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(1) The aim of the thesis was to investigate the presence of anatomical sphincters at the ileo-caeco-rectal junction and recto-coprodeal junction of the large intestine of birds using the domestic duck (Anas platyrhynchos) as the subject (2) The methods used to study the anatomy of the digestive tract sphincters, the evidence for the existence of sphincters in the large intestine of birds, and the anatomical evidence for the existence of sphincters at the gastro-oesophageal, gastro-duodenal, and ileo-caecal junctions of mammals were outlined. (3) The detailed objectives were as follows. (a) To study the arrangement of the muscle at the ileo-caeco-rectal junction and recto-coprodeal junction using light and transmission electron microscopy and, in the case of the ileo-caeco-rectal junction, by the construction of 3-D models of the circular muscle and by scanning electron microscopy. (b) To study the ultrastructure of the muscle cell at the junctions and to provide quantitative data on their size by estimating the length and volume. (c) To study the ultrastructure of the nerve bundles at the junctions and to provide quantitative data on the number of nerve bundles and axon profiles in the circular muscle layer. (4) The arrangement of the muscle at the ileo-caeco-rectal and recto-coprodeal junctions was investigated in adult birds by means of gross observations, light microscopy and reconstruction models. (a) The muscle layers in the wall of the terminal part of the ileum consisted of four closely apposed layers including the inner longitudinal layer, the inner circular layer, the outer circular layer and the outer longitudinal layer. (b) The muscle in the wall of the caeca and rectum consisted of three layers including the muscularis mucosae (inner longitudinal muscle layer), the circular muscle layer and the outer longitudinal muscle layer. The inner portion of the circular muscle layer was found to be absent in the large intestine. (c) The muscle layer in the ileal papilla was composed only of the musculario mucosae and the circular layer, the outer longitudinal muscle layer being absent. There was no evidence for the existence of cells which were characteristic of the inner portion of the circular layer. The muscle layers around the caecal orifice consisted of the muscularis mucosae, the circular layer and the longitudinal layer. (d) The muscularis mucosae in the terminal part of the ileum was thickened at the base of the ileal papilla and was continuous laterally with the muscularis mucosae of the caeca. At the base of each caecum it was thickened and became continuous medially with the muscularis mucosae of the papilla and laterally with the muscularis mucosae of the rectum. At the recto-coprodeal junction the muscularis mucosae was slightly thickened and became continuous distally with the muscularis mucosae of the coprodeum. (e) The longitudinal muscle of the ileum did not extend into the ileal papilla. Immediately proximal to the base of the papilla it was thickened and became continuous caudally with the longitudinal muscle of the rectum. At the base of each caecum the longitudinal muscle was thickened and became continuous caudally with the longitudinal muscle of the rectum. The longitudinal muscle of the recto-coprodeal junction was not thickened. It was continuous caudally with the longitudinal muscle of the coprodeum. (f) The circular muscle of the terminal part of the ileum was massively thickened at the base of the ileal papilla forming a thick muscular ring, the ileal sphincter. The circular muscle of the caeca was thickened forming muscular rings at the caecal orifices, the right and left caecal sphincters. These three muscular rings were continuous. The circular muscle layer in the caudal part of the rectum was slightly thickened 1-2 mm cranial to the recto-coprodeal junction forming an obliquely orientated sphincter. (5) The ultrastructure of the muscle cells was investigated in adult birds, by means of transmission electron microscope. (a) The ultrastructure of the muscle cells was basically similar to the structure of the mammalian visceral muscle cells. The main part of the cell was occupied by three different types of myofilaments. Dense bodies were scattered throughout the cytoplasm among the myofilaments and associated with the thin myofilaments. Dense bands were wide and attached to large areas of the cell membrane. The plasma membrane of the cell was lined with numerous regular vesicles, the caveolae. Smooth endoplasmic reticulum was observed in the form of long tubules or as a lace-like network. Other cytoplasmic contents included free ribosomes, rough endoplasmic reticulum, microtubules and centrioles. (b) Two types of intercellular junction were identified. The gap junctions (nexuses) were found mainly in the circular muscle layer of the junction regions whilst the intermediate junctions were formed between the dense bands of cytoplasm of adjacent cells. (c) These findings were discussed in relation to the available ultrastructural information of the muscle cells in the chicken and other classes of vertebrates. (6) The size of the muscle cells in three adult birds was calculated by estimating their length and volume using the stereological method of Gabella ( 1976 ). (a) The longest muscle cells at the ileo-caeco-rectal junction occurred in the circular muscle layer at the base of the ileal papilla and around the caecal orifices. The muscle cells at the base of the ileal papilla were tended to belonger than those in the ileum and rectum 5 mm from the junction.The muscle cells around the caecal orifice were longer than those in the caecum and rectum 5 mm from the junction. At the recto-coprodeal junction the muscle cells in the circular layer were evidently longer than those in the rectum and coprodeum 5 mm from the junction. (b) The largest volume of the muscle cells occurred at the junctional regions of the large intestine. The muscle cells at the base of the ileal papilla had an apparently larger volume than those in the ileum and rectum 5 mm from the ileo-caeco-rectal junction. The muscle cells around the caecal orifices had a generally larger volume than those in the caecum and rectum 5 mm from the junction. The muscle cells at the recto-coprodeal junction had a somewhat larger volume than those in the rectum and coprodeum 5 mm from the junction. (c) The differences in the cell length and volume between the sphincter and non-sphincter regions were discussed and compared with similar data in other vertebrates. (7) The distribution and the ultrastructure of the nerve bundles in the muscle layers was investigated in adult birds by means of the transmission electron microscope. (a) Nerve bundles and vesiculated axon profiles were distributed mainly throughout the circular muscle layer and were rarely seen in the inner and outer longitudinal layers. (b) Numerous nerve bundles, the plexus muscularis profundus, were observed in the connective tissue between the inner and outer portions of the circular muscle in the terminal part of the ileum. (c) Small and large axon profiles were identified. Small axon profiles contained mainly microtubules and neurofilaments, whilst large profiles contained mainly granular and agranular vesicles. Three types of axon profile terminals were described. The first type of profile contained predominantly agranular vesicles and was probably cholinergic. The second type of axon contained small granular vesicles and a varying number of agranular vesicles and was probably adrenergic. The third type of profile contained mainly large granular vesicles and many agranular vesicles and was probably peptidergic. (d) The non-neuronal cells were Schwann cells, interstitial cells and fibroblasts. Schwann cells gave rise to many long, thin processes which ensheathed many axons. Some structural differences between the interstitial cells and fibroblasts were observed. (e) These observations were discussed in the light of the available ultrastructural information in other classes of vertebrates. (8) The density of innervation in the circular muscle of the junctional regions was estimated in ten adult birds by counting the number of nerve bundles and axon profiles, and the percentage of vesiculated axon profiles per approximately 1000 muscle cells., (a) The muscular rings at the base of the ileal papilla and around the caecal orifices were more innervated than the non-thickened circular muscle of the ileum, caecum and rectum 5 mm from the ileo-caeco-rectal junction. The innervation of the circular muscle at the recto-coprodeal junction was denser than the innervation of the circular muscle in the rectum and coprodeum 5 mm from the junction. (b) The total number of axon profiles was greater at the base of the ileal papilla and around the caecal orifices than in the circular muscle of the ileum, caecum and rectum 5 mm from the junction. At the recto-coprodeal junction the total number of axon profiles was also greater at the junction than in the rectum and coprodeum 5 mm from the junction. (c) The number of vesiculated axon profiles and their percentage of the total number of axon profiles was significantly greater in the circular muscle at the base of the ileal papilla and around the caecal orifice than in the circular muscle of the ileum, caecum and rectum 5 mm from the junction. At the recto-coprodeal junction the number of the vesiculated axon profiles and their percentage was also significantly greater than in the rectum and coprodeum 5 mm from the junction. (d) The innervation of the thickened rings at the base of the ileal papilla and around the caecal orifices was significantly denser and contained more vesiculated axon profiles than in the ileum, caecum and rectum 5mm from the junction.