CAPILLARIES OF AORTA AND PULMONARY ARTERY. 245 



vence cavce. The system of the pulmonary artery is filled 

 with black blood, the system of the pulmonary veins is filled 

 with red blood. The cavities of the right side of the heart 

 namely, the right auricle and the right ventricle are filled with 

 black blood ; the cavities of the left side of the heart namely, 

 the left auricle and left ventricle are filled with red blood. 

 Thus of the sanguiferous organs, the system of the pulmonic 

 veins, the left cavities of the heart, and the aortic system, 

 transmit red blood ; the system of the vence cavce, the right 

 cavities of the heart, and the system of pulmonic arteries, 

 transmit black blood. Between the ramifications of the aortic 

 arterial system and the radicles of the venous system of the 

 vence cavce are interposed capillary blood-vessels. These capil- 

 lary blood-vessels are situated between the finest ramifications 

 of the aortic system and the finest twigs of the system of the 

 vence cavce. They are not all of one uniform size ; yet they do 

 not differ much from each other in calibre, the largest of them 

 not exceeding the 2000th part of an inch in diameter, while 

 they inosculate with each other, so" as to form a general net- 

 work. In these capillary vessels there are no apertures or 

 open mouths, so that the interior of the aortic system and the 

 interior of the system of the vence cavce are one continuous 

 cavity, the communication between the two divisions being 

 established by the uninterrupted cavity of the capillary blood- 

 vessels. In its passage from the fine ramifications of the 

 aortic system to the fine radicles of the system of the vence 

 cavce, through the capillary blood-vessels, the red blood changes 

 to black. In like manner between the ramifications of the 

 pulmonary artery and the extreme radicles of the system of 

 the pulmonary veins, there is interposed a system of capillary 

 vessels of equal or even greater fineness, giving the same con- 

 tinuity between the interior of the system of the pulmonary 

 artery and the interior of the system of pulmonary veins. 



