The branching patterns in the blood vascular system in the avian hind limb have been studied comparatively in 43 species representing 16 orders of the class Aves. Six main arterial patterns are distinguishable in the upper part of the limb. Interspecific variations in the patterns include differences in the origin of three larger arteries (a. femoralis caudalis, a. genicularis medialis and a. cruralis medialis) and the differences in the size relationships of a. femoralis and a. ischiadica. The last artery constitutes the main supply to the leg in most birds, except Spheniscus, Musophaga, Chiroxiphia, Procnias and Pipra where the entire hind limb is supplied by the a. femoralis. This is made possible by the presence of an anastomosis between the distal part of the a. ischiadica and a. femoralis medialis. Six arterial patterns are distinguishable in the feet of birds, which could all be derived from a basic, hypothetical pattern. The interspecific differences in the pedal arterial pattern include differences in the number of dorsal metatarsal arteries and differences in the origin of the digital arteries. The variability of the venous system is treated more superficially. The findings are discussed and compared with analogous differences in the branching pattern in the vascular system in the head of birds. The significance of the leg vasculature in phylogenetic studies is briefly discussed. Summary Patterns in the hind‐limb vascular system were studied in 43 species of birds, using plastic corrosion casts and dissection of Indian ink injected legs. The sciatic artery was the main artery of the leg, except in Spheniscus, Musophaga, Procnias and Pipra, wherein the hind limb is supplied by the femoral artery through a highly developed anastomosis between the a. femoralis medialis and the distal part of the a. ischiadica. The following arteries are branches either of the femoral artery system or of the ischiadic‐popliteal system: a. circumflexa femoris, a. nutricia femoris proximalis, a. femoralis caudalis, a. genicularis medialis and a. cruralis medialis. On the basis of these differences, six arterial patterns are distinguishable in the upper part of the avian hind limb. Differences in the six main pedal arterial patterns are determined by the number of dorsal metatarsal arteries and the point of origin of the digital arteries. One dorsal metatarsal artery is present in most birds, two such occur in penguins, and three in the pigeon and owl. The main pedal vein is a large medial metatarsal vessel in all species, except in the penguin where a large vein has evolved on the dorsal aspect of the foot. Vascular interspecific differences are discussed and their significance in phylogenetic studies is assessed. A haemodynamic principle may explain some of these differences. Sincere gratitude is tendered to Professor J. J. Baumel, Department of Anatomy, Creighton University, Omaha, Nebraska, who willingly read the manuscript and gave many helpful comments and to Professor K. G. Wingstrand for his interest in the work and fo...

The structure and development of the blood vascular system in the head of the herring gull (Larus argentatus) have been studied using injection techniques and histological sections. Three different but interconnected divisions of the arterial system are recognized in the adult: the cerebral carotid artery system, the external ophthalmic artery system, and the external carotid artery system. Embryologically, the arterial system is characterized by changes in the relative development of these divisions; the cerebral carotid system being the most prominent in the first half of the embryonic period. The venous system is divided into two parts, the rostral cephalic system and the caudal cephalic system, which drain separate regions of the head. The Rete ophthalmicum , which is an arteriovenous network associated with the external ophthalmic artery system, can be identified from the fifth day of incubation, and its development appears to be coupled with changes in the arterial supply to the eye. The possibility of a homology between the Rete ophthalmicum of birds and the Rete caroticum of mammals is briefly discussed.

A morphometric analysis has been made of the rete ophthalmicum which is a functional arteriovenous heat exchange system in the heads of birds. The morphological data were obtained from histological sections of the rete from 40 species of birds covering a weight range of 20,000-fold. The maximal number of retial arteries, the length, the diameter of the arteries, the total arterial cross-sectional area, and the maximal exchange area have been expressed in terms of allometric equations. Other parameters important for the countercurrent heat exchange effect have been estimated or calculated. By use of simple physical laws for ideal heat exchangers, it is shown that the temperature decrease of the arterial blood passing the rete is rather constant over a wide range in body weight, but larger than previously reported body-to-brain temperature differences. The results are consistent with the heat exchange function of the rete and provide support for the idea that the rete, besides serving as a cooling system for the brain, also may be important in reducing heat loss from the eye.

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Midtgird, U . 1981. The rete tibiotarsale and arterio-venous association in the hind limb of birds: a comparative morphological study on counter-current heat exchange systems. (Institute of Comparative Anatomy, University of Copenhagen, Denmark.) -Acta zoot. (Stockh.) 62 (2) : 67-87.Arterio-venous association was investigated in the tibia1 region of 66 bird species representing 21 orders. An intermingled network of arteries and veins (rete tibiotarsale) was found in all stages of complication, ranging from the most simple with only 3 arteries and 5-7 veins (tinamou and owl) to the most elaborate types with more than 60 arteries and 40 veins (flamingo). As opposed to what has hitherto been believed, elaborate retia are present in many shortlegged species, while most of the long-legged wading birds only have retia of a very simple type. Arctic forms of eider ducks were found to possess elaborate retia comparable to those in other ducks. In species lacking the rete tibiotarsale, the single artery and its counter-current veins are forming a venae comitantes system. The number of counter-current veins increases with foot size and in ravens and crows up to 8 veins were surrounding the artery, resembling the pattern described in the fins of whales. In species of gulls and guillemots, the contact surface between the veins and the artery appeared to be higher in arctic forms than in more southerly distributed birds. The findings are discussed in relation to heat economy, foot size, and ecology of the species. Further, theories for the evolution of the different types of heat exchange systems are presented. It is concluded that no single selective factor can explain the evolution and occurrence of the fete tibiotarsate. Uffe Midtga'rd, Institute of Comparative Anatomy, Uniuersitetsparken 15, DK-2100 Copenhagen 0, Denmark.the pattern in the Herring Gull (Larus argentutus) applies to most birds.Arteries. At the back of the knee, the popliteal artery gives rise to two arteries, which emerge on the cranial part of the shank through the proximal interosseos foramen ( F p , Fig. 1A). These small arteries chiefly supply the musculature on the cranial and lateral aspect of the shank ( m . extensor digitorum longus, m. tibialis anterior, and the peroneal muscles). One of the arteries, J. Fjeldsi, Zoological Museum, Copenhagen;