Karana in Hindu philosophy is the sense organs (Indriya). There are in all thirteen Indriyas or Karanas in Hinduism. This is mentioned in Samkhya Karika (verses 32 – 33).
Ten of the sense organs are the external organs (bahya karana). They are:
Five sense organs – organs of sight (eye or Chakshu), organ of touch (skin or twacha), organ of sound (ear or shrotra), organ of smell (nose or ghrana), and organ of taste (tongue or rasana).
The five motor organs (karmendriya) – hands (hasta), feet (pada), anus (payu), sex organ (upastha) and organ of speech (jivha).
There are three internal organs (antraindriya or antah – karana). These are
Manas or mind
Asmita or ahamkara (ego)
Budhi or mahat-tattva or lingamatra (intellect).
The manas (mind) is connected with the ten external organs as well as with the buddhi (intellect). It grasps the stimuli gathered by the sense organs and submits them to intellect, which recognizes them, understands them, names them, decides the action to be taken and nature of them, stores the traces of experiences in itself, and can revive these traces later, on particular occasions. Its decisions for action are carried by the manas to the motor organs. Thus the manas is said to be working in two ways (ubhayatmaka) (Samkhya Karika verse 27)
The external organs work only in the present tense, not in the past or future. But the internal organ is said to be able to be in contact with objects which may be past, present or future.
Hence the external organs are called doors through which the internal organs goes out (Samkhya karika verse 35)
Source – Encyclopedia of Hinduism Volume V page 496