The United States expelled immigrants from Central America.

Feeling world

1. Humans have different senses and receptors, and can feel all kinds of stimuli, which are introduced into the nerve center through nerves, resulting in different senses. For example:

The organ that perceives sound-the ear

Nose-the organ that senses smell

The organ that perceives light-the eye

Tongue, the organ that senses the taste of food

Skin-the organ that feels cold and hot pain

2. Skin has sensory functions such as touch, pain, cold and hot sensation, especially the sensory function of pain, which is beneficial to people's healthy life.

3. Different parts of the skin have different sensitivities to various stimuli. For example, fingertips are generally used for touching, and the tactile nerve endings in this part are relatively concentrated. To test the temperature of the patient's forehead, generally use the back of the hand, because the nerve endings on the back of the hand are sensitive to heat.

4. The process of olfactory formation: The external odor actually enters the nasal cavity, stimulates the olfactory nerve endings, and then reaches the olfactory center of the brain through the olfactory nerve. After brain analysis, the smell will be known.

(1) "I don't know if it stinks for a long time" This is because the olfactory center of the brain has adapted (fatigue).

(2) The olfactory sensitivity of different animals is very different.

(3) With the increase of age, the sensitivity of sense of smell gradually decreases.

(4) Animals have different sensitivities to the smells of different substances.

5. There are taste receptors on the tongue, which can feel the stimulation of various foods.

(1) Taste cells on the tongue can feel the stimulation of different substances, especially liquid substances.

(2) Different parts of the tongue have different sensitivities to sweetness, sourness, bitterness and saltiness.

(3) When people catch a cold, the sensitivity of sense of smell decreases, and the sensitivity of taste sometimes decreases.

(4) The formation of taste: Some chemicals in food are dissolved in saliva, which stimulates the taste cells on the tongue, and then spread to the brain through the taste nerve. After brain analysis, the taste of food is known.