HED 
further change the blood returns by the 
veins, and again passes through the course 
of circulation. From his experiments on 
the capacities of arterial and venous blood, 
Dr. Crawford found the capacity of the 
former for heat to be 1.030, and that of the 
latter only 0.892, whence he concludes, that 
though heat must be given out in conse- 
quence of the diminished capacity of the 
combined oxygen absorbed by respiration, 
yet the increased capacity of the arterial 
blood will prevent its becoming sensible 
immediately in the lungs ; instead of which, 
it will be given out at the smaller ramifica- 
tions where the blood becomes changed in 
its nature, and in its capacity for heat by 
its conversion to the venous state. It has 
also been established by the experiments of 
the same philosopher, that the process of 
absorption of oxygen is less in a higher than 
in a low temperature ; the difference be- 
tween the arterial and venous blood being 
at the same time less, and consequently the 
augmentation of temperature in the animal 
less considerable. This law of the animal 
economy, assisted by the increased evapo- 
ration, and by the slow conducting power 
of an animal body, and perhaps by the per- 
manency of the enlarged capacity, seems 
sufficient to account for the' power which 
animals possess of maintaining their na- 
tural temperature without any remarkable 
change in an atmosphere greatly heated, 
as was shewn in the experiments of For- 
dyce and Blagden. (See Philos. Trans. 
1775.) It must be cbnfessed, however, 
that some farther investigations seem want- 
ing on this subject. 
Though the lungs appear to be the great 
organ of oxygenation in the larger animals, 
jt is well ascertained that a process of nearly 
the same nature is carried on at the skin ; 
and in many of the smaller or less perfect 
animals there appears to be no other means 
for effecting this absorption. 
HEATH. See Erica. 
HEAVINESS, in general, the same with 
weight or gravity. See Gravity and 
Weightv 
HEBENSTREITIA, in botany, a genus 
of the Didynamia Angiospermia class and 
order. Essential character: calyx emar- 
ginate, cleft underneath ; corolla one-lipped, 
lip ascending, four-cleft ; stamens inserted 
into the edge of the border of the corolla ; 
capsule containing two seeds. There are 
six species, all natives of the Cape. 
HECTIC. See Medicine. 
HEDERA, in botany, English ivy, a ge- 
HED 
nus of the Pentandria Monogynia class and 
order, Natural order of Hederace®. Ca- 
prifolia, Jussieu. Essential character : pe- 
tals five, oblong; berry five-seeded, sur- 
rounded by the calyx. There are six species, 
with several varieties. 
HEDERACEjE, in botany, the name of 
the forty-sixth order of Linnaeus's “ Frag- 
ments ofaNatural Method,’’ consisting of the 
ivy, vine, and a few other genera, which from 
their general habit and appearance seem 
nearly allied. This order consists ofherba- 
ceous and shrubby plants, most of which, 
particularly the ivy and vine just mention- 
ed, have creeping branches, that attach 
themselves by tendrils to the bodies in their 
neighbourhood. The roots are long; the 
stems and young branches commonly cy- 
lindric. The leaves are alternate, sometimes 
simple, as in the ivy and vine ; sometimes 
winged, as in the zanthoxylum, or tooth-ach 
tree, in which the surface of the leaves is 
covered with points. On each side of the 
foot-stalk of the leaves of the vine are placed 
two pretty large stipul®, or scales ; from 
the side opposite to the leaves proceeds a 
branching tendril, which serves to fasten 
the plant to the bodies in its neighbourhood. 
The flowers are either hermaphrodite, as in 
the ivy and vine ; male and female upon 
different roots, as in the ginseng; or herma- 
phrodite and male upon different roots, as 
in the zanthoxylum. The calyx, or proper 
flower cup, consists of one leaf divided into 
five parts, which are small, and generally 
permanent. The petals are commonly five ; 
but in the cissus four, and in the zanthoxy- 
lum none. There are five stamina ; but 
the cissus has only four. The anthers, or 
tops of the stamina, are roundish : in the 
ivy they are attached to the filaments by 
the sides. In the zanthoxylum the filaments 
are Crowned with twin anthers. The seed 
bud is of different shapes ; the seed-vessel is 
of the berry kind, with one, two, or five 
cells, and the seeds are from one to five in 
number, placed either in distinct cells, or, 
as in the case of the ivy and vine, dispersed 
through the pulp without any partition. See 
Panax, &’c. 
HEDGES, in agriculture, are either 
planted to make fences round inclosures; or 
to divide the several parts of a garden. 
When they are designed as outward fences, 
they are planted either with hawthorn, 
crabs, or blackthorn; but those hedges 
which are planted in gardens, either to sur- 
round wilderness-quarters, or to screen the 
other parts of a garden from sight, are 
