TRANSACTIONS OF SECTION I. 729 
Thirdly, physiology seeks in the nervous system how by its ‘ conductivity ’ the 
separate units of an animal body are welded into a single whole, and from a mere 
collection of organs there is constructed an individual animal. 
This third line of inquiry, though greatly needing more data from the second 
and the first, must in the meantime go forward of itself. It is at present busied 
with many questions that seem special—hence its work is generally catalogued as 
Special Physiology. But it includes general problems. In the time before us 
I would venture to put before you one of these. 
When we regard the nervous system as to this, which I would term its 
integrative function, we can distinguish two main types of system according to the 
mode of union of the conductors—(i.) the nerve-net system, such as met in Medusa 
and in the walls of viscera, and (ii.) the synaptic system, such as the cerebro-spinal 
system of Arthropods and Vertebrates. Inthe integrative function of the nervous 
system the unit mechanism is the reflex. The chain of conduction in the reflex is a 
neryous arc, running from a receptor organ to an effector organ, e.g. from a sense- 
organ toalimb-muscle. We may still, I think, conveniently accept the morphological 
units termed neurones as units of construction of the reflex arc. It may be that 
these neurones are in some cases not unicellular but pluricellular, That question 
need not detain us now. Accepting the neurone as the unit of structure of the 
reflex chain, the characteristic of the synaptic system is that the chain consists of 
neurones jointed together in such a way that conduction along the chain seems pos- 
sible in one direction only. These junctions of the neurones are conveniently termed 
synapses. The irreversible direction of the conductivity along the neurone chain 
is probably referable to its synapses. This irreciprocity of conduction especially 
distinguishes the synaptic nervous system from the nerve-net system. 
The first link of each reflex chain is a neurone which starts in a receptor organ, 
e.g. a sense-organ, A receptive field, eg. an area of skin, is always analysable into 
receptive points, and the initial nerve-path in every reflex arc starts from a receptive 
point or points. A single receptive point may play reflexly upon quite a number of 
different effector organs. It may be connected through its reflex path with many 
muscles and glands in various parts. Yet all its reflex arcs spring from the one 
single shank, so to say; that is, from the one afferent neurone that conducts from 
the receptive point at the periphery into the central nervous organ. This neurone 
dips at its deep end into the great central nervous organ, the cord or brain. There 
it enters a vast network of conductive paths. In this network it forms mani- 
fold connections. So numerous are its potential connections there, that, as shown 
by the general convulsions induced under strychnia-poisoning, its impulses can 
discharge practically every muscle and effector organ in the body. Yet under 
normal circumstances the impulses conducted by it to this central network do 
not irradiate there in all directions. Though their spread over the conducting 
network does, as judged by the effects, increase with increase of stimulation of 
the entrant path, the irradiation remains limited to certain lines. Under weak 
stimulation of the entrant path these lines are sparse. The conductive network 
affords, therefore, to any given path entering it some communications that are 
easier than others, This canalisation of the network in certain directions from 
each entrant point is sometimes expressed, borrowing electrical terminology, by 
saying that the conductive network from any given point offers less resistance along 
certain circuits than along others. This recognises the fact that the conducting 
paths in the great central organ are arranged in a particular pattern. The pattern 
of arrangement of the conductive network of the central organ reveals somewhat 
of the integrative function of the nervous system. It tells us what organs work 
together in time. The impulses are led to this and that effector organ, gland or 
muscle, in accordance with the pattern. The success achieved in the unravelling 
of the conductive patterns of the brain and cord is shown by the diagrams furnished 
by the works of such investigators as Edinger, Exner, Flechsig, van Gehuchten, 
v. Lenhossek, v. Monakow, Ramon, and Schafer. Knowledge of this kind stands 
high among the neurological advances of our time. 
But we must not be blind to its limitations. The achievement may, though 
more difficult, be likened to tracing the distribution of blood-vessels after Harvey’s 
