204 ANIMAL BIOLOGY. [Parti. 



3. (a) The resistance to the blood-flow is maximum in the 

 carotid trunk owing to the small size of the 

 vessels and the carotid gland ; 



(b) The resistance to the blood-flow is minimum in the 



pulmo-cutaneous trunk owing to the short course 

 and large capillaries of the lungs ; 



(c) The resistance in the systemic aorta being medium ; 

 therefore it follows that 



1. Venous blood passes first along the line of minimum 



resistance, to the left of the swing-valve, to the 

 lungs ; then the capillaries of the lungs being 

 gorged with blood, 



2. Mixed blood flows next along the line of medium 



resistance to the system ; 

 finally, as these capillaries also become gorged, 



3. Arterial blood flows last along the line of maximum 



resistance to the brain and head. 



It will be understood that, as the capillaries of the lungs 

 become gorged, the resistance in the pulmo-cutaneous artery rises 

 so as to exceed that in the systemic aorta ; wherefore the blood 

 passes along that branch. And as it flows along the right of the 

 longitudinal swing-valve, it tends to fold it down to the left 

 over the pulmo-cutaneous orifice. Then as the systemic capil- 

 laries in turn become gorged, the resistance along that line 

 becomes greater than that which was at first the maximum in 

 the carotid; which thus becomes the channel of the most 

 arterial blood. 



The Rabbit. The student should lay open the thoracic cavity 

 of the rabbit and dissect away the thymous gland and other 

 tissues which hide the origin of the great vessels, so as to 

 display the heart and vessels as shown diagrammatically in 

 Fig. 63. 



The heart has two auricles and two ventricles. From the 

 right ventricle (r. v.) there arises the pulmonary artery (pul. a.), 

 the part of the ventricle leading up into it being known as the 

 conus arteriosus. The pulmonary artery passes over towards 



