FINAL REMARKS 49 1 



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 Aminocoetes. 

 Also, so far as I know, the distribution of unstriped muscle in 

 Ammocoetes 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.e. 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 mammal 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 Ammocoetes 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 Ammocoetes corresponding 

 to the ganglia of the sympathetic system in other vertebrates, it is 



