Full context:   The brains of vertebrates are made of very soft tissue. Living brain tissue is pinkish on the outside and mostly white on the inside, with subtle variations in color. Vertebrate brains are surrounded by a system of connective tissue membranes called meninges, which separate the skull from the brain. Cerebral arteries pierce the outer two layers of the meninges, the dura and arachnoid mater, into the subarachnoid space and perfuse the brain parenchyma via arterioles perforating into the innermost layer of the meninges, the pia mater. The endothelial cells in the cerebral blood vessel walls are joined tightly to one another, forming the blood–brain barrier, which blocks the passage of many toxins and pathogens (though at the same time blocking antibodies and some drugs, thereby presenting special challenges in treatment of diseases of the brain). As a result of the osmotic restriction by the blood-brain barrier, the metabolites within the brain are cleared mostly by bulk flow of the cerebrospinal fluid within the glymphatic system instead of via venules like other parts of the body.