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鱼际 is a ying(荥) point, useful for clearing heat, especially that related to the lung meridian e.g. sore throat. The 鱼际 point of the lung meridian lies beneath this muscle group called the thenar eminence. The thenar eminence is made up of three muscles, OAF, ie ‘O’ = opponens pollicis, ‘A’ = abductor pollicis brevis, ‘F’ = flexor pollicis brevis.

There are many methods to locate 鱼际, but for me, your needle enters between the flexor pollicis brevis (拇短屈肌) and abductor pollicis brevis (拇短屈肌), around the midpoint of the first metacarpal (see http://classes.kumc.edu/sah/resources/handkines/bone/1mc.html). The opponens pollicis (拇对掌肌), also part of the thenar group, lies deep to abductor pollicis brevis. Probably both of them sandwhich 鱼际 if an acupoint is imagined to be an entity between two muscles. One point about this covered muscle: It brings your thumb towards fingers in a grasping manner, an action humans have and most animals don’t.

All three are innervated by the median nerve (正中神经).

There is one more muscle on the palm area around the thumb - the adductor pollicis (拇收肌) - which is not part of the thenar eminence group. It is innervated by the ulna nerve.

References:
1. http://www.eatonhand.com/ner/ner075.htm
2. http://en.allexperts.com/e/t/th/thenar_eminence.htm