Page:A manual and atlas of medical ophthalmoscopy.djvu/23



no other structure of the body are the termination of an artery and the commencement of a vein presented to view, and information regarding the general state of the vascular system is often to be gained from an inspection of their size, texture, and the conditions of the circulation within them. It must be remembered, however, that the vessels there seen are of very small size. One of the primary divisions of the retinal artery, large as it appears to direct ophthalmoscopic examination, is actually so small as to be scarcely visible to the unassisted eye, being less than the 1/100th an inch in diameter, and the smallest vessels visible with the ophthalmoscope are not more than the y-^th of an inch in diameter. But these, it must also be remembered, are considerably larger than capillaries. The retinal capillaries are always invisible, and, away from the optic disc, they are never so numerous as to occasion any recognizable reddish tint.

A second point to be remembered is that the red lines spoken of as the retinal arteries or veins are not the vessels themselves, but the columns of blood within them. The walls of the vessels are, as a rule, invisible; they are always invisible to the indirect method of examination, but by the direct method the walls of the larger branches may be sometimes seen, as fine white translucent lines along the sides of the red column of blood, most distinct where one vessel passes over another. They are best seen by feeble illumina-