i6o 



ORGANIZATION OF THE HEAD 



atrium, and ventricle, all of which are muscular, and there is also a 

 muscular base to the ventral aorta, the conus arteriosus, provided with 

 valves (Fig. 102). 



The five afferent branchial arteries carry blood to the gill lamellae, 

 whence it is collected by a system of four efferents and connecting 

 vessels into a median dorsal aorta, carrying blood to all parts of the 

 body. Oxygenated blood is supplied to the head from three sources. 

 (1) From the top of the first gill a carotid artery leaves the efferent 

 branchial and runs forwards and towards the midline: it then divides 



f. eF pef 



Fig. 102. Diagram of the branchial circulation of an elasmobranch fish. 



aa. median anterior prolongation of aorta; ac. anterior carotid; afa. afferent vessel of spiracu- 

 lar gill; aef. afferent vessel from last hemibranch; af. anterior efferent vessel; a/. 2-6 , five 

 afferent vessels from ventral aorta; afa. afferent artery of spiracular gill; c. conus leading to 

 ventral aorta; cl. coeliac artery; d. ductus Cuvieri; da. dorsal aorta; ef. epibranchial artery; 

 h. a. hyoid afferent vessel; hp. hepatic veins; ht. heart; pc. posterior carotid; pef. posterior 



efferent vessel; s. spiracle; va. ventral artery; I-V, branchial slits. 



(From Goodrich, Vertebrata, A. & C. Black, Ltd., after Parker.) 



into an external carotid to the upper jaw and internal carotid to the 

 brain. (2) The dorsal aorta divides at its front end into branches, 

 which join the carotids before their division. (3) From the vessel 

 that collects blood from the first gill arises a hyoidean artery, carry- 

 ing oxygenated blood to the spiracle. From here the hyoidean artery 

 runs on as the anterior carotid (Fig. 102) across the floor of the orbit 

 to join the internal carotid within the brain-case. 



The heart is supplied by a cardiac artery arising from the dorsal 

 aorta behind the gills. The blood-pressure in the ventral aorta is 30-40 

 mm Hg and there is a drop of 10-20 mm Hg across the gills. The 

 circulation is slow, with a mean circulation time as low as 2 minutes. 

 The venous return of the fishes is ensured by a system of very large 

 sinuses. The pericardium is almost completely enclosed in a carti- 

 laginous framework by the basibranchial plate above and pectoral 

 girdle below it. It may be that this produces a negative pressure in 



