364 MANUAL OF ELEMENTARY ZOOLOGY 



and inferior jugular sinuses. At the outer end of each 

 ductus Cuvieri a subclavian sinus enters from the fore-fin. 

 On its hinder side a very large posterior cardinal sinus 

 brings back blood from the trunk. The two posterior 

 cardinal sinuses converge backwards, growing narrower, 

 and lie side by side between the kidneys, from which blood 

 passes into them by numerous renal veins. On each flank 

 two lateral sinuses return blood from the body-wall, and into 

 one of these open vessels from the fins. Blood from the 

 tail is returned by the caudal vein ; this divides opposite the 

 hinder ends of the kidneys into two renal portal veins, which 

 run forwards along the outer sides of the kidneys and 

 supply them with blood. Blood from the alimentary canal 

 and spleen is conveyed to the liver by a hepatic portal vein, 

 and thence, after passing through capillaries, is discharged 

 into the hepatic sinuses. It will be noticed that the circula- 

 tion of the dogfish contains a single circuit only, the blood 

 from the respiratory organs being carried directly to the 

 rest of the body without returning to the heart in the 

 interval. Its general course is summed up in the table on 

 p. 365. The blood cells of the dogfish resemble those of 

 the frog. 



The spinal cord of the dogfish resembles that of the frog 



in most respects and need not be described 

 Central here. The brain, although in general features 



system. it is like that of the frog, shows considerable 



differences in detail. The foremost region in 

 the middle line is the cerebrum, which corresponds to the 

 cerebral hemispheres of the frog, but is single and some- 

 what globular in shape; its double nature is shown out- 

 wardly by a shallow longitudinal groove and internally by 

 the presence of two lateral ventricles. The two olfactory 

 lobes lie at the sides of the cerebrum, each arising from it 

 by a short, stout stalk, which expands into a large mass 

 against the olfactory capsule. The lateral ventricles of the 

 cerebrum are continued into the olfactory lobes. The 

 cerebrum is followed by a thalamencephalon which is some- 

 what longer than that of the frog. From the hinder part of 

 its thin roof arises the long, hollow, slender pineal stalk, 

 which runs forward over the cerebrum to end in a small 

 swelling below the membrane which covers the anterior 



