292 JOHN C. DONALDSON 
diameters, and the outlines of the -entire section and of the 
medulla were traced. The area of each of these figures was 
measured with a planimeter. From these areas and the known 
thickness of the section the actual volume of each section 
traced and of its medulla was calculated. The nine sections 
between each of those traced were considered as each having 
the same volume as the nearest traced section. By adding 
the volumes determined together the total volumes of the 
gland and of the medulla were found. Blood sinuses and 
blood-vessels were considered as a part of the tissues in which 
they were found. In other words, no attempt was made to 
subtract their volume from that of the gland. The thin capsule 
was included in the area of most of the sections. In those 
toward the end of the gland, where the capsule appeared thick 
owing to the plane of section passing tangentially to the surface 
of the gland, the capsule was omitted from the areas. 
Somewhat similar methods were used by Elliot and Tuckett 
(06) in their study of the adrenals of a number of mammals. 
They examined the cat, the guinea-pig, and the rabbit, and found 
in these animals that the relative volume of the medulla, com- 
pared with the volume of the whole gland, decreased with age. 
There were marked differences in the relative amount of med- 
ullary tissue in the gland, the maximum being 14 per cent of 
the entire gland, in a young guinea-pig, and the minimum 1 per 
cent in an old one. The values for the cat and the rabbit fell 
between these extremes. These authors give the value of 9 per 
cent for the amount of medullary tissue in the gland of a male 
rat of 120 grams, and 6 per cent for that in one of 190 grams. 
They do not state the variety of rat. They admit that the method 
they used would not give accurate results with so small a gland 
as that of the rat. In his study of the adrenal of the rabbit, 
Bertel Bager (’17) finds that the medulla forms 20 per cent of 
the total volume of the gland at birth. At twelve months it 
makes up a little less than 2 per cent. The ratio rises to 3 per 
cent as the animal gets older. The medulla is relatively smaller 
in the females. 
