and runs, back to a
ganglionated nervous network, just behind the coeliac artery, into
which the vagus also enters; this is the coeliac ganglion, and together
with a similar superior mesenteric ganglion around the corresponding
artery, makes up a subsidiary visceral nervous network, the solar
plexus. A similar and smaller nervous tangle, bearing an inferior
mesenteric ganglion, lies near the inferior mesenteric artery.

Section 135. Finally, we may note the pineal gland and the pituitary
body, as remarkable appendages above and below the
thalamencephalon. Their function, if they have a function, is altogether
unknown. Probably, they are inherited from ancestors to whom they
were of value. Such structures are called reduced or vestigial
structures, and among other instances are the clavicles of the rabbit,
the hair on human limbs, the little pulpy nodule in the corner of the
human eye, representing the rabbit's third eyelid, and the caudal
vertebrae at the end of the human spinal column. In certain lowly
reptiles, in the lampreys, and especially in a peculiar New Zealand
lizard, the pineal gland has the most convincing resemblance to an
eye, both in its general build and in the microscopic structure of its
elements; and it seems now more than probable that this little
vascular pimple in our brains is a relic of a third and median eye
possessed by ancestral vertebrata. The pituitary body is probably
equivalent to a ciliated pit we shall describe in the lacelet
(Amphioxus).

8. _Renal and Reproductive Organs_

Section 136. We have now really completed our survey of the
individual animal's mechanism. But no animal that was merely
complete in itself would be long sanctioned by nature. For an animal
species to survive, there must evidently, also, be proper provision for
the production of young, and the preservation of the species as well
as of the individual. Hence in an animal's physiology and psychology
we meet with a vast amount of unselfish provision, and its structure
and happiness are more essentially dependent on the good of its kind
than on its narrow personal advantage. The mammalia probably owe
their present dominant position in the animal kingdom to the
exceptional sacrifices made by them for their young. Instead of
laying eggs and abandoning them before or soon after hatching, the
females retain the eggs within their bodies until the development of
the young is complete, and thereafter associate with the