Among fishes the anus varies, in the osseous and cartilaginous divisions of this class ; in the former it usually presents the appearance of a round opening leading into a longitudinal groove ; it is placed in front of the anal fin, and of the urinary and genital aperture, contrary to what occurs in all other vertebral animals. In the cartilaginous fish, as the ray and shark, this groove is deeper, and has the appearance of a true cloaca, through which are discharged, as in the sepice and in birds, not only the alvine, but also the urinal and seminal excretions.
In reptiles the anus serves as the opening of a cloaca, or common receptacle of the re siduum of the food, as well as of the urine, semen, or ova; in the batrachia, as the frog, it is situated at the end of the back, and there fore above the body of the animal. In the chelonia, as the tortoise, it is under the tail. In the sauria and ophidia it is a transverse cleft, but in the salamander it is a longitudinal fissure with two prominent lips.
In birds the rectum expands above the anus into the cloaca, which also receives the ter minations of the ureters, the ends of the vasa deferentia, and the penis (when the latter exists); also the openings of the oviducts, and of the bursa Fabricii. In all the maminalia the rectum terminates in a distinct anal open ing, which is placed at the posterior or in ferior extremity of the trunk, directly under the origin of the tail, and usually in a direc tion opposite to the mouth, and in all it is placed behind, and not, as in fish, before the urinary and sexual orifice ; in some few of the quadrumana, as the mandril, it is directed upwards. In almost all mammalia it is a dis tinct orifice, giving passage to the faces only ; in the beaver and sloth, however, the rectum and urethra have a common termination. The monotreinatous animals also, such as the echidni and ornithorhynci, form a complete ex ception to this statement ; in these singular and anomalous creatures a single opening gives exit to the faecal and urinary secretions, and also subserves sexual purposes. (See INTES TINAL CANAL.) ANUS (in human anatomy). In the present article we propose to examine not merely the structures which immediately bound this open ing in man, in their normal and healthy state, as well as in their abnormal and diseased condi tions, but we shall also examine the parts which enclose and surround it, and which can exert an influence, direct or indirect, on its functions ; that is, we shall consider the anatomy, normal and abnormal, of the parts contained in the Anal Region.
The Anal Region is synonymous with the posterior portion of the perinum ; its triangular area is denoted by the following outlines : the apex, which is posterior and superior, is marked by the extremity of the os coccygis ; its base, which is before and below the latter, is defined by an imaginary line extending transversely from one tuber ischii to the other, and each side is denoted by a line drawn from the last named process to the point of the coccyx : these lateral boundaries correspond to the mar gins of the gluti maximi muscles, which over lap the inferior or the great sacro-sciatic liga ments ; the base or the transverse line before mentioned, separates the anal from the anterior perineal or urethral region : in the adult male this line will be found to be about three inches, or nearly three inches and a quarter in length ; in the female it is about half an inch longer, and more certainly so if the individual ex amined have borne children ; great variety, how ever, has been found to exist in this measure ment, the extremes of which may be stated at two and four inches. In children under twelve years of age this transverse diameter of the perinum is considerably less, in consequence of the extreme narrowness of the pelvis prior to puberty.
The anal region contains the lower portion of the intestinum rectum, several muscles, and fasciae, some nerves and vessels of importance, and an abundance of adipose substance. The quantity and consistence of the adipose substance found in this region vary considerably in dif ferent individuals at the several periods of life, and under various conditions of health ; a fact most important for the surgeon to bear in mind, inasmuch as this diversity causes corresponding differences in the physical characters which this region presents under these particular circum stances. In children, and in the female, in youth and middle age, as also in the robust and healthy male, this region will be found plump, or convex around the anus, whereas in the ema ciated, the sickly, or the old, it often presents the very opposite appearances; and a proportional diflerence may be observed in the depth of the perinum, or in the distance between the neck of the bladder and the surface : the greatest extremes of this difference have been found between two and four inches, a circumstance which bears materially on the lateral operation of lithotomy.