Objective:

To trace the experimental and conceptual evolution of the blood–brain barrier (BBB) through the work of Ehrlich, Biedl, Kraus, Lewandowsky, and Goldmann, highlighting how each contributed to defining the barrier’s existence and physiologic significance. 

Background:

In the late 19th century, neurologists and physiologists puzzled over a simple question: why did chemical dyes stain nearly every organ except the brain and spinal cord? Between 1885 and 1913, a sequence of experiments, beginning with Paul Ehrlich and culminating with his student Edwin Goldmann, revealed a physiologic boundary between blood and brain. This discovery, later termed the blood–brain barrier, changed how neurologists understood brain physiology, drug delivery, and disease. 

Design/Methods:

Primary historical sources were reviewed, including Paul Ehrlich’s late 19th-century dye perfusion studies, Biedl and Kraus’s toxin permeability experiments, Lewandowsky’s reports introducing the term blood–brain barrier, and Edwin Goldmann’s early 20th-century cerebrospinal dye investigations.

Results:

Ehrlich observed that intravenously injected aniline dyes colored all organs except the brain, which he misattributed to weak tissue affinity rather than vascular selectivity. Biedl and Kraus later showed that certain toxins produced neurologic effects only when introduced directly into the CNS, suggesting restricted permeability. Lewandowsky unified these findings by proposing a protective vascular barrier and introducing the term blood–brain barrier. Goldmann later demonstrated this visually by injecting trypan blue into the cerebrospinal fluid, which stained the brain while peripheral tissues remained colorless. Together, these experiments turned an unexplained staining observation into clear evidence of a selective barrier between blood and brain.

Conclusions:

Through the meticulous experiments of Ehrlich, Biedl, Kraus, Lewandowsky, and Goldmann, the brain’s chemical isolation was recognized decades before its anatomic basis was visualized. Their discoveries bridged 19th-century experimental physiology with modern neuropharmacology and continue to guide efforts to deliver therapeutics across the blood–brain barrier.