1. A method for determining the oxyhemoglobin content in a blood vessel of a mammalian subject comprising: directing a first incident beam of laser light into an ocular blood vessel having an ocular blood vessel outer wall and an ocular blood vessel inner wall such that said first incident beam of laser light has a radius that is less than the radius of the ocular blood vessel inner wall; measuring the Doppler shift power spectrum of the scattered laser light from said first incident beam with a detector; directing a second incident beam of laser light at a different wavelength than said first incident beam into the same ocular blood vessel having an ocular blood vessel outer wall and an ocular blood vessel inner wall such that said second incident beam of laser light has a radius that is less than the radius of the ocular blood vessel inner wall; measuring the Doppler shift power spectrum of the scattered laser light from said second incident beam with a detector; comparing the total Doppler shift power of said scattered beam of laser light from said first incident beam with the total Doppler shift power of said scattered laser light from said second incident beam to obtain a differential between the total Doppler shift power of said two scattered beams of laser light of different wavelength; choosing the wavelength of the first said laser light to be at an isobestic point and the wavelength of the second said laser light to be at a point where the extinction coefficient of oxy- and deoxyhemoglobin is maximally different; choosing the wavelength of the first said laser light to be at a point where the extinction coefficient of oxy- and deoxyhemoglobin is maximally different and the wavelength of the second said laser light to be at a point where the extinction coefficient of oxy- and deoxyhemoglobin is maximally different in the opposite direction; and, correlating said differential to a reference value of oxyhemoglobin content, determined from a theoretical model of a blood vessel of a similar size, with that of the measured ocular blood vessel; thereby determining the oxyhemoglobin content in a blood vessel.