The role of the lymphatic system in drainage of cerebrospinal fluid and aqueous humour.

1. The jugular lymphatic trunks were cannulated in anaesthetized rabbits and cats. Over 6-8 hr, the mean lymph flow was 2.3 microliters min-1 in the rabbit (one side only) and 5.0 microliters min-1 in the cat (sum of both sides). 2. After a single injection of radio-iodinated albumin into a lateral cerebral ventricle without significant rise in pressure, a mean of 14.4% of the radioactivity was recovered in deep cervical lymph of one side in the rabbit and of 12.9% in that of both sides in the cat. 3. During slow infusion of [125I]albumin and fluorescent dextran of 150,000 mol. wt. into a lateral ventricle of the cat at 20 microliters min-1, radioactivity and fluorescence reached plateaus in deep cervical lymph at 47.4 and 50.0% of their concentrations in cisternal c.s.f. respectively. 4. No significant radioactivity, other than from blood, was detected in superficial cervical lymph after intraventricular injection of radio-iodinated albumin in the cat. 5. No significant radioactivity, other than from blood, was detected in deep cervical lymph of the rabbit or in deep and superficial cervical lymph of the cat within 6 hr after injection of radio-iodinated albumin into the aqueous humour or orbital fat. 6. Gradients of radioactivity in tissues within the orbit suggested that there is a small flow of c.s.f., 0.05-0.15 microliters min-1 in the rabbit, passing centrifugally along the subarachnoid space of the optic nerve, through the posterior part of the globe and into the orbital tissue. Also a small proportion of aqueous humour, 1-2% or more, drains through the anterior sclera into the surrounding tissue. 7. A substantial quantity of cerebrospinal fluid drains into the deep cervical lymphatic system of the rabbit, 30% or more, and of the cat, 10-15% or more. The small component of aqueous humour drainage passing through the wall of the glove does not enter cervical lymph within 6 hr, if at all.