The Magic School Bus Inside the Human Body is the third book in the book series.
Ms. Frizzle's class takes a field trip which is supposedly to a museum, but after they stop at a park for lunch she suddenly gathers all the students back onto the bus except for Arnold. Then she makes the bus magically shrink itself until it is small enough for Arnold to absentmindedly swallow. This leads to the development of two parallel stories, inside and outside of Arnold. With the bus inside, the class discovers the process of digestion from the mouth to the stomach and the small intestine. Then the bus enters a blood vessel where readers can see through the plasma where red and white blood cells fight germs and they flow into the heart where the class, now out of the bus and garbed in operating room smocks and masks, flow with the red blood cells, now with fresh oxygen, to the brain. From there they climb down the bones of the spine, follow nerves to see muscles work, and then finally catch up with the bus to emerge into the nasal cavity. Meanwhile, Arnold has coped with being lost and alone and has made it back to the school. With one sudden sneeze, he sends the bus out into the parking lot where it returns to full size. The class can now chart the body from actual experience.
- Unlike in the episode "For Lunch", the students never realize that they are inside Arnold's body. However, Phoebe says that she feels as though he is close by.
- While the first half of the field trip in the book serves as basis of "For Lunch", the second half likely inspired both "Inside Ralphie" and "Works Out".
- Ralph starts appearing regularly with the other students and for the first time wears his trademark red baseball cap.
- This is the second book to not have the Bus transform into anything. The first time was At The Waterworks.
- In the republished version, the Bus had GURGLE! as its overhead sign.
- On the cover of the first release, the title logo is seen in the font Cambria.
- On the back of the original book cover, Arnold is seen holding his stomach in sadness (Similar to Pg.13), with the robin sitting beside him, (Except in paperback version, the robin is blocked out by the rectangular box filled with names, credits and list of other Magic School Bus book titles).
- On page 8, Arnold does a report, titled "Your Tongue is Covered with Thousands of Taste Buds". on the four flavors: bitter, sour, salty, and sweet (sans umami). However, there is a difference between the first release and the republished edition. In the original, the taste buds that taste "bitter" are located at the back of the tongue, and in the republished version, "bitter" is located on the roof of the mouth. This is because research shows that taste buds exist outside the tongue, ranging from the cheeks to the soft palate. (Arnold mentioned it in the new report) The report's title changed to "Your Mouth Has Thousands of Taste Buds!".