En-face analysis of short posterior ciliary arteries crossing the sclera to choroid using wide-field swept-source optical coherence tomography.

To study the topographic distribution of the short posterior ciliary arteries (SPCA) entry sites into the choroid in normal eyes using structural en-face swept source optical coherence tomography (SS-OCT). Retrospective analysis of SS-OCT scans (wide-field structural SS-OCT 12 × 12 mm) of 13 healthy subjects was performed. Cross-sectional swept-source OCT scans derived from a volume scan were represented as en-face image display following the Choroid-Scleral Interface to obtain en-face OCT. SPCAs in their last scleral location before choroidal entrance were identified manually, counted and localized by two masked observers. Correlations between two masked observers were analyzed using inter- and intra-class correlation. Accuracy for the choroidal inner and outer border segmentation was 95-99%. Eighteen eyes from 13 normal subjects were included for SPCA analysis. The mean number of arteries was 13.8 ± 3.5 per eye. Thirty-six percent were in the center of the posterior pole image; however, 21% were in the temporal part of the posterior pole. Median accuracy of the detection is 0.94. The correlation between the two observers was fair (0.54). Our algorithm allows visualization of the SPCA at the posterior pole of the eye using wide-field en-face SS-OCT. It can also help the clinicians to study the SPCAs in numerous ocular diseases, particularly its relationship with focal choroidal diseases.

Exploring choroidal angioarchitecture in health and disease using choroidal vascularity index.

The choroid is one of the most vascularized structures of the human body and plays an irreplaceable role in nourishing photoreceptors. As such, choroidal dysfunction is implicated in a multitude of ocular diseases. Studying the choroid can lead to a better understanding of disease pathogenesis, progression and discovery of novel management strategies. However, current research has produced inconsistent findings, partly due to the physical inaccessibility of the choroid and the lack of reliable biomarkers. With the advancements in optical coherence tomography technology, our group has developed a novel quantitative imaging biomarker known as the choroidal vascularity index (CVI), defined as the ratio of vascular area to the total choroidal area. CVI is a potential tool in establishing early diagnoses, monitoring disease progression and prognosticating patients. CVI has been reported in existing literature as a robust marker in numerous retinal and choroidal diseases. In this review, we will discuss the current role of CVI with reference to existing literature, and make postulations about its potential and future applications.

Inter-Relationship of Arterial Supply to Human Retina, Choroid, and Optic Nerve Head Using Micro Perfusion and Labeling.

Purpose: The prevailing view is that the human retina is supplied by the central retinal artery (CRA), the short posterior ciliary arteries (SPCAs) support the choroid, and both the CRA and the SPCAs are so-called "end artery" systems. In this study, we investigate whether vascular connections among the retina, choroid, and the optic nerve head (ONH) exist, using selective cannulation and microperfusion-labeling techniques. Methods: The CRA and/or one or more of the SPCAs were selected for cannulation in 18 human donor eyes. Fluorescent probes with different excitation wavelengths were perfused through different arteries on the same eye to distinguish the supply sources of different vascular beds. After labeling and fixation, the ONH region was dissected either longitudinally or transversely as thick sections for confocal microscopy. Retina, choroid, and ONH were imaged from whole-mount specimens. Results: Probes perfused through the CRA or the SPCA alone labeled the microvessels in the retina, choroid, and ONH regions, as well as the optic nerve trunk. The vessels of the lamina cribrosa and the optic nerve trunk were labeled when probes were perfused through the SPCA. Perfusion through both the CRA and SPCA produced double labeling of vessels in the retina, the choroid, and the ONH. Conclusions: The results indicate an inter-relationship of arterial supply to the retina, choroid, and ONH in the human eye. This has important implications in understanding clinical observations and disease mechanisms such as that of glaucoma and ischemic optic nerve disease.

Características clínicas de la cabeza del nervio óptico en recién nacidos sanos / Clinical features of optic nerve head in healthy newborns

OBJETIVO: Determinar el tamaño y características clínicas del disco óptico en recién nacidos sanos a término. METODOLOGÍA: Estudio descriptivo, observacional, transversal y retrospectivo basado en expedientes clínicos completos de recién nacidos en el Hospital Ángeles de Pedregal (Ciudad de México) entre mayo y agosto del 2015. Las medidas del disco óptico y las características clínicas fueron obtenidas de fotografías digitales y evaluadas por dosexpertos. Para los datos obtenidos utilizamos el cálculo de frecuencias, medias y desviación estándar con el programa SPSS versión 17.0 para Windows. RESULTADOS: Tuvimos un total de 121 pacientes y 121 ojos elegidos, 65 fueron varones. El diámetro vertical promedio fue de 1,60 ± 0,17 mm, el diámetro horizontal de 1,13 ± 0,13mm, el área de disco óptico de 1,43± 0,26 mm2, la excavación de 0,25 ± 0,11. El color del anillo neurorretiniano era naranja en 116 (95,9%) ojos. La lámina cribosa no era visible en ningún paciente, en 90 ojos (74,4%) la aparición de los vasos en la papila era central y nasal en los restantes. En 38 ojos (31,4%) estuvo presente la arteria ciliorretiniana. La presencia de un anillo peripapilar hiper- e hipopigmentado se evidenció en 114 ojos (94,2%). CONCLUSIÓN: El promedio de área del nervio óptico es 1,43 ± 0,26 mm2, ligeramente superior a lo reportado por otros autores; la excavación promedio es menor a 0,3, como se informa en la literatura y llama la atención que todos los ojos tienen un anillo peripapilar hiperpigmentado e hipopigmentado

OBJECTIVE: To determine the size and clinical features of the optical disc in healthy term infants. METHODOLOGY: Descriptive, observational, cross-sectional, and retrospective study based on complete medical records of newborns at the Angeles Pedregal Hospital, Mexico DF, from May to August 2015. The measurements of the optical disc and clinical features were obtained from digital photographs and evaluated by 2experts. Frequencies, means and standard deviation were calculated using the SPSS version 17.0 for Windows. RESULTS: The study included a total of 121 patients and 121 eyes. There were 65 males. The mean vertical diameter was 1.60±0.17mm, horizontal diameter 1.31±0.13mm, optical disc area 1.43±0.26mm2, and cup to disc ratio 0.25±0.11. The neuroretinal ring colour was orange in 116 (95.9%) eyes. The cribriform plate was not visible in any patient, and in 90 (74.4%) eyes the appearance of the vessels in the papilla was central, and nasal in the rest. The cilioretinal artery was present in 38 (31.4%) eyes. A hyper- and hypo-pigmented peripapillary ring was observed in 114 eyes (94.2%). CONCLUSION: The mean area of the optic nerve is 1.43±0.26mm2, slightly higher than previously reported. The mean cup to disc ratio is less than 0.3, as reported in the literature, and it is noteworthy that all eyes have a hyper-pigmented and hypo-pigmented peripapillary ring

An optical coherence tomography-based analysis of choroidal morphologic features and choroidal vascular diameter in children and adults.

PURPOSE: To analyze choroidal sub-layers and vascular diameter in children, and to compare these choroidal features with those of adults. DESIGN: Retrospective observational study. METHODS: This study included 96 eyes from 48 healthy children and 54 eyes from 27 healthy adults. The subfoveal choroidal thickness, large choroidal vessel layer thickness, medium choroidal vessel layer-choriocapillaris layer thickness, and large choroidal vessel diameter were estimated. The ratio of thickness of the large choroidal vessel layer to total choroidal thickness was calculated. The association between subfoveal choroidal thickness and large choroidal vessel layer, as well as ratio of thickness of the large choroidal vessel layer to total choroidal thickness, was analyzed. Furthermore, the ratio and choroidal vascular diameter were compared between children and adults. RESULTS: The mean age was 6.7 ± 1.9 years and 30.7 ± 4.3 years in children and in adults, respectively. In children, the mean ratio was 0.71 ± 0.08 and the mean choroidal vascular diameter was 103.1 ± 16.0 µm. In adults, the values were 0.73 ± 0.08 and 122.5 ± 20.7 µm, respectively. The subfoveal choroidal thickness in children was significantly associated with the ratio (P < .001), whereas the association was not significant in adults (P = .173). The choroidal vascular diameter was significantly greater in adults than in children (P < .001). However, the ratio was not different between the 2 groups (P = .391). CONCLUSIONS: Choroidal morphologic features are generally comparable between children and adults. Some differences between the 2 groups may reflect changes in choroidal morphology associated with aging.

Angiography of the retina and the choroid with phase-resolved OCT using interval-optimized backstitched B-scans.

In conventional phase-resolved OCT blood flow is detected from phase changes between successive A-scans. Especially in high-speed OCT systems this results in a short evaluation time interval. This method is therefore often unable to visualize complete vascular networks since low flow velocities cause insufficient phase changes. This problem was solved by comparing B-scans instead of successive A-scans to enlarge the time interval. In this paper a detailed phase-noise analysis of our OCT system is presented in order to calculate the optimal time intervals for visualization of the vasculature of the human retina and choroid. High-resolution images of the vasculature of a healthy volunteer taken with various time intervals are presented to confirm this analysis. The imaging was performed with a backstitched B-scan in which pairs of small repeated B-scans are stitched together to independently control the time interval and the imaged lateral field size. A time interval of ≥ 2.5 ms was found effective to image the retinal vasculature down to the capillary level. The higher flow velocities of the choroid allowed a time interval of 0.64 ms to reveal its dense vasculature. Finally we analyzed depth-resolved histograms of volumetric phase-difference data to assess changes in amount of blood flow with depth. This analysis indicated different flow regimes in the retina and the choroid.

Variable velocity range imaging of the choroid with dual-beam optical coherence angiography.

In this study, we present dual-beam Doppler optical coherence angiography with variable beam separation. Altering beam distance, independently of the scanning protocol, provides a flexible way to select the velocity range of detectable blood flow. This system utilized a one-micrometer wavelength light source to visualize deep into the posterior eye, i.e., the choroid. Two-dimensional choroidal vasculature maps of a human subject acquired with different beam separations, and hence with several velocity ranges, are presented. Combining these maps yields a semi-quantitative visualization of axial velocity of the choroidal circulation. The proposed technique may be useful for identifying choroidal abnormalities that occur in pathological conditions of the eye.

A restudy of the surgical anatomy of the posterior aspect of the globe: an essential topography for exact macular buckling.

PURPOSE: To study the anatomy of the posterior aspect of the globe comparing it with previous textbook descriptions, to find constant anatomical landmarks for macular buckling surgery. DESIGN: This is an anatomical study on cadaver eyes with additional in vivo measurements during the macular buckling procedure. METHODS: Twelve cadaver eyes were carefully dissected identifying important structures on the posterior aspect of the globe and measuring their cross-diameters and relative distances. The distance of the medial end of inferior oblique insertion from the foveal position projected on the scleral surface and the border of the optic nerve sheath were measured. The intrascleral course of temporal long posterior ciliary artery was identified and measured, and its distance from the optic nerve sheath was also measured. RESULTS: The macula consistently lies on the horizontal meridian (foveal horizontal meridian) defined by the course of the long posterior ciliary arteries, which cross the optic nerve somewhere between its center and its lower border. CONCLUSION: The horizontal meridian (foveal horizontal meridian), defined by the entry site and course of the long posterior ciliary arteries, serves as an important surgical landmark if precise external scleral buckling of the fovea is the objective. For practical purposes, foveal horizontal meridian, geometrical horizontal meridian, and optic nerve horizontal meridian need to be distinguished.

Basic technique and anatomically imposed limitations of confocal scanning laser Doppler flowmetry at the optic nerve head level.

Many studies have suggested an association between blood flow dysregulation and glaucomatous damage to the optic nerve. Confocal scanning laser Doppler flowmetry (CSLDF) is a technique that measures the capillary blood flow of the retina and optic nerve head and provides a two-dimensional map of ocular perfusion in these areas. This review discusses the anatomy of the anterior optic nerve vasculature and the capabilities and limitations of the CSLDF. Methods to minimize error and to acquire more reliable measurements of capillary blood flow are also outlined.

Histomorphometry of the circular peripapillary arterial ring of Zinn-Haller in normal eyes and eyes with secondary angle-closure glaucoma.

PURPOSE: To examine the location and size of the peripapillary arterial circle of Zinn-Haller (PACZH) and its associations with other eye measures in normal eyes and eyes with secondary angle-closure glaucoma. METHODS: The study included 29 human globes enucleated because of malignant choroidal melanoma (n = 19) (control group) or because of secondary angle-closure glaucoma (n=10). Anterior-posterior histological sections were morphometrically evaluated. RESULTS: The PACZH was present in all eyes. The glaucoma group and the control group did not vary significantly in the distance from the PACZH to Bruch's membrane (297 ± 67 versus 270 ± 67 µm; p = 0.29), optic disc border (281 ± 103 versus 391 ± 170 µm; p=0.07), optic disc centre (1059 ± 191 versus 978 ± 205µm; p = 0.30) and retrobulbar cerebrospinal fluid space (173 ± 58 versus 172 ± 81 µm; p = 0.97) nor in the minimal PACZH diameter (39± 18 versus 36 ± 18 µm; p = 0.74) and maximal PACZH diameter (78 ±37 versus 65 ± 25 µm; p = 0.36). The PACZH location, measured as distance from Bruch's membrane, disc border, disc centre and cerebrospinal fluid space, was not significantly associated with axial length (p > 0.39), horizontal globe diameter (p > 0.17) and vertical globe diameter (p > 0.22). Both diameters were statistically independent of axial length (p = 0.72 and p = 0.58, respectively), horizontal globe diameter (p = 0.60 and p = 0.41, respectively) and vertical globe diameter (p = 0.64 and p = 0.52, respectively). All parameters were statistically independent of age (p > 0.10) and gender (p > 0.10). CONCLUSIONS: The PACZH was present in all human eyes examined and did not vary significantly in location and diameter between eyes with secondary angle-closure glaucoma and nonglaucomatous eyes, nor between myopic versus hyperopic eyes.

The structural relationship between the microvasculature, neurons, and glia in the human retina.

PURPOSE: To develop a new technique for detailed study of the spatial distribution of retinal and choroidal microvasculature and their relationship to neurons and glial cells at the cellular level in human cadaveric eyes. METHODS: Twenty-six human donor eyes were used. Wherever possible, the central retinal artery and a branch of the posterior ciliary artery were individually cannulated and perfused with oxygenated Ringer's solution with 0.5% bovine serum albumin. The perfusion pressure was continuously monitored. Once residual blood was washed out, the perfusate solutions were switched to fixative, membrane-permeabilizing solution and selected labeling solutions. The eyes were then immersion fixed and the retina and choroid flat-mounted for immunolabeling and confocal imaging before cryosectioning. The microstructures of vascular, glial, and neuronal cells in the retina and the stroma in the choroid were studied. RESULTS: The retinal microvasculature was fully perfused and stained by cannulation of the central retinal artery. Regional distribution of choroidal vasculature perfusion was dependent on the specific feeder artery cannulated. The detailed spatial relationship between endothelial cells, glial cells, and neurons at the cellular and subcellular levels was identified with confocal microscopy and immunohistochemical labeling of retinal sections. In the choroid, endothelial cells were clearly identifiable down to the level of the intracellular cytoarchitecture of the choriocapillaris, along with their relationship to Bruch's membrane and the feeding and drainage vessels. CONCLUSIONS: A microperfusion fixation and staining technique has been developed that allows studies of the structural relationships of vascular, glial, and neuronal elements at the cellular level in human donor eyes.

Morphology and histological annual changes of the oviduct of chaetophractus villosus (Mammalia, Xenarthra, Dasypodidae) / Morfología y cambios histológicos anuales del oviducto de Chaetophractus villosus (Mammalia, Xenarthra, Dasypodidae)

The morphological features of the oviduct in the armadillo Chaetophractus villosus (Desmarest, 1804) were studied. In the simple columnar epithelium, ciliated cells, secretory cells, and cells of the bottom of the folds are described. The most remarkable findings are the increase in number of ciliated cells, when present, from the fimbriae to the uterus, being the isthmus practically lined by ciliated cells, and that secretory cells are more abundant in the apical zone of the ampulla and fimbriae, than in the isthmus. Morphological changes along the year in the different portions are described.

Se estudiaron las características morfológicas del oviducto del armadillo Chaetophractus villosus (Desmarest, 1804). En el epitelio, columnar simple, se describen células ciliadas, secretoras y células del fondo de los pliegues. Las células ciliadas, cuando están presentes, aumentan en número desde las fimbrias hacia el útero, estando el istmo prácticamente tapizado por células ciliadas, las células secretoras son más abundantes en la zona apical de la ampolla y en las fimbrias, que en el istmo. Se describen los cambios morfológicos en las diferentes porciones a lo largo del año.

Anterior ciliary circulation from the horizontal rectus muscles.

PURPOSE: To quantify the anterior ciliary arteries that accompany horizontal rectus muscles and to correlate these findings with analysis of arterial cross-sectional area in distal tendons of these muscles. PATIENTS AND METHODS: In 31 consecutive patients, previously unoperated horizontal rectus muscles were photographed intraoperatively. Major and minor anterior ciliary arteries (ACAs) were counted from the photographs by a masked independent observer and compared between medial and lateral rectus muscles. Cross sections from medial and lateral rectus muscle resection specimens were stained for elastin to quantify arterial volume in the distal tendons. RESULTS: Twenty lateral and 22 medial rectus muscles from 31 patients were included. There was no significant difference in the mean number of major anterior ciliary arteries between lateral rectus muscles (2.25 +/- 0.79, range 1-4) and medial rectus muscles (2.05 +/- 0.65, range 1-3) (p = 0.46). The mean number of minor anterior ciliary arteries was 6.7 in both medial and lateral rectus muscles (p = 0.97). Resected rectus muscle tendons showed no significant difference between lateral and medial rectus muscles for arterial vessel number or cross-sectional area. CONCLUSIONS: These results show the contribution from the lateral rectus muscle to the anterior segment circulation may be more robust than is commonly taught.

Vascular anatomy of the optic nerve head.

Although intraocular pressure remains the main modifiable risk factor for open-angle glaucoma, other factors such as vascular perfusion likely play a significant role. It is not clear how mechanical deformation, axonal damage, glial responses, and ischemia interact to lead to the tissue remodeling seen clinically as glaucomatous cupping. To understand the potential role of vascular risk factors in glaucoma, it is important to understand the vascular anatomy of the optic nerve head (ONH). The focus of this review is to provide a description of the vascular anatomy of the ONH and to describe recent work in the central nervous system that suggests that astrocytes play a key role in vascular regulation. Finally, the evidence for vascular regulation in the ONH and retina is reviewed.

Accurate course and relationships of the intraorbital part of the ophthalmic artery in the sagittal plane.

INTRODUCTION: Knowledge of variations in the course and distribution of the intraorbital part of ophthalmic artery (OA) is necessary for the diagnosis and treatment of anterior cranial and orbital disorders. MATERIAL: 38 human cadaver dissections to demonstrate the microsurgical anatomy of the intraorbital part of the OA were studied in three stages, considering its neighbourhood with the optic nerve in the sagittal plane. RESULTS: The first part of the OA was located on the inferolateral aspect of the optic nerve in 89.47%. The diameter and the length of the first part of the OA were 1.69+/-0.34 mm and 7.58+/-0.89 mm. 73.68% of the cases crossed the optic nerve superiorly, and 26.31% inferiorly. The diameter and length of the second part of the OA were as 1.52+/-0.29 mm and 4.12+/-0.85 mm. The diameter and length of the third part of the OA were 1.07+/-0.18 mm and 4.12+/-0.85 mm. The first branch of the intraorbital part of the OA was the central retinal artery in 26.31% of the specimens. CONCLUSION: A better understanding of the vascular anatomy of the orbit should allow for the modification of surgical techniques to reduce bleeding during biopsy or excision of orbital structures.

Topography of the posterior arteries supplying the eye and relations to the optic nerve.

PURPOSE: The aim of the study was to investigate the arterial blood supply of the intraorbital part of the optic nerve. METHODS: The location, course, length and diameter of the central retinal artery (CRA) and posterior ciliary arteries were studied in 19 adult white male preserved cadavers of between 35 and 75 years of age. RESULTS: In right eyes, the first branch of the intraorbital part of the ophthalmic artery was the CRA in 26.3% (5/19) and the CRA and medial posterior ciliary artery in 21% (4/19) of eyes. In left eyes, the first branch of the intraorbital part of the ophthalmic artery was the CRA in 47.4% (9/19) and the CRA and medial posterior ciliary artery in 26.3% (5/19) of eyes. The CRA was observed as a single branch in 57.9% and a trunk in 42.1% of right eyes, and as a single branch in 52.6% and a trunk in 47.3% of left eyes. The outer diameter of the CRA measured 0.6 +/- 0.1 mm (min-max 0.5-0.9 mm) in right eyes and 0.6 +/- 0.2 mm (min-max 0.4-0.9 mm) in left eyes. The CRA entered the optic nerve 7.5 +/- 2.2 mm (min-max 5.3-12.5 mm) behind the ocular bulb in right eyes and 7.4 +/- 2.3 mm (min-max 5.3-14.1 mm) behind it in left eyes, at its lower and medial side. The posterior ciliary arteries ran forward, divided into multiple branches and pierced the sclera close to the optic nerve medially, laterally or superiorly. The longitudinal capillaries ran between the optic nerve and the CRA antero-posteriorly, while the transverse capillaries surrounded the optic nerve. Collaterals from both the longitudinal and transverse capillaries joined to form a complicated capillary plexus. CONCLUSION: This article confirms the well known variability of the arterial circulation of the intraorbital part of the optic nerve. Better understanding of the arterial anatomy of the intraorbital part of the optic nerve should enable appropriate modification of surgical techniques.

A case of a cilioretinal artery supplying the entire retina.

The cilioretinal artery commonly supplies a small area of the retina, usually the macula. It can serve a valuable purpose in preserving the central vision in central retinal artery occlusion. We describe a case, where the total retinal circulation is from the cilioretinal artery in one eye and an abnormal central retinal artery branching in the other eye. The clinical relevance of cilioretinal artery in the eye is discussed.

Heritability of cilioretinal arteries: a twin study.

PURPOSE: To determine whether the presence of one or more cilioretinal arteries, a distinct element of the pattern of fundus vessels, is genetically programmed, influenced by environmental factors, or the result of random mechanisms of vascular development. METHODS: The fundi of 112 pairs of healthy monozygotic and dizygotic twins were examined using digital fundus photography and visual assessment of grayscale fundus photographs and color transparencies to detect the presence of cilioretinal arteries. RESULTS: Cilioretinal arteries were present in 45.1% of participants and 28.8% of eyes. The majority of cilioretinal arteries, 88.2%, were located temporally, and 11.8% were located nasally. Monozygotic twins had higher concordance rates for cilioretinal arteries than dizygotic twins. Tetrachoric correlations and Mantel-Haenszel odds ratios demonstrated statistically significant evidence of a genetic effect underlying the presence of cilioretinal arteries (P < 0.01). Statistical analysis supported the hypothesis that additive genetic factors influenced the presence of cilioretinal arteries with a heritability of 71.4%, the remaining variance being attributable to nonshared or random environmental factors. CONCLUSIONS: The presence or absence of one or more cilioretinal arteries in healthy persons is markedly influenced by genetic factors.

Angioarchitecture and innervation of the primate anterior episclera.

PURPOSE: To investigate the primate episcleral vasculature and its innervation with respect to morphological specializations. METHODS: Serial sections of the anterior episclera of 8 monkey eyes and 20 human eyes were investigated enzyme- and immunohistochemically using antibodies against smooth-muscle alpha-actin (SMA), neurofilament, synaptophysin, substance P (SP), calcitonin gene-related peptide (CGRP), vesicular acetylcholine transporter (VACHT), vasoactive intestinal polypeptide (VIP), neuropeptide Y (NPY), tyrosine hydroxylase (TH), vesicular monoamine transporter II (VMAT II), as well as the NADPH-diaphorase reaction. Arteriovenous anastomoses (AVA) were quantified. RESULTS: All episcleral vessels including veins showed intense staining for SMA. Capillary loops were only seen in the limbal arcades, not in the episclera itself. Instead, AVA connected the episcleral arteries with the veins, which formed an interlacing vascular network. In the monkey episclera, 4-6/mm2 AVA were found; in the human episclera, 0.5-1/mm2. Numerous nerve endings staining for NADPHd (NADPHdiaphorase) and TH surrounded all episcleral vessels including anastomoses and veins. NPY, VIP, and VACHT-immunoreactive (IR) nerve terminals were less numerous. CGRP and SP-IR terminals were seen both at the vessels and in the intervascular connective tissue. CONCLUSIONS: The episcleral vasculature shows a specialized morphology with absence of capillaries, numerous arteriovenous anastomoses, a muscle-rich venous network, and intense innervation by vasodilative and vasoconstrictive nerves. This might allow regulation of blood flow and volume in the episcleral vessels and Voigt's capillaries for thermoregulation and modulation of episcleral venous pressure and thereby outflow facility.