Question

the \boxed{} is the space between two neurons where the axon of a sending neuron communicates with the dendrites of a receiving neuron by using chemical messages.

Explanation:

In neuroscience (a subfield of Biology, which is part of Natural Science), the synapse is the junction between two neurons. At the synapse, the axon of the presynaptic (sending) neuron releases neurotransmitters (chemical messages) that cross the synaptic gap to interact with the dendrites of the postsynaptic (receiving) neuron. This matches the description in the question about the space between two neurons for chemical communication.

Answer:

synapse