now with respect to the digestive system the bulk of the viscera contributes to something
called the gastrointestinal tract or the GI T this is the space starting with the mouth and
the oral cavity and ending with the anus and all structures between that people refer to as the
GI T the digestive tract the alimentary canal or the gut sometimes you hear it as the gut tube but
oftentimes that's in the context of development all of these names reference the structures that
contribute to this pathway now there are different ways in which we can organize how we conceptualize
the the GI T one very easy way is that there are upper and lower portions to the pathway those
upper and lower portions to the pathway really are going to find a junction right about there so the
mouth the oral cavity the esophagus the stomach and the first two parts of the duodenum would be
the upper GI whereas the the lower GI would be the the second two parts of the duodenum the remainder
of the small intestines the large intestine the sigmoid colon the rectum and the anal canal now
we can also conceptualize there being a narrow digestive portion of the pathway so that would
start with the the mouth and the oral cavity and the the pharynx or the throat so this would
be a common pathway for both the respiratory and the digestive systems and then there's
the digestive pathway that would be everything beyond so the esophagus down to the anal canal
now that digestive pathway we conceptualize as having three parts so there's the foregut which
would be the esophagus stomach and first two parts of the duodenum the mid gut which would
be the second two parts of the duodenum the remainder of the small intestines the a sending
colon and about two-thirds of the transverse colon and then the hindgut which would be the
remainder of the transverse colon the descending colon the sigmoid colon the rectum and the anus
there are also accessory organs of the digestive system so these consist of the
liver and gallbladder as well as the pancreas these are glands that are going to secrete
products into the lumen of the duodenum to enhance digestion so that we may have good
absorption of materials now the layers of the GI tract are in four parts there's the mucosa which
is surrounding the lumen so the mucosa consists of epithelial or in this case endothelial tissue
and surrounding that mucosa is going to be the lamina propria and within that lamina propria
we're going to have mucosal associated lymphoid tissue and then surrounding the lamina propria
as the deepest layer of the mucosa is going to be the muscularis mucosae which is a layer of
longitudinal smooth muscle that helps to keep the mucosa in contact with the materials inside the
lumen surrounding the mucosa is going to be the submucosa the submucosa you going to be very rich
with vasculature so arteries veins some lymphatic vessels as well and mixed in there is going to be
the submucosal plexus oftentimes referred to as the plexus of Meissner so this submucosal
plexus is going to be the autonomic component specifically the parasympathetic component
that is going to innervate this muscularis mucosae and the endothelium of the mucosa
surrounding the submucosa are various layers of smooth muscle that we call
the muscularis so these will constitute a very thick layer and some of these are
going to be in circular layers surrounding the submucosa and some of these are going to
be in longitudinal layers so the longitudinal layers are what's responsible for peristalsis
whereas the circular layers are what's responsible for segmentation contraction
now innervating the muscularis is going to be the my enteric plexus oftentimes called the plexus
of Auerbach and that myenteric plexus nestles itself between the longitudinal and the circular
fibers of the muscularis and then finally surrounding all of this is going to be the
serosa and the serosa is going to be the visceral peritoneum that surrounds the gastrointestinal
tract and other intraperitoneal organs and this is going to conduct the neurovasculature from
the body to the gastrointestinal tract now different portions of the gastrointestinal
tract are going have variations of these different layers so we're going to see specializations of
these layers in particular specializations of the mucosa and the muscularis and we'll
talk about some of these specializations as we talk about these specific layers thank you