Ancient Epitome of Canon XVII.
In time of service lessons shall be interspersed with the Psalms.
It was well to separate the Psalms by lessons when the congregation was gathered in church, and not to keep them continuously singing unbroken psalmody, lest those who had assembled might become careless through weariness.
This was an ancient custom which has been laid aside since the new order of ecclesiastical matters has been instituted. 
Here it may be remarked we find the real reason why in our present rite, the lections, verses, etc., of the nocturns are placed between the Psalms, so as to repel weariness.
 I do not understand this note, as to-day in the Divine Office of the Greek Church the Psalms are still divided by Lessons. Vide The Horologion(horologion to mega) and an English translation by G. V. Shann, entitled Euchology, A Manual of Prayers of the Holy Orthodox Church.