The cartilage of the third eyelid: a comparative macroscopical and histological study in domestic animals.

The purpose of this comparative study was to evaluate morphological differences between the cartilages of the third eyelid in dogs, cats, pigs, cows, small ruminants and horses. For that reason a total of 83 third eyelids were investigated. By the aid of a modified maceration technique, the three-dimensional form of the cartilage could be demonstrated for the first time. Generally, the cartilage consists of a long narrow appendix which is followed by a variable crossbar. In dogs the appendix is cone shaped in the basal end and extends to form a triangular plate. The former is crescent-like in shape and has a marked bulge. The cartilage of the cat consists of an appendix which is enlarged in the proximal end as compared to the dog. The crossbar resembles a reverse s-form with ends tapering off to a point. In contrast pig and cow cartilage possess a typical anchorform whereas the cartilage of small ruminants starts with a thin rod which extends in a slightly curved form ending in an oval plate. The crossbar is crescent-like in these animals. In the horse the base of the cartilage is surrounded by a massive fatty tissue and the crossbar has a characteristic hook-form. Moreover, there are significant differences in regard to the quality of the cartilage, especially concerning the presence and distribution of elastic fibres. In cats and horses the elastic fibres of the adjacent connective tissue penetrate the perichondrium. Additionally, the centre of the cartilage shows a very dense network consisting of fine elastic fibres. In dogs, pigs, cows and small ruminants the cartilage consists of hyaline quality and only in the neighbouring connective tissue are some elastic fibres detectable.