XI.--The branches of the systemic aorta form frequent anastomoses with each other in all parts of the body. This anastomosis occurs chiefly amongst the branches of the main arteries proper to either side. Those branches of the opposite vessels which join at the median line are generally of very small size. There are but few instances in which a large blood vessel crosses the central line from its own side to the other. Anastomosis at the median line between opposite vessels happens either by a fusion of their sides lying parallel, as for example (and the only one) that of the two vertebral arteries on the basilar process of the occipital bone; or else by a direct end-to-end union, of which the lateral pair of cerebral arteries, forming the circle of Willis, and the two labial arteries, forming the coronary, are examples. The branches of the main arteries of one side form numerous anastomoses in the muscles and in the cellular and adipose tissue generally. Other special branches derived from the parent vessel above and below the several joints ramify and anastomose so very freely over the surfaces of these parts, and seem to pass in reference to them out of their direct course, that to effect this mode of distribution appears to be no less immediate a design than to support the structures of which the joints are composed. XII.--The innominate artery. When this vessel is tied, the free direct circulation through the principal arteries of the right arm, and the right side of the neck, head, and brain, becomes arrested; and the degree of strength of the recurrent circulation depends solely upon the amount of anastomosing points between the following arteries of the opposite sides. The small terminal branches of the two occipital, the two auricular, the two superficial temporal, and the two frontal, inosculate with each other upon the sides, and over the vertex of the head; the two vertebral, and the branches of the internal carotid, at the base and over the surface of the brain; the two facial with each other, and with the frontal above and mental below, at the median line of the face; the two internal maxillary by their palatine, pharyngeal, meningeal, and various other branches upon the surface of the parts to which they are distributed; and lastly, the two superior thyroid arteries inosculate around the larynx and in the thyroid body. By these anastomoses, it will be seen that the circulation is restored to the branches of the common carotid almost solely. In regard to the subclavian artery, the circulation would be carried on through the anastomosing branches of the two inferior thyroid in the thyroid body; of the two vertebral, in the cranium and upon the cervical vertebrae; of the two internal mammary, with each other behind the sternum, and with the thoracic branches of the axillary and the superior intercostal laterally; lastly, through the anastomosis of the ascending cervical