90 THE GENERAL CIRCULATION. 



blood, by the pulmonary veins, proceeding from thence to 

 the left side of the heart, thence by the aorta, to every part 

 of the body. The second division beginning with the black 

 blood, in the extremities and circumference of the body, 

 and proceeding thence to the lungs, its place of termination. 



It will be seen that this mode of division includes an- 

 other and distinct circulation in the lungs, to be noticed in 

 another place. 



The direction of the vessels belonging to the vascular 

 system, with some exceptions, is generally straight, and 

 proceeds in right lines. 



All its different divisions have free communication, the 

 one with the other, as the capillary, portal, general, and 

 pulmonary. So likewise with the several parts composing 

 each division. This mode of connection is called Anasto- 

 mosis, (from cwa, through, and $<yia, mouth,) where the ves- 

 sels open like mouths, and run into each other. By means 

 of injections, the arteries are often traced into the veins 

 the lymphatics also; thus demonstrating the free inter- 

 course between the arterial, venous, and lymphatic systems. 



J. F. Meckel points out three different forms of anasto- 

 mosis in the arteries: 



1st. Where two arteries run into each other and form an 

 arch, the place of union not being known. This is the most 

 common form, and is always found at the different articu- 

 lations, and among the branches of the mesenteric arteries 

 in the abdomen. A most striking example of this form is 

 the beautiful curve formed by the union of two branches of 

 the superior and inferior mesenteric arteries, called the 

 great meso-colic arch, and a hundred similar arcades can 

 be seen in the intestinal arteries. 



The 2d form of anastomosis, is where two vessels unite 

 by a small transverse branch, as in the anterior and poste- 

 rior cerebral arteries forming the circle of Willis, and in 

 the pulmonary artery and aorta joined by the ductus arte- 

 riosus. 



The 3d form is where two vessels come together at an 

 acute angle, to form one common trunk, as we see at the 



