Refractive cornea
In the eyes of most mammals, birds, reptiles, and most other terrestrial vertebrates (along with spiders and some insect larvae) the vitreous fluid has a higher refractive index than the air. In general, the lens is not spherical. Spherical lenses produce spherical aberration. In refractive corneas, the lens tissue is corrected with inhomogeneous lens material (see Luneburg lens), or with an aspheric shape. Flattening the lens has a disadvantage; the quality of vision is diminished away from the main line of focus. Thus, animals that have evolved with a wide field-of-view often have eyes that make use of an inhomogeneous lens.As mentioned above, a refractive cornea is only useful out of water. In water, there is little difference in refractive index between the vitreous fluid and the surrounding water. Hence creatures that have returned to the water—penguins and seals, for example—lose their highly curved cornea and return to lens-based vision. An alternative solution, borne by some divers, is to have a very strongly focusing cornea. Reflector eyes
An alternative to a lens is to line the inside of the eye with "mirrors", and reflect the image to focus at a central point. The nature of these eyes means that if one were to peer into the pupil of an eye, one would see the same image that the organism would see, reflected back out.Many small organisms such as rotifers, copepods and flatworms use such organs, but these are too small to produce usable images. Some larger organisms, such as scallops, also use reflector eyes.
Post-thrombotic syndrome (PTS), also called postphlebitic syndrome and venous stress disorder is a medical condition that may occur as a long-term complication of deep vein thrombosis (DVT). Signs and symptoms
Signs and symptoms of PTS in the leg may include:
pain (aching or cramping)
heaviness
itching or tingling
swelling (edema)
varicose veins
brownish or reddish skin discoloration
ulcerThese signs and symptoms may vary among patients and over time. With PTS, these symptoms typically are worse after walking or standing for long periods of time and improve with resting or elevating the leg.PTS lowers a persons quality of life after DVT, specifically with regards to physical and psychological symptoms and limitations in daily activities. Cause
Despite outgoing research, the cause of PTS is not entirely clear. Inflammation is thought to play a role as well as damage to the venous valves from the thrombus itself. This valvular incompetence combined with persistent venous obstruction from thrombus increases the pressure in veins and capillaries. Venous hypertension induces a rupture of small superficial veins, subcutaneous hemorrhage and an increase of tissue permeability. That is manifested by pain, swelling, discoloration, and even ulceration. Risk factors
The following factors increase the risk of developing PTS:
age > 65
proximal DVT
a second DVT in same leg as first DVT (recurrent ipsilateral DVT)
persistent DVT symptoms 1  month after DVT diagnosis
obesity
poor quality of anticoagulation control (i.e. dose too low) during the first 3 months of treatment

Diagnosis
When physicians find a DVT in the clinical history of their patients, a postthrombotic syndrome is possible if the patients have suggestive symptoms.