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There is of necessity co-ordination between the activity of the caudal region of the large gut and the behaviour of the sphincters which control the anal canal. There are, however, two sphincters. The internal anal sphincter of smooth muscle receives its motor supply through the hypogastric nerves from the sympathetic outflow and its inhibitory supply through the pelvic nerves from the parasympathetic outflow. The external anal sphincter of striped muscle is controlled by the somatic pudendal nerves. In this case relaxation can only be due to a reduction in frequency of existing motor impulses in the pudendal nerves. Since the two sphincters are superimposed, one upon the other, it is difficult to disentangle the roles played by these two in the control of the anal canal. Garry (1933aGarry ( , 1934 gives diagrams of the anatomical arrangement, with the nerve supply, both in the cat and in man. Floyd & Walls (1953) have an accurate drawing of the actual relationships in man. The nerve supply to the distal portion of the colon and to the anal sphincters of the cat is shown schematically in Fig. 1 of this paper. Garry (1933b) was able to show by mechanical recording in the cat that movement of an object within the distal colon, when the pelvic and pudendal nerves were both intact, led to dilatation of the anal canal. Such dilatation could still be evoked after section of both pudendal nerves had paralysed the external anal sphincter. Thus stimulation of the colon must lead to inhibition of the internal anal sphincter by impulses travelling in the pelvic nerves. But there was no actual proof for inhibition of the external anal sphincter when the colon was stimulated. Such inhibition had to be assumed, otherwise dilatation of the anal canal could not have taken place when the pudendal nerves were intact. This belief that the two sphincters relaxed simultaneously was further supported by the observations of Barrington (1921, 1931) on the reciprocal behaviour of the detrusor muscle of the urinary bladder and the striped

To find the influence of a nervous outflow on an organ in the body one may either cut the nerve, with subsequent stimulation of the cut ends, or one may simply divide the nerve. In the former case the response of the organ is to nerve impulses set up by an abnormal stimulus which may elicit impulses in quantity and in quality different from the normal, while in the latter case, after the disturbances due to trauma have subsided, the response exhibited by the viscus is due to the absence of normal impulses.The latter method has found little favour in the past. Barrington [1915], however, used just such a method, often in acute preparations, to elucidate the normal nervous control of the urinary bladder. The close developmental and physiological connection between the large bowel and the urinary bladder suggests that a similar method may be successful in investigating the normal nervous control of the large bowel. NOMENCLATURE.Because of present uncertainty about the constitution and function of the nervous outflows to the large bowel the purely morphological terms "lumbar" and "sacral" are used in preference to "sympathetic" and "parasympathetic" respectively. The names applied to the subdivisions of the outflows are those recommended by Langley and Anderson [1896]. The lumbar outflow arises from the second, third, fourth and sometimes fifth lumbar nerves. The fibres pass without interruption through the sympathetic chains and run as four or five strands on either side to the inferior mesenteric ganglia. These strands are the spinal rami of the inferior mesenteric ganglia. The inferior mesenteric ganglia, usually four in number, form a ring round the inferior mesenteric artery about 1* cm. from the origin of the artery from the aorta. The majority of the fibres of the lumbar outflow form synapses in these ganglia. From the ganglia arise the lumbar colonic nerves which accompany the inferior mesenteric artery to the colon. There also arise from the inferior mesenteric ganglia the hypogastric nerves which run caudad to join the pelvic plexus.

Barclay-Smith [1904] point out, to divide the colon into regions corresponding to those of human anatomy. Nevertheless, it is convenient to refer to the cranial and caudal regions of the large bowel and to regard the anal canal as that part encircled by the anal sphincters. The name external anal sphincter should be applied to the entire striped muscle innervated by the pudendal nerves, and the name internal and sphincter confined to the circular aggregation of smooth muscle fibres at the anus. The external anal sphincter overlaps the internal anal sphincter and surrounds the anal glands. The levator ani in man is the homologue of the pelvo-caudal muscles which flex and abduct the tail in the cat.

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