GATE OF LOS

It appears that Los has a gate through which one may pass between the world of Satan and the world of Jesus the Imagination. 
Inside is Golgonooza, outside are Satan's mills.

 Milton, Plate 35 [39], (E 126)
"But the Larks Nest is at the Gate of Los, at the eastern
Gate of wide Golgonooza & the Lark is Los's Messenger"

Jerusalem, Plate 12. (E 155)
"The curtains, woven tears & sighs, wrought into lovely forms
For comfort. there the secret furniture of Jerusalems chamber    
Is wrought: Lambeth! the Bride the Lambs Wife loveth thee:
Thou art one with her & knowest not of self in thy supreme joy.

Go on, builders in hope: tho Jerusalem wanders far away,
Without the gate of Los: among the dark Satanic wheels.

 Albion fled through the gate of Los. Efforts to return him to Eden were futile because he would not return voluntarily. 

 
Jerusalem, Plate 35 [39], (E 181)
"By Satans Watch-fiends tho' they search numbering every grain
Of sand on Earth every night, they never find this Gate.
It is the Gate of Los. Withoutside is the Mill, intricate, dreadful
And fill'd with cruel tortures; but no mortal man can find the Mill
Of Satan, in his mortal pilgrimage of seventy years              

For Human beauty knows it not: nor can Mercy find it! But
In the Fourth region of Humanity, Urthona namd[,]
Mortality begins to roll the billows of Eternal Death
Before the Gate of Los. Urthona here is named Los.
And here begins the System of Moral Virtue, named Rahab.  
Albion fled thro' the Gate of Los, and he stood in the Gate.

Los was the friend of Albion who most lov'd him."

Jerusalem, Plate 38 [43],(E 186)
"So Los spoke. Pale they stood around the House of Death:      
In the midst of temptations & despair: among the rooted Oaks:
Among reared Rocks of Albions Sons, at length they rose
Plate 39 [44]
With one accord in love sublime, & as on Cherubs wings
They Albion surround with kindest violence to bear him back
Against his will thro Los's Gate to Eden: Four-fold; loud!
Their Wings waving over the bottomless Immense: to bear
Their awful charge back to his native home: but Albion dark,
Repugnant; rolld his Wheels backward into Non-Entity
Loud roll the Starry Wheels of Albion into the World of Death
And all the Gate of Los, clouded with clouds redounding from
Albions dread Wheels, stretching out spaces immense between
That every little particle of light & air, became Opake       
Black & immense, a Rock of difficulty & a Cliff
Of black despair; that the immortal Wings labourd against
Cliff after cliff, & over Valleys of despair & death:
The narrow Sea between Albion & the Atlantic Continent:
Its waves of pearl became a boundless Ocean bottomless,       
Of grey obscurity, filld with clouds & rocks & whirling waters
And Albions Sons ascending & descending in the horrid Void.

But as the Will must not be bended but in the day of Divine
Power: silent calm & motionless, in the mid-air sublime,o
The Family Divine hover around the darkend Albion.          

Such is the nature of the Ulro: that whatever enters:
Becomes Sexual, & is Created, and Vegetated, and Born."

British Museum 

Night Thoughts 

Leaving Eden or Beulah or Golgonooza through Los' gate changes one's perspective. Fourfold Eternal vision becomes threefold Sexual vision; the unifying integrating force which Blake sometimes referred to as the lineaments is lost leaving the head, heart and loins. 

To be created means to be divided from the Eternal: to lose the ability to return to the integration of Eden. 

To be Vegetated means to acquire the characteristics of a being which is capable of reproducing itself in the material world, with the implication of self-absorption and self-perpetuation. 

To be Born means to enter the world of generation and come under the pedagogue of experience as an alienated, autonomous individual whose ability to perceive reality is attenuated and distorted.