This section is from the book "Applied Anatomy: The Construction Of The Human Body", by Gwilym G. Davis. Also available from Amazon: Applied anatomy: The construction of the human body.
The small intestines are frequently resected and anastomosed with themselves or other portions of the gastro-intestinal canal. Gastro-enterostomy has been alluded to on page 406.
On opening the abdomen, if it is desired to find the upper end of the small intestine, the omentum is pulled out, drawing with it on its under surface the transverse colon. The hand is to be passed backward on the under surface of the transverse mesocolon until the spine is reached; on its left side will be felt the duodenojejunal flexure. On drawing the jejunum to the right, the ligament of Treitz will be seen. A loop 40 cm. (16 in.) down may be taken and brought up in front of the omentum and used for an anterior gastro-enterostomy, or the intestine immediately below the flexure may be used for a posterior gastro-enterostomy (see page 406). If one desires to find the lower end of the small intestine, then a search is made for the colon in the right iliac region. It is recognized by its longitudinal bands and is followed down to the ileocaecal junction. If the case is one of typhoid fever, a rapid examination is then made from the ileocaecal valve upward for perforations. It is desirable at times to determine which is the proximal and which the distal end of an intestinal loop. The best way to do so is to follow the loop down to the mesenteric attachment, as advised by Monks; if the mesentery proceeds up and to the left you have the proximal end; if, however, it is passing down to the right you have the distal end.
The intestine receives its nourishment from the mesentery and will die when detached, hence it is necessary to avoid injury or detachment of the mesentery or its vessels; when this detachment has occurred the involved portion of intestine is resected and removed.