[No Sanskrit text]
| de ltar bsams pa de yis ni | | thugs kyis rnam dpyod gyur pa ste |
| gaṅ du gyur na ṅes sam źes | | rga bas ’chi ba mṅon par soṅ |
de ltar: thus (iti, evam)
bsams pa: think
thugs: mind, heart (citta)
kyis: [instrumental particle]
rnam dpyod: discernment
gyur pa: become, arise
gang du: in which, where (yatra)
gyur na: if it so happened, if it became so
nges: certainly, verily
rga ba: aging
'chi ba: death, dying (死)
mngon par song: involved, fully occupied, engrossed
EHJ's translation from the Tibetan:
52. After thus considering, he reflected in his mind, “What is it verily, whose existence causes the approach of old age and death? ”
Rightly considering, inwardly he reflected from what source birth and death proceed ; (SB)
In his right mindfulness, he inwardly considered the origination of birth and death. (CW)
In Aśvaghoṣa presentation of the 12 links, he is going to work backwards (against the grain; pratilomam) from link no. 12 (sufferings of aging & death) towards link no. 1 (ignorance).
So in this verse the bodhisattva asks what is the cause of link no. 12 (sufferings of aging and death), and in the next verse he will answer by identifying link no. 11 (birth).
In the maṇḍala copied below from the BBC website, going with the grain (anulomam) would mean working around the rim of the wheel in a clockwise direction.
Going against the grain (anulomam) means working anti-clockwise, from aging and death  at 11 o' clock, back to birth  at 10 o' clock.