FINAL REMARKS AQgI 
the different systems of the striated voluntary musculature—for these 
have been for the most part compared in the two groups of animals 
in previous chapters—as the involuntary unstriped musculature, 
about which no word has been said. The origin of the different 
systems of unstriped muscles in the vertebrate is bound up with 
the origin of the sympathetic system and its relation to the cranial 
and sacral visceral systems. The reason why I have not included in 
this book the consideration of the sympathetic nervous system is on 
account of the difficulty in finding any such system in Ammoceetes. 
Also, so far as I know, the distribution of unstriped muscle in 
Ammoccetes has not been worked out. 
One clue has arisen quite recently which is of great importance, 
and must be worked out in the future, viz. the extraordinary con- 
nection which exists between the action of the sympathetic nervous 
system and the action of adrenalin. This substance, which is 
obtained from the medullary part of the adrenal or suprarenal glands, 
when injected into an animal produces the same effects as stimulation 
of the nerves, which belong to the lumbo-thoracic outflow of visceral 
nerves, i.¢@. the system known as the sympathetic nervous system, 
which is distinct from both the cranial and sacral outflows of visceral 
nerves. The similarity of its action to stimulation of nerves is 
entirely confined to the nerves of this sympathetic system, and never 
resembles that of either the cranial or sacral visceral nerves. 
Another most striking fact which confirms the great importance 
of this connection between the adrenals and the sympathetic nervous 
system from the point of view of the evolution of the latter system is 
that the extract of the adrenals always produces the same effect 
as that of stimulation of the nerves of the sympathetic system, 
whatever may be the animal from which the extract is obtained. 
Thus adrenalin obtained from the elasmobranch fishes will produce 
in the highest inammal all the effects known to occur upon stimula- 
tion of the nerves of its sympathetic system. 
Further, the cells, which are always associated with the presence 
of this peculiar substance—adrenalin—stain in a characteristic manner 
in the presence of chromic salts. In Ammoccetes patches of cells 
which stain in this manner have been described in connection with 
blood-vessels in certain parts, so that, although I know of no definite 
evidence of the existence of cell-groups in Ammoccetes corresponding 
to the ganglia of the sympathetic system in other vertebrates, it is 
