v. iz VASCULAR SYSTEM OF ELASMOBRANCHS 161 



the veins. There is a passage of unknown function, the pericardio- 

 peritoneal canal, leading from the pericardium to the abdominal 

 coelom and the hinder end of this is very narrow. 



A caudal sinus from the tail opens into a renal portal system above 

 the kidneys. From the latter, and from the muscles of the back, blood is 

 collected into the pair of very large posterior cardinal sinuses, lying on 

 the dorsal wall of the coelom. Above the heart these receive the open- 

 ings of other large sinuses, such as the anterior cardinal sinus, running 

 above the gills and collecting blood from the head by way of an orbital 

 sinus, and the jugular, lateral cardinal, subclavian, and other sinuses from 

 the body wall. Blood then passes round the oesophagus in the two 

 ductus Cuvieri into the sinus venosus, where hepatic sinuses also open. 



The resistance offered by a vessel to flow within it decreases with 

 approximately the fourth power of the diameter, therefore the large 

 size of these vessels substantially assists in allowing return to the 

 heart. The heart-muscles, like any others, require antagonists; they 

 can contract in one direction only, and each chamber therefore needs 

 to be actively dilated. It will be noted that the fish heart consists of 

 a series of three muscular chambers, presumably because the low 

 venous pressure is able to dilate only a chamber with very thin walls, 

 such as the sinus venosus. Contraction of the sinus then inflates the 

 auricle, and the auricle inflates the ventricle, which thus constitutes 

 the third step in this serial pressure-raising system. In the land animals, 

 where most of the blood only passes through a single set of capillaries, 

 a two-step system (auricle and ventricle) is sufficient for each part of 

 the circulation. 



Little is known of the control of the circulation but it is probably 

 less effective than in higher animals. There is a cardiac branch from 

 the vagus ending in an elaborate plexus in the sinus venosus (Fig. 104). 

 Stimulation of this nerve slows the heart. There is no anatomical or 

 physiological evidence of a sympathetic nerve to the heart, but 

 abundant sympathetic fibres run to the arteries. Small doses of adren- 

 aline cause prolonged rise of blood-pressure. 



There are receptors in the efferent branchial vessels and in the post- 

 branchial plexus above the cardinal veins (p. 175). Nerve impulses 

 from these receptors can be recorded in the vagus at each systole and 

 are increased by raising the blood-pressure. Their reflex effects are 

 to slow the heart and respiration and decrease the blood-pressure, 

 perhaps for protection of the gill capillaries. These reflexes are pre- 

 sumably the ancestors of the carotid sinus and similar reflexes of land 

 vertebrates. 



