SUPRA-RENAL CAPSULES. THYMUS GLAND. 



521 



a. In the Supra-Renal Capsules, as in the Kidneys, there is an obvious difference between 

 the cortical and the mf'litllnnj substances. The former is of a yellowish colour ; and presents 

 an appearance, when cut into, as if it were made up of straight parallel libres, arranged side 

 by side. Of these straight libres, however, a part are branches of arteries, whicb enter this 

 body at every point of its exterior, from a capillary network covering its surface; and others 

 are corresponding branches of veins, that receive the blood from these arteries, and convey 

 it into a venous plexus which forms the centre of the .organ. Between the radiating blood- 

 vessels, there are found lying, in the cortical substance, numerous parallel cylinders or elongat- 

 ed cones, formed by closed sacs of basement-membrane, including nuclei and cells in various 

 stages of development, with fat-cells. The medullary substance is partly made up of the ve- 

 nous plexus, dilated into a sort of cavernous texture, together with empty cavities or lacuna, 

 that seem destitute of a lining membrane, and contain only a thick grayish-white fluid ; and 

 partly of an intervening parenchyma, consisting of cells in various stages of development. 

 In the Human adult, there is a great predominance of nuclei, which seem as if they did not 

 attain their full development; but in Ruminant animals, and in the Human subject in early 

 life, the cells are more or less developed, and then resemble the ordinary lymph-corpuscles 

 in size and appearance. The Lymphatics are of large size, like those of the Spleen ; and 

 probably convey away the matter which has been elaborated by these organs, that it may be 

 mingled with that which is being taken up and prepared by other parts of the Absorbent 

 system. The Supra-Renal capsules attain a very large size early in fcetal life, surpassing 

 the true Kidneys in dimension, up to the tenth or twelfth week: but they afterwards dimi- 

 nish relatively to the latter, and are evidently subordinate organs during the whole remainder 

 of life. 



It does not seem unlikely that these bodies, like the Spleen, have a double 

 function ; and that, besides participating in the general actions of the Absorb- 

 ent glandulse, they may serve as a diverticulum tor the Renal circulation, when 

 from any cause the secreting function of the Kidneys is retarded or checked, 

 and the movement of blood through them is stagnated. 



687. The Thymus Gland is another body which seems referrible to the 

 same group ; having all the essential characters of a true gland, save an excre- 

 tory duct; and its function being evidently connected, during the early period 

 of life at least, with the elaboration of nutritive matter, which is to be re- 

 introduced into the circulating current. 



a. Its elementary structure may be best understood from the simple form it presents when 

 it is first capable of being distinguished in the embryo. It then consists of a single tube, 

 closed at both ends, and filled with granular matter ; and its subsequent development consists 



[Fig. 209. 



A section of the Thymus gland at the eighth month, showing its anatomy ; from a preparation of 

 Sir A. Cooper's; 1, the cervical portions of the gland; the independence of the two lateral glands is 

 well marked; 2, secretory follicles seen upon the surface of the section; these are observed in all 

 parts of the section ; 3, 3, the pores or openings of the secretory follicles and pouches ; they are seen 

 covering the whole internal surface of the great central cavity or reservoir. The continuity of the 

 reservoir in the lower or thoracic portion of the gland with the cervical portion, is seen in the figure.] 



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