112 TOPOGRAPHICAL ANATOMY OF THE 



often possible to distinguish oval patches of mucous membrane character- 

 ised by the presence of numerous punctiform depressions. These are 

 the aggregated lymph nodules of Peyer^ (noduli lymphatici aggregati), 

 and should be looked for along the border of the intestine opposite the 

 mesenteric attachment. Their length may range from 2 to 6 

 centimetres, and their breadth from 5 to 15 millimetres; but they 

 vary considerably in size in different parts of the intestine, becoming 

 larger as the termination of the ileum is approached. They also vary 

 greatly in different individuals, being generally more conspicuous in 

 young animals. 



Structure of the intestines. — A certain amount of the structure 

 of the intestines can be determined by the naked eye, and there will be 

 no difficulty in detecting three tunics : (1) A serous tunic (tunica serosa), 

 consisting of visceral peritoneum on the exterior; (2) a muscular tunic 

 (tunica muscularis) ; and (3) a mucous membrane (tunica mucosa) lining 

 the interior. 



To facilitate the examination of the wall of the gut, a piece should be 

 pinned on a sheet of cork or paraffin with the mucous membrane down- 

 wards, and the thin serous investment then carefully removed. The 

 dissector will notice that the serous covering of the intestine is thinner 

 than the membrane forming the mesentery with which it is continuous. 



Now turn the piece of intestine over and examine the mucous lining. 

 If the piece of gut belongs to the small intestine, and if it be .sunk in 

 water and its surface thoroughly cleared of mucin, a velvety character, 

 due to the presence of delicate projections (villi), will be noticeable. 



Next remove the mucous tunic and note that the connective tissue 

 under it and between it and the muscular tunic — the submucous tissue 

 or tela mucosa — is loose in texture. 



An examination of the muscular tunic shows that the fibres com- 

 posing it are arranged in two strata. The more superficial fibres are 

 longitudinal, and, in the large intestine, gathered together to form bands 

 (tsenise) ; the deeper stratum consists of circular fibres, particularly 

 obvious at the end of the ileum, where they form what may be regarded 

 as a sphincter muscle, and at the commencement of the colon. 



The stomach (Ventriculus). — The stomach is the most dilated part 

 of the alimentary canal, intervening between the oesophagus and the 

 small intestine. In view of the herbivorous habit of its possessor, the 

 stomach of the horse is remarkably small, its capacity being scarcely 

 ever greater than 15 litres. In form it is an elongated sac very sharply 



^ Johann Conrad Peyer, a Swiss anatomist, 1653-1712. 



