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Digestive system (apparatus digestorius) 195

the gastrointestinal tract. The tela submucosa thus serves cle) and result in vasoconstriction. Parasympathetic fibres
VetBooks.ir both to supply the physiological requirements of the adja- acting on the plexus nervorum myentericus increase gut
cent layers, and to enable these layers to move and flex motility and promote glandular secretion.
The muscle layers are responsible for aboral transport
with respect to one another. It also supports the longitu-
dinal progression of ingesta through the digestive tract. of ingesta, via waves of contraction that take place along
The tela submucosa contains abundant free immune varying lengths of the tract. Peristaltic contractions also
cells (macrophages, lymphocytes and plasma cells), dif- promote mixing of the ingesta with digestive enzymes.
fuse lymphatic tissue and lymphoid follicles, reflecting the
continuous immune responses occurring in the tubular Tunica adventitia
digestive organs. Together with the lamina propria, the The tunica adventitia consists of loose connective tissue
tela submucosa plays an important role in immune defence and serves to connect the tubular digestive organs with
because the mucosa is constantly exposed to microorgan- surrounding tissues. It encloses vessels, nerve fibres, nerve
isms including bacteria and viruses. Foreign animal and plexuses and adipose tissue. In regions outside the pleural
vegetable proteins entering the gastrointestinal tract may and peritoneal cavities (neck, caudal pelvic cavity) it forms
also elicit innate and adaptive immune responses. Proteins the outermost layer of the digestive tract.
lose their antigenic effect only after they have been enzy-
matically degraded into amino acids, which are then Tunica serosa
absorbed across the mucosa. Within the pleural and peritoneal cavities (including the
cranial pelvic cavity) the exterior of the tubular digestive
Tunica muscularis organs is coated with tunica serosa. This is composed of
Except for species-specific modifications in the oesophagus a superficial layer of simple squamous epithelium which,
and anus, the tunica muscularis is composed of smooth due to its mesodermal origin, is also referred to as meso-
muscle comprising an inner circular layer (stratum thelium. The primary functions of the mesothelium are
circulare) and an outer longitudinal layer (stratum lon- pinocytotic absorption and phagocytosis of substances
gitudinale). In the oesophagus, smooth muscle may be in the body cavities, and secretion of serous fluid. The
replaced by striated muscle. Blood and lymphatic vessels mesothelium is connected to the underlying layers of the
course between the muscular layers. Autonomic ganglia digestive tract by a thin connective tissue lamina propria
(Auerbach plexus, plexus nervorum myentericus) are serosae, which also facilitates movement of adjacent layers
also present. with respect to one another. A tela subserosa composed of
Chemical and mechanical stimuli activate the plexus loose connective tissue lies between the tunica serosa and
nervorum myentericus, from which efferent impulses the tunica muscularis.
act upon muscle cells of the tunica muscularis and vessel
walls, and on glands. This intramural autonomic system Oesophagus
is regulated by sympathetic and parasympathetic nerve The oesophagus transports food from the pharynx to the
fibres. Sympathetic fibres have an inhibitory effect on the stomach. This is achieved by peristaltic contraction of the
plexus nervorum myentericus (relaxation of smooth mus- muscles in its wall. Accordingly, the tunica muscularis is

10.21 Oesophagus (horse). A lamina muscularis mucosae
is present throughout. The tunica muscularis is striated
to the level of the root of the lung. Haematoxylin and
eosin stain (x12).

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