926 THE URINARY ORGANS. 



lying chiefly on the upper part of the pericardium, and presenting under the micro- 

 scope a distinct tubulo-vesicular structure filled with polygonal cells; at the twelfth 

 week the thymus is broad, and its surface is entirely covered with lobules ; it then 

 increases rapidly until birth, but not with uniform rapidity, for it grows especially 

 during the seventh, eighth, and ninth months of intra-uterine existence. 



A fter birth, the thymus, as already stated, continues to grow to near the end of the 

 second year. According to the observations of Haugstedt and Simon upon the weight 

 of this organ in young animals, it appears for a short time after birth to increase not 

 merely absolutely, but even faster than the rest of the system, and during the next 

 period only to keep pace with the increase of the body. After the second year it ceases 

 to grow, and becomes gradually converted by the eighth or twelfth year into a fatty 

 mass. In this condition the corpuscles of the thymus disappear, forming, according 

 to Simon's opinion, the nuclei of cells which become developed into the cells of adipose 

 tissue. At puberty the thymus is generally reduced to a mere vestige which has 

 entirely lost its original structure, and consists of brownish tissue occupying the 

 upper part of the anterior mediastinum. Occasionally it is still found in good condi- 

 tion at the twentieth year; but generally only traces of it remain at that time, and 

 these are rarely discoverable beyond the twenty-fifth or thirtieth year. 



The thymus gland presents no difference in the two sexes. It exists, according to 

 Simon, in all animals breathing by lungs, and is persistent in those which hybernate, 

 though only as a mass of fat. 



THE URINARY ORGANS. 



THE urinary organs consist of the kidneys, the glandular organs by which 

 the urine is secreted, and of the ureters, bladder, and urethra, which are 

 the organs of its excretion and evacuation. As being locally connected, the 

 suprarenal capsules are usually described along with these organs, though 

 they have no relation, as far as is known, to the secretion of urine. 



THE KIDNEYS. 



The kidneys, two in number, are deeply seated in the lumbar region, 

 lying one on each side of the vertebral column, at the back part of the 

 abdominal cavity, and behind the peritoneum. They are situated on a level 

 with the last dorsal and the two or three upper lumbar vertebrae, the right 

 kidney, however, being placed a little lower down than the left, probably in 

 consequence of the vicinity of the large right lobe of the liver. They are 

 maintained in this position by their vessels, and also by a quantity of sur- 

 rounding loose areolar tissue, which, usually contains much dense fat (tunica 

 adiposa). The size of the kidneys varies in different instances. Ordinarily, 

 they measure about four inches iu length, two and a half inches in breadth, 

 and an inch and a quarter or more in thickness. The left kidney is usually 

 of a longer and thinner shape, whilst the right is shorter and wider. 



Weight. The average weight of the kidney is usually stated to be about 4^ oz. in 

 the male, and somewhat less in the female. According to Clendinning, the two kid- 

 neys of the male weigh on an average 9| oz., and those of the female 9 oz. The 

 estimate of Rayer is 4^ oz., for each organ in the male, and 3 oz. in the other sex. 

 Reid's observations (made on sixty -five males and twenty-eight females, between the 

 ages of twenty -five and fifty -five) would indicate a higher average weight, viz., rather 

 more than 5^ oz. in the former, and not quite 5oz. in the latter, the difference 

 between the two sexes being therefore upwards of half an ounce. The prevalent weights 

 of the kidney, as deduced from the tables of Reid, are, in the adult male (160 

 observations) from 4^ oz. to 6 oz., and in the adult female (74 observations) from 

 4 oz. to 5^ oz. The tables more recently published by Peacock give still higher 

 average results as to the weight of these organs. The two kidneys are seldom of 



