414 THE DIGESTIVE APPARATUS IN MAMMALIA. 



STRUCTURE, The serous membrane does not envelop the whole of the 

 organ ; that portion which traverses the bottom of the pelvic cavity being 

 left uncovered by it. The muscular layer is very thick, and composed 

 of large, longitudinal, and slightly spiral fasciculi, beneath which are 

 annular fibres. The mucous membrane, loosely attached to the muscular 

 tunic, shows longitudinal and transverse rugae. The small mesenteric and the 

 internal pudic artery (artery of the bulb) supply these membranes with blood. 

 The nerves are derived from the pelvic or hypogastric plexus. 



ANUS. The anus, or posterior opening of the digestive tube, is situated 

 at the posterior extremity of the rectum, under the base of the tail, where, 

 in Solipeds, it can be seen forming a rounded prominence which diminishes 

 with age. It is the border or margin of this orifice, which is corrugated 

 like the mouth of a draw-purse, where the intestinal mucous membrane 

 meets with, and is continued by, the external skin. 



In proceeding from within outwards, there are found the following 

 elements entering into tho structure of the anus : 1, The mucous membrane 

 of the rectum ; 2, The prolongation of the circular and longitudinal fibres 

 of the muscular layer, forming what is named the internal sphincter; 

 3, A sphincter muscle, composed of red fibres, which receives the insertion of 

 a retractor ; 4, The fine, hairless, and closely-attached skin, which covers 

 the sphincter ; though destitute of hair, it is rich in sebaceous follicles. We 

 will only notice the muscles. 



The SPHINCTER OF THE ANUS, (sphincter ani) is formed of circular fibres, 

 some of which are fixed above, under the base of the tail, and are confounded, 

 below, with the muscles of the perineal region. Comprised between the 

 skin and the prolongation of the muscular layer of. the rectum, this muscle 

 is (during life) in a state of almost permanent contraction to keep the anal 

 aperture closed, it being only relaxed during the expulsion of faecal matters. 



The RETRACTOR OP THE ANUS, or iscHio-ANAL muscle (retractor ani), is a 

 wide band attached to the internal surface of the ischiatic ligament (sacro- 

 ischiatic), and even to the supercotyloid crest, by aponeurotic fibres. The 

 fasciculi composing this band are all parallel to each other, while their 

 posterior extremities are insinuated beneath the sphincter, and are confounded 

 with its fibres. This arrangement of the retractor ani clearly indicates that 

 it pulls the anus forwards, re-establishing it in its normal position after 

 expulsion of the faeces : an act which always results in carrying the posterior 

 extremity of the rectum backwards. 



These two muscles are of a red colour, and belong to those of animal 

 life. Their vessels are derived from the same sources as those of the rectum. 

 The hfflmorrhoidal nerve supplies them both with filament 



DIFFERENTIAL CHARACTERS OF THE INTESTINES IN OTHER THAN SOLIPED ANIMALS. 



In the domesticated animals, the intestines vary as much in their dimensions, length, 

 and diameter, as in their general disposition. 



1. The Intestines of Ruminants. (Fig. 208.) 



The small intestine of the Ox floats at the extremity of a broad mesenteric lamina, 

 which is narrower in front than behind, plane throughout, except at its intestinal border, 

 which is of considerable length, and is folded into a multitude of festoons. Twice the 

 length of the small intestine of the Horse averaging about 49 j'ards it is about 

 one-half its diameter. The duodenum, at first sustained by the' omentum, which 

 attaches the small curvature of the fourth compartment of the stomach to the posterior 

 fissure of the liver, forms a particular loop, which is in contact with the sublumbar 

 region before it goes to be suspended by the large mesentery, and to be continued by the 

 cjuvol-utions of the floating portion. The ileum terminates as in the Horse. Tho 



