSVERSE SECTION OP A LOBULE 

 OP AN INJECTED INFANTILE THYMUS GLAND 

 (from Kolliker). ^ 



a, capsule of connective tissue surrounding 

 the lobule ; 6, membrane of the glandular 

 vesicles ; c, cavity of the lobule, from which 

 the larger blood-vessels are seen to extend to- 

 wards and ramify in the spheroidal masses of 

 the lobule. 



common cavity. (Astley Cooper, Ana- 

 tomy of the Thymus Gland, Lond., 

 1832 ; Simon, Physiological Essay on 

 the Thymus Gland, Lond., 1845 ; His, 

 on the Lymphatics of the Thymus, in 

 Zeitsch. f. wissensch. Zoologie, X. and 

 XI. ; Kolliker and Henle in their respec- 

 tive Handbooks.) 



Fig. 648. 



Vessels. The arteries of the thymus are 

 derived from various sources, viz., from the 

 internal mammary arteries, the inferior and 

 superior thyroid, the subclavian and carotid 

 arteries. They terminate in capillary vessels, which form a vascular envelope around 

 and within each vesicle. 



The veins pursue a different course from the arteries, and, for the most part, open 

 into the left innominate vein. 



The lymphatics are large. A ccording to the observations of His on the calf, the 

 larger blood-vessels passing to the central canal are each accompanied by two or more 

 lymphatic stems. He finds that these arise from an interlobular plexus of lymphatic 

 spaces destitute of walls, and that this plexus receives its roots from the interior of the 

 lobules; and he advances the opinion that they communicate directly with the central 

 spaces of the lobules; he has not, however, actually observed such a connection. 



The nerves are very minute. Haller thought they were partly derived from the 

 phrenic nerves, but according to Cooper, no filaments from these nerves go into the 

 gland, though they reach the investing capsule, as does also a branch from the 

 descendens noni. Small filaments, derived from the pneumogastric and sympathetic 

 nerves, descend on the thyroid body, to the upper part of the thymus. Sympathetic 

 nerves also reach the gland along its various arteries. 



Development. The early development of the thymus has been carefully studied by 

 Simon, whose researches were chiefly conducted in the embryos of swine and oxen. 

 In embryos about half an inch in length, it may be seen with the aid of a high 

 magnifying power; and in those of one and a half inch, with the aid of a simple lens. 

 When first distinguishable, it consists of a simple tube closed in all directions, lying 

 along the carotid vessels. The contents'of this tube are granular, but do not show 

 regular corpuscles; its walls are delicate and homogeneous. The tube has no con- 

 nection with the respiratory mucous membrane, as was supposed by Arnold; and so 

 soon as it is discoverable, it is found to be perfectly distinct from the thyroid body. 

 At intervals along the sides of this tube small vesicles bud out, so as to form lateral 

 diverticula, which contain nucleated corpuscles, and which go on subsequently 

 branching out into groups of two or four, the formation of the permanent vesicles 

 being merely the last repetition of this process. In the human foetus at the seventh 

 week, Kolliker has seen the thymus lobate at its lower end, and single above; at 

 about the ninth week, the thymus consists of two minute elongated parallel parts 



