Cecum
Encyclopedia
The cecum or caecum is a pouch, connecting the ileum
with the ascending colon
of the large intestine
. It is separated from the ileum by the ileocecal valve
(ICV) or Bauhin's valve, and is considered to be the beginning of the large intestine. It is also separated from the colon
by the cecocolic junction. The appendix
is connected to the cecum . The cecum is usually peritoneal, while the ascending colon is retroperitoneal .
species, and also in lungfish
, but not in any living species of amphibian
. In reptile
s, it is usually a single median structure, arising from the dorsal side of the large intestine. Bird
s typically have two paired ceca, as, unlike other mammals, do hyrax
es.
Most mammalian herbivore
s have a relatively large cecum, hosting a large number of bacteria
, which aid in the enzymatic
breakdown of plant materials such as cellulose
; in many species, it is considerably wider than the colon. In contrast, obligatory carnivore
s, whose diets contain little or no plant material, have a reduced cecum, which is often partially or wholly replaced by the vermiform appendix
. Over 99% of the bacteria in the gut flora
are anaerobes, but in the cecum
, aerobic bacteria reach high densities.
Many fish have a number of small outpocketings, called pyloric ceca, along their intestine; despite the name they are not homologous
with the cecum of amniotes, and their purpose is to increase the overall area of the digestive epithelium. Some invertebrates, such as squid, may also have structures with the same name, but these have no relationship with those of vertebrates.
In dissection
s by the Greek
philosophers
, the connection between the ileum
of the small intestines and the cecum was not fully understood. Most of the studies of the digestive tract
were done on animals and the results were compared to human structures
.
The junction between the small intestine and the colon, called the ileocecal valve, is so small in some animals that it was not considered to be a connection between the small and large intestines. During a dissection, the colon could be traced from the rectum
, to the sigmoid colon
, through the descending
, transverse
, and ascending
sections. The colon seemed to dead-end into the cecum, or cul-de-sac.
The connection between the end of the small intestine (ileum) and the start of the colon (cecum) is now clearly understood, but the name has not changed.
Ileum
The ileum is the final section of the small intestine in most higher vertebrates, including mammals, reptiles, and birds. In fish, the divisions of the small intestine are not as clear and the terms posterior intestine or distal intestine may be used instead of ileum.The ileum follows the duodenum...
with the ascending colon
Ascending colon
The ascending colon is smaller in caliber than the cecum.It passes upward, from its commencement at the cecum, opposite the colic valve, to the under surface of the right lobe of the liver, on the right of the gall-bladder, where it is lodged in a shallow depression, the colic impression; here it...
of the large intestine
Large intestine
The large intestine is the third-to-last part of the digestive system — — in vertebrate animals. Its function is to absorb water from the remaining indigestible food matter, and then to pass useless waste material from the body...
. It is separated from the ileum by the ileocecal valve
Ileocecal valve
The ileocecal valve, or ileocaecal valve, is of a bilabial papilla structure with physiological sphincter muscle situated at the junction of the small intestine and the large intestine, with recent evidence indicating an anatomical sphincter may also be present in humans) Its critical function is...
(ICV) or Bauhin's valve, and is considered to be the beginning of the large intestine. It is also separated from the colon
Colon (anatomy)
The colon is the last part of the digestive system in most vertebrates; it extracts water and salt from solid wastes before they are eliminated from the body, and is the site in which flora-aided fermentation of unabsorbed material occurs. Unlike the small intestine, the colon does not play a...
by the cecocolic junction. The appendix
Vermiform appendix
The appendix is a blind-ended tube connected to the cecum , from which it develops embryologically. The cecum is a pouchlike structure of the colon...
is connected to the cecum . The cecum is usually peritoneal, while the ascending colon is retroperitoneal .
Variation Across Species
A cecum is present in most amnioteAmniote
The amniotes are a group of tetrapods that have a terrestrially adapted egg. They include synapsids and sauropsids , as well as their fossil ancestors. Amniote embryos, whether laid as eggs or carried by the female, are protected and aided by several extensive membranes...
species, and also in lungfish
Lungfish
Lungfish are freshwater fish belonging to the Subclass Dipnoi. Lungfish are best known for retaining characteristics primitive within the Osteichthyes, including the ability to breathe air, and structures primitive within Sarcopterygii, including the presence of lobed fins with a well-developed...
, but not in any living species of amphibian
Amphibian
Amphibians , are a class of vertebrate animals including animals such as toads, frogs, caecilians, and salamanders. They are characterized as non-amniote ectothermic tetrapods...
. In reptile
Reptile
Reptiles are members of a class of air-breathing, ectothermic vertebrates which are characterized by laying shelled eggs , and having skin covered in scales and/or scutes. They are tetrapods, either having four limbs or being descended from four-limbed ancestors...
s, it is usually a single median structure, arising from the dorsal side of the large intestine. Bird
Bird
Birds are feathered, winged, bipedal, endothermic , egg-laying, vertebrate animals. Around 10,000 living species and 188 families makes them the most speciose class of tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from...
s typically have two paired ceca, as, unlike other mammals, do hyrax
Hyrax
A hyrax is any of four species of fairly small, thickset, herbivorous mammals in the order Hyracoidea. The rock hyrax Procavia capensis, the yellow-spotted rock hyrax Heterohyrax brucei, the western tree hyrax Dendrohyrax dorsalis, and the southern tree hyrax, Dendrohyrax arboreus live in Africa...
es.
Most mammalian herbivore
Herbivore
Herbivores are organisms that are anatomically and physiologically adapted to eat plant-based foods. Herbivory is a form of consumption in which an organism principally eats autotrophs such as plants, algae and photosynthesizing bacteria. More generally, organisms that feed on autotrophs in...
s have a relatively large cecum, hosting a large number of bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
, which aid in the enzymatic
Enzyme
Enzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...
breakdown of plant materials such as cellulose
Cellulose
Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to over ten thousand β linked D-glucose units....
; in many species, it is considerably wider than the colon. In contrast, obligatory carnivore
Carnivore
A carnivore meaning 'meat eater' is an organism that derives its energy and nutrient requirements from a diet consisting mainly or exclusively of animal tissue, whether through predation or scavenging...
s, whose diets contain little or no plant material, have a reduced cecum, which is often partially or wholly replaced by the vermiform appendix
Vermiform appendix
The appendix is a blind-ended tube connected to the cecum , from which it develops embryologically. The cecum is a pouchlike structure of the colon...
. Over 99% of the bacteria in the gut flora
Gut flora
Gut flora consists of microorganisms that live in the digestive tracts of animals and is the largest reservoir of human flora. In this context, gut is synonymous with intestinal, and flora with microbiota and microflora....
are anaerobes, but in the cecum
Cecum
The cecum or caecum is a pouch, connecting the ileum with the ascending colon of the large intestine. It is separated from the ileum by the ileocecal valve or Bauhin's valve, and is considered to be the beginning of the large intestine. It is also separated from the colon by the cecocolic...
, aerobic bacteria reach high densities.
Many fish have a number of small outpocketings, called pyloric ceca, along their intestine; despite the name they are not homologous
Homology (biology)
Homology forms the basis of organization for comparative biology. In 1843, Richard Owen defined homology as "the same organ in different animals under every variety of form and function". Organs as different as a bat's wing, a seal's flipper, a cat's paw and a human hand have a common underlying...
with the cecum of amniotes, and their purpose is to increase the overall area of the digestive epithelium. Some invertebrates, such as squid, may also have structures with the same name, but these have no relationship with those of vertebrates.
Etymology
The term cecum comes from the Latin caecum, literally "blind", here in the sense "blind gut" or "cul de sac".In dissection
Dissection
Dissection is usually the process of disassembling and observing something to determine its internal structure and as an aid to discerning the functions and relationships of its components....
s by the Greek
Ancient Greece
Ancient Greece is a civilization belonging to a period of Greek history that lasted from the Archaic period of the 8th to 6th centuries BC to the end of antiquity. Immediately following this period was the beginning of the Early Middle Ages and the Byzantine era. Included in Ancient Greece is the...
philosophers
Greek philosophy
Ancient Greek philosophy arose in the 6th century BCE and continued through the Hellenistic period, at which point Ancient Greece was incorporated in the Roman Empire...
, the connection between the ileum
Ileum
The ileum is the final section of the small intestine in most higher vertebrates, including mammals, reptiles, and birds. In fish, the divisions of the small intestine are not as clear and the terms posterior intestine or distal intestine may be used instead of ileum.The ileum follows the duodenum...
of the small intestines and the cecum was not fully understood. Most of the studies of the digestive tract
Gastrointestinal tract
The human gastrointestinal tract refers to the stomach and intestine, and sometimes to all the structures from the mouth to the anus. ....
were done on animals and the results were compared to human structures
Human anatomy
Human anatomy is primarily the scientific study of the morphology of the human body. Anatomy is subdivided into gross anatomy and microscopic anatomy. Gross anatomy is the study of anatomical structures that can be seen by the naked eye...
.
The junction between the small intestine and the colon, called the ileocecal valve, is so small in some animals that it was not considered to be a connection between the small and large intestines. During a dissection, the colon could be traced from the rectum
Rectum
The rectum is the final straight portion of the large intestine in some mammals, and the gut in others, terminating in the anus. The human rectum is about 12 cm long...
, to the sigmoid colon
Sigmoid colon
The sigmoid colon is the part of the large intestine that is closest to the rectum and anus. It forms a loop that averages about 40 cm...
, through the descending
Descending colon
The descending colon of humans passes downward through the left hypochondrium and lumbar regions, along the lateral border of the left kidney....
, transverse
Transverse colon
The transverse colon, the longest and most movable part of the colon, passes with a downward convexity from the right hypochondrium region across the abdomen, opposite the confines of the epigastric and umbilical zones, into the left hypochondrium region, where it curves sharply on itself beneath...
, and ascending
Ascending colon
The ascending colon is smaller in caliber than the cecum.It passes upward, from its commencement at the cecum, opposite the colic valve, to the under surface of the right lobe of the liver, on the right of the gall-bladder, where it is lodged in a shallow depression, the colic impression; here it...
sections. The colon seemed to dead-end into the cecum, or cul-de-sac.
The connection between the end of the small intestine (ileum) and the start of the colon (cecum) is now clearly understood, but the name has not changed.
Development
The cecum and appendix are formed by the enlargement of the postarterial segment of the midgut loop.The proximal part of the bud grows rapidly to form the cecum. The lateral wall of the cecum grows much more rapidly than the medial wall with the result that the point of attachment of the appendix comes to lie on the medial side.Diseases
A cecal carcinoid tumor is a carcinoid tumor of the cecum. A appendiceal carcinoid tumor (a carcinoid tumor of the appendix) is sometimes found next to a cecal carcinoid.External links
- Photo at mgccc.cc.ms.us - "Abdominal organs in situ." - "The larger intestine." - "The cecum with the distal portion of the ileum." - "Incisions of the Cecum"
- Video clip of worms in the Cecum