Where this function is defined?

This is the code for timer example in esp8266..(got from a website)

My doubt is where the function timer1_enable(TIM_DIV16, TIM_EDGE, TIM_SINGLE) is defined ??
I am look in Ticker.cpp, but not found.

#include < ESP8266WiFi.h>
#include < Ticker.h >

Ticker blinker;

#define LED 2  //On board LED

//=======================================================================
void ICACHE_RAM_ATTR onTimerISR(){
    digitalWrite(LED,!(digitalRead(LED)));  //Toggle LED Pin
    timer1_write(600000);//12us
}

void setup()
{
    Serial.begin(115200);
    Serial.println("");

    pinMode(LED,OUTPUT);

    //Initialize Ticker every 0.5s
    timer1_attachInterrupt(onTimerISR);
    timer1_enable(TIM_DIV16, TIM_EDGE, TIM_SINGLE);
    timer1_write(600000); //120000 us
}

void loop()
{
}