348 MANUAL OF ZOOLOGY SECT. 



arterial blood, i.e. blood which contains abundance of oxygen. 

 The veins have thin, non-elastic walls, which collapse when 

 the vessel is empty, and are provided with valves ; the 

 contained venous blood is darker in colour than the arterial, 

 and has been deprived of oxygen in the tissues. Both 

 arteries and veins ramify extensively, the ultimate branches 

 being of very small size. Connecting together the ultimate 

 branches of the arteries and the ultimate branches of the 

 veins is a system of microscopic vessels the capillaries. 



The heart is ventral and anterior in position. In the 

 Dog-fish it will be found to lie in a space, the pericardial 

 cavity, between the two rows of gills, and separated behind 

 from the general body-cavity (abdomen) in which the majority 

 of the internal organs are contained, by a transverse fibrous 

 partition. It consists of four chambers, the sinus venosus, 

 auricle, ventricle, and conus arteriosus. The venous blood 

 enters the sinus venosus from the great veins and passes 

 through the other three chambers in succession in the order 

 given. All the chambers contract rhythmically, and by 

 their contractions the blood is propelled from chamber to 

 chamber, and finally driven out from the heart, its passage 

 in the opposite direction being prevented by the presence of 

 valves. These are placed in the openings leading from cham- 

 ber to chamber, and are so arranged that while they permit 

 the ready passage of the blood in the direction above men- 

 tioned, they close up the opening when pressure is exerted in 

 the opposite direction : thus, for example, when the auricle 

 contracts the valve guarding the opening leading back into 

 the sinus venosus closes that opening, while the valve in the 

 aperture leading into the ventricle opens freely, and the 

 blood passes readily in that direction. The ventricle is by 

 far the most muscular of the four chambers, since it is 

 mainly by its contractions that the blood is forced through 



