Pulsatile Pressures in the Cranial Fluids of Heterodontus Portusjacksoni

Abstract

ABSTRACT It has been shown in mammals (Hamilton, Woodbury & Harper, 1936, 1944) that dilation of the craniospinal blood vessels by the pressure pulses caused by straining and coughing is largely prevented by the direct communication of the raised intra-thoracic pressure to the cerebrospinal fluid (CSF). The pathways through which pressure is communicated include the loose areolar tissue around the spinal nerves, and the venous blood in the paravertebral plexuses which communicate with the large veins at the back of the thorax and abdomen. McIntosh, Estes & Warren (1956) have shown by means of myelograms that as the intrathoracic pressure rises, the spinal nerves move medially and the subarachnoid space narrows. The Port Jackson shark, Heterodontus portusjacksoni exhibits spontaneous, rhythmic, and evoked coughing during which the pharynx is contracted and water is forced from the mouth and gill slits (Satchell & Maddalena, 1972). During an evoked cough water pressure in the pharynx may exceed 70 cm H2O ; this is more than thirty times that of normal respiration. Some preliminary observations indicated that here too the rise of pressure in the pharynx is communicated via the dorsal aorta to the craniospinal vessels, and, by some more direct route to the fluid surrounding the brain. The totally different anatomy of the respiratory system of a fish compared with that of a mammal suggested that the phenomenon required investigation.