The anatomy of the teleost lymph system is considered in relation to the observed flow of lymph. The plaice Pleuronectes platessa L. was studied.
The vessels described by anatomists as longitudinal ventral, dorsal and lateral lymph ducts do not pass lymph to the head but act as collecting sinuses; the lymph flows from these sinuses to the neural lymph duct via the interspinal ducts.
At the junction of each interspinal duct and the neural lymph duct there is a simple valve. Lymph drains through this system as a result of a negative pressure developed in the neural canal by the respiratory movements of the gill apparatus.
The rigid nature of the neural lymph duct and the valves where the interspinal ducts join it allow the negative pressure to develop from a negative positive gill generated pres-sure cycle in the cephalic lymph sinuses. Valves Unking cephalic lymph sinuses aid the return of lymph to the duct of Cuvier using the same pressure cycles.
The drainage system is different in the salmonidae where the neural lymph duct is degenerate and the network of peripheral ducts serve as lymph return ducts. This can probably be explained by the occurrence of higher circulation pressures in the salmonidae.
Lymph from the neural lymph duct is a clear fluid having a composition similar to that of blood plasma. Like blood, lymph clots, and contains normal blood levels of mobile leucocytes and thrombocytes and lymph reflects blood levels of lactate. Lymph contains nearly no erythrocytes and the protein concentration of neural duct lymph is 80–4% that of blood plasma.