Picture: In the neck the main vessels into and from the brain (carotis communis, jugularis interna) right under the muscle that is controlling the positions of the neck (muscular sternocleidomastoides) very easily loose some of their circulation when the neck muscles are tight or tense, due to typical poor head posture in slouched sitting. The prevention is good posture which is the easiest to attain by sitting on a swinging two-part saddle chair.
Original source of picture: Paulsen, Waschke, Sobotta Atlas of Human Anatomy, 15th Edition 2011©Elsevier GmbH, Urban & Fischer, Munich.
Up to 25 % of the total circulation goes into the head. The vessels are in a narrow space between the neck vertebrae, throat and outer neck muscles (mainly a muscle called Sternocleidomastoides). All static poor postures of the head or the neck make muscles tense and reduce circulation in the main vessels. The main vein (Jugularis Interna) is actually leaning to Sternocleidomastoides. If the muscle is tense, the circulation slows down, the blood pressure in the brain increases and headache easily follows. Good posture of the neck can be attained only when the whole back is in a good posture. Soft elbow pads close to the body help the shoulder area muscles to relax. The two-part saddle seat enables a good posture and prevents the aforementioned problems.
When the circulation in the head is functioning properly also the eyes get enough oxugen. They don't get tired and their degeneration is slower.
Picture: Notice the place of the vessels under the muscle. Even the smallest tension (bowing the head, etc.) blocks the circulation into and from the head.