Complex sentence: The operation on his spinal cord allowed Sternberg to breathe deeper and easier, to speak more clearly and in greater volume and to remain upright for longer periods, which had the effect of improving his feeling of well being.
Split into simpler sentences: The operation on his spinal cord allowed Sternberg to breathe deeper and easier, to speak more clearly and with greater volume. Sternberg was also able to remain upright for longer periods which improved his feeling of well being.