5 
As the various parts of the feather ripen and open out, the difference 
between the day and night growth largely disappears, even where 
conspicuous in the early stages of the unopened feather. A plume can be 
considered as perfectly formed only when this is the case, and ripe well- 
grown feathers often show no trace of the daily alternation. Most plumes, 
however, when held in certain positions reveal indistinct interruptions or 
weaknesses in the flue corresponding with those in the unopened feather, 
and sometimes the shaft is more or less indented on the outer surface, 
the rings becoming complete on the quill. These shaft indentations can 
easily be felt by passing the finger up and down the stem. In the best 
examples it will be found that the weakenings in the flue and also the 
indentations in the shaft occur at intervals of about a quarter of an 
inch, which, as stated above, is the rate of daily growth of the wing 
feathers. After one has become acquainted with the appearance of the 
day and night intervals, on both the growing and expanded parts of the 
feather, traces of them can be recognized on almost every feather, both 
on the flue and on the stem; they are not sufficiently strong, however, 
to be represented in photographic reproductions. 
How tHE Day anno Nicut RINGS ARE PRODUCED. 
The further question presents itself: how is the alternation of day 
and night growth produced, and what is its influence on the feather? 
Without entering into details it may be stated that it has been proved 
experimentally that in most birds the temperature of the body is lowered 
at night and along with it the blood-pressure. This signifies that less 
physiological activity is taking place during the night than during the day, 
and consequently less feather material is then being formed; the feather 
continues to grow both day and night, but less vigorously during the 
night repose. The diminished activity in feather-growth at night 
particularly affects the barbules and the outside sheath, on account of their 
cells being furthest from the blood supply in the middle of the feather ; 
the nutritive fluids do not ooze to their cells as freely as they do under 
the stronger pressure of the day. Hence these parts are the first to 
suffer in any lowered vitality. Though the shaft and barbs may grow 
uniformly day and night the barbules and sheath yet suffer from the 
diminished blood supply at night. From the above it follows that the 
night growth, if only for a few hours, is normally not as strong as that 
during the day, and therefore we get the alternation of the narrow and 
broad rings seen on the unexpanded feather. 
Owing to their less density it is clear that the night rings will 
represent so many places of weakness in the feather, both as regards the 
sheath and the internal feather-forming material. With a bird in a 
high nutritive condition, and under ordinary conditions of day and night, 
the difference in the two growths is, however, barely perceptible ; and even 
where it can be distinguished on the unopened feather it is scarcely 
noticeable when the plume matures and unfolds. Under other circum- 
stances, particularly those of impaired or deficient nutrition, it has been 
proved that the difference between the two growths becomes intensified, 
and then the rings on the unopened feather remain as bands of different 
density on the expanded flue. 
WRINKLINGS PRopUcED at Nieut Rincs. 
Having now shown how the night rings are places of weakness in the 
growing parts of the feather the next step is to establish the connection 
between them and the wrinklings, the latter being always present when 
barring is at all conspicuous. If within the soft growing feather the 
