162 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



The heart lies in a pericardial cavity which is separated from 

 the abdominal or peritoneal cavity by a transverse septum, but 

 the two cavities communicate with each other by a pair of 

 pericardio-peritoneal canals. The pericardium is immediately 

 above the ventral portion of the pectoral girdle, and its dorsal 

 wall is supported by the basi-branchial cartilaginous plate. 



The heart, which contains only impure blood (i.e., blood poor 

 in oxygen, and loaded with C0 2 ), consists of sinus venosus, 

 a single auricle, a single ventricle, and a conus arteriosus. 

 The sinus venosus is a transverse tube which receives blood at 

 its two ends from the chief venous trunks. From this the blood 

 passes through a valved aperture into the auricle, which is by 

 far the largest division of the heart, and occupies the dorsal 

 half of the pericardial cavity. The thin walls of the auricle are 

 provided with a plentiful meshwork of muscles, which by their 

 contraction drive the blood through a valved auriculo-ventricular 

 aperture into the ventricle which lies on the ventral wall of the 

 pericardium. The ventricle is a nearly globular sac with a very 

 thick muscular wall, which by its contraction drives the blood 

 forwards into the conus through a valved aperture. The conus 

 arteriosus is a muscular tube running horizontally forward from 

 the ventricle to the anterior wall of the pericardium, from which 

 point it is continued forwards by the cardiac aorta. Within 

 the conus are two series of valves, viz.: the series already 

 referred to guarding the aperture from the ventricle, and a 



Fig. 46. DOGFISH. General dissection of $ , semi-diagrammatic (re- 

 duced). na, Nostrils; s.t, openings of sensory tubes; <jl, gill-slits, 

 widened in both directions by means of scissors ; p.f, right pectoral 

 fin; pl.J, right pelvic fin the left one is cut away; d, cloacal aper- 

 ture ; ab.p, abdominal pore ; the tail has been cut off. The floor of 

 the mouth-cavity and pharynx has been cut through and its left half 

 removed, and the walls of these cavities have been dissected to show 

 blood-vessels; g, cut end of gullet; st, stomach; int, intestine cut open 

 to display spiral valve; ret, rectum; Ir, liver, the bile-duct, b.d, is 

 seen crossing the bursa Entiana; pa, pancreas, the pancreatic duct is 

 seen at x ; rct.gl, rectal gland. Heart, lit, with ventral aorta, v.ao, 

 and afferent branchial arteries (shaded with transverse lines). The 

 dorsal aorta is seen to be formed by the union of efferent branchial 

 arteries (darkly shaded), arising from loops surrounding gill-clefts, 

 x x x x x ; sp, internal opening of spiracle; ca, carotid; a.ca, 

 anterior carotid ; set, subclavian ; cos, cceliac ; a. m, anterior mesenteric. 

 The ovary has been removed ; ovd, right oviduct with oviducal gland, 

 od ; ovd ', common abdominal opening of oviducts. 



