Meru, the Axis of the Lung-Globe

 The two primary bronchi serve as the front and back of Meru. The portion being spoken of in reference to the movement of the sun (to and from Meru) would be viewed as the front of the mountain and automatically, the other bronchus will be its back. 

Meru which is code for the two primary bronchi, is imagined by the ancient poet-anatomists as situated at the centre of the (respiratory) world. It divides the world into two parts, the world lying to the north of Meru and the world lying to its south. 

The sun, in either part, cannot cross the summit of Meru which is a kind of an axis for the microcosm. Its movement is only towards the primary bronchus and back (towards the distalmost end of the lung, relative to the hilum). But of course, when the earth rotates, as it were, with reference to the primary bronchus, the "front" and "back" also are interchanged. The movement of the sun now become defined with respect to the other bronchus. 

Speaking with reference to the bronchus that is "front," as the sun cannot cross this portion of Meru, the other ("back") portion always remains enveloped by darkness. 

Mid-day is defined at the top of Meru. As the sun approaches Meru, its brightness goes on increasing and as it takes the reverse route, towards the alveoli, its brightness goes on diminishing. It is midnight at the level of the alveoli. 

When the sun starts from one side of Meru it is noon there and midnight in the opposite location on the globe ie at the other primary bronchus. Then, as it travels in its chariot through the various spots of the lung, it is as if he gradually moves away from the corresponding locations. His intensity diminishes and he sets and eventually night falls and there is midnight at the level of the alveoli. In the other part of the lung-globe, the denizens of the locations opposite to those where noon, evening, night and midnight occur, experience respectively midnight, dawn, morning and noon! 

Change in the Direction of Movement 

Again, as the sun starts from the region of the alveoli, it is midnight there (and noon in the opposite side of the globe). His intensity increases as he moves towards the primary bronchus (Meru) and he now rises and the denizens of the locations falling on his path witness dawn, morning and noon. The denizens in the opposite part of the lung-globe would now experience evening, night and midnight!