238 
we move to one side, it follows that the 
same object will still seém to be in-a line 
with us, exactly as in the front view—seem- 
ing, as we moye, to turn from their first 
direction. In portraits, the permanence of 
direction, with reference to the spectator, 
depends on the same principles. So the 
nose, drawn in front with its central line 
upright, continues directed to the spectator, 
though viewed obliquely ; or, if the right 
side of the nose is represented, it must ap- 
pear directed to the right of the spectator, 
in all situations. 
The temporary derangement of vision, 
which very commonly follows acidity or 
flatulency in the stomach, and as com- 
monly precedes sick-headach, or else un- 
usual sleepiness, in great numbers of per- 
sons (the writer amongst the number), 
has lately attracted the attention of Dr. 
Wollaston, who, with his wonted sagacity, 
has discovered that, usually, one half only, 
either to the right or to the left of each eye, 
is in these cases temporarily affected with 
blindness. From a careful consideration 
of the circumstances attending five cases 
of temporary half-blindness, which are de- 
tailed in the Philosophical Transactions just 
published, the doctor has been led to an 
important anatomical discovery, as to the 
semi-decussation of the optic nerves in the 
human subject ; that is, instead of the en- 
tire optie nerves from the two opposite 
thalami of the brain, crossing each other, 
and proceeding entire to the eyes, on op- 
posite sides, as has generally been sup- 
posed, that portion of nerve which proceeds 
from the right thalamus to the right side of 
the right eye, passes to its destination 
without interference; and, in a similar 
manner, the left thalamus supplies the 
left side of the left eye with one part of 
its fibres; whilst the remaining halves of 
both nerves, in passing over to the eyes of 
ithe opposite sides, intersect each other, 
either with or without intermixture of their 
fibres. On this principle, Dr. Wollaston 
most ingeniously explains how single 
vision is produced by two eves—how in- 
fants are enabled to avoid squinting, Xe. 
The migration of birds was a subject 
which, during many years, engaged the at- 
‘tention of the late celebrated Dr. Jenner, 
having been early in life stimulated to the 
inquiry by the investigation on this subject 
which the great John Hunter was carrying 
on, whilst Mr. Jenner was resident in his 
house as a medical pupil. The son of Dr. 
Jenner has, since his decease, communi- 
nicated to the Royal Society his father’s 
manuscripts on. the subject, which have 
been printed in the Philosophical Transac- 
tions,—a recapitulation of which is as fol- 
lows, vix.—First, Dr. Jenner adduces some 
arguments in support of migration, be- 
cause of the fact itself not being generally 
admitted by naturalists of celebrity, and 
also against the hypothesis of a state of 
torpor, or what may be termed the hyber- 
Spirit of Philosophical Discovery. 
[Oct. I, 
nating system. He next shews, from re- 
peated observations, that the swallow tribe, 
and many other birds that absent them- 
selves at stated periods, return annually to 
the same spot to build their nests; and 
that any inference drawn from this fact, in 
support of a state of torpor, would be fal- 
lacious, upon physiological principles. In 
corroboration and continuation of the ob- 
servations of John Hunter, Dr. Jenner 
shews, that certain periodical changes of 
the testes and ovaria are the exciting causes 
of migration,—and states many facts, hi- 
therto unnoticed, with respect to the cause 
which excites the migrating birds, at cer- 
tain seasons of the year, to quit one coun- 
try. for another, viz., the enlargement of the 
testes of the male, and ovaria in the fe- 
male, and the need of a country where 
they can, for a while, be better accommo- 
dated with succours for their infant brood 
than in that from which they depart. It 
is attempted to be shewn by Dr, Jenner, 
that their departure from this country is 
not in consequence of any disagreeable 
change in the temperature of the air, or 
from a scarcity of their common food,—but 
the result of the accomplishment of their 
errand, i.e., the incubation and rearing of 
their young, and the detumessence of the 
testes and ovaria; that successive arrivals 
of migrating birds are attributable to the 
progressive developement of the generative 
system in the male and female ; that pro- 
gressive developements are wise provisions 
of the Creator; that premature arrivals 
and departures are frequently to be ac- 
counted for on the same principle; that 
the departure of the spring migrators is 
owing to a change in the testes and ovaria, 
the very opposite of that which took place 
in the spring; that the departure of the 
young birds is not guided by the parents, 
but the result of an unknown principle. 
In the second part of the doctor’s paper 
some observations are made on the winter 
birds of passage: —that they quit their 
homes in this country, in the spring, in 
quest of a country better suited to their 
intended purpose than this; that they are 
actuated by the same impulse in quitting 
this couutry that causes the spring birds to 
come to it, and that want of food cannot be 
the inducement; that the emigration of 
the winter birds is less complete than that 
of the others, or spring migrators; that 
some species breed here, especially the 
wild-duck and wood-pigeon ; that the red- 
wings and fieldfares are the most regular 
and uniform in their appearance and dis- 
appearance, and most probably never 
risk the trial of incubation here, or at least 
not in the part of Gloucestershire where 
Dr. Jenner resided; that they quit the 
country temporarily, im severe and long- 
continued frost, through want of food, and 
return to it again at the approach of more 
temperate weather; that the arrival of 
water-birds forbodes the approach of in- 
tense 
