Oct. is, 1924 Postnatal Growth of the Single-Comb White Leghorn 
385' 
It is not, however, so evident for the 
heart as. for the lungs. This chart 
shows that the heart increases a little 
more slowly during the first and middle 
portions of the curve but continues to 
increase in the heavier chickens. 
After 1,400 gm. in gross body weight, 
the increase in the heart weight is 
marked, while in the digestive sj^stem, 
for example, there is a much slower 
rate of increase after a body weight of 
1,600 gm. is reached. This is in 
accord with the condition found in man, 
namely, that the heart increases slowly 
in weight after maturity. 
The percentage weights of the heart 
show a very brief period of increase 
erel, for as Joseph (10) has suggested, 
the activity of an animal is correlated 
with the size of the heart. 
Jackson (8) found that in the rat 
the heart forms 0.65 per cent at birth,- 
with a slight postnatal increase, gradu¬ 
ally decreasing to about 0.4 per cent 
in the adult. 
DUCTLESS GLANDS 
Thyroid gland. —The thyroid gland 
in the chicken is a rather small oval 
gland located on each side at the base 
of the neck, or at the bifurcation of the 
common carotid artery. It consists 
of a single lobe on each side. Figure 21 
shows the absolute and the percentage 
Fig. 21.—Absolute and relative weights of the thyroid gland plotted on gross body weight. All the cases 
above 2,200 gm. gross bodv weight or the adults, and the pullet at 1,290, and the hen at 1,920 gm. gross 
body weight, are shown in’the chart though not included in the data used in determining the curve 
from 0.51 per cent at hatching to a 
maximum of 1 per cent on the fourth 
day. The curve of relative growth 
does not show this very brief period of 
initial increase but steadily decreases 
from about 0.85 per cent to 0.47 per 
cent at 1,400 gm. gross body weight. 
It then rises to a value of 0.55 per cent 
at 2,100 gm. gross body weight and 
remains nearly at this level to the end. 
In the individual percentage values 
(not shown on chart) there seems to be 
an indication of a relatively heavier 
heart in the male after about 1,200 gm. 
of gross body weight. 
There is no apparent explanation for 
the relatively heavier heart in the cock¬ 
erel any more than there is for the 
lungs. Possibly it may be associated 
with the increased activity of the cock- 
weights of the gland plotted against 
gross body weight. The formula is: 
y=0.01[0.01(X + 100)] l * 23 -0.000075X 
-0.006 
from 30-2,100 gm. gross body weight. 
Y represents the weight of the thy¬ 
roid in grams and X represents the 
gross body weight. 
In the smaller chicks it shows a much 
slower growth than do most of the 
organs, except the sex glands and the 
secondary sexual structures, such as 
the comb and wattles. The female at 
1,290 gm. in body weight had an unusu¬ 
ally heavy thyroid,’ of 0.417 gm. This 
weight was not averaged in the series, 
being excluded as either an abnor¬ 
mality or an error. All three of the 
older cockerels had much heavier thy- 
