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Intradermal injections are somewhat difficult to correctly administer because the skin layers are so thin that it is easy to accidentally punch through to the deeper subcutaneous layer.
Normal urine is sterile. It contains fluids, salts, and waste products. It is free of bacteria, viruses, and fungi.
Barbituric acid, the base material of barbiturates, was first synthesized in 1863 by Adolph von Bayer. His company later went on to synthesize aspirin for the first time, and Bayer aspirin is still a popular brand today.
Hippocrates noted that blood separates into four differently colored liquids when removed from the body and examined: a pure red liquid mixed with white liquid material with a yellow-colored froth at the top and a black substance that settles underneath; he named these the four humors (for blood, phlegm, yellow bile, and black bile).
The strongest synthetic topical retinoid drug available, tazarotene, is used to treat sun-damaged skin, acne, and psoriasis.
![(A) The structure of a neuron, showing the dendrites, nerve cell body, and axon. (B) Photomicrograph](https://biology-forums.com/gallery/47/medium_14755_27_08_12_4_56_11_821215.jpeg)
![Cerebral angiography. The four angiograms each reveal the distribution of blood vessels supplying th](https://biology-forums.com/gallery/47/medium_14755_14_10_12_6_48_58_94111673.jpeg)