Evolutionary Trends In The Mammalian Digestive System


General Functions of the Digestive System

With the emergence of mammals, the digestive system was modified in a variety of ways depending on the animal's diet.  For example, the cat has simple large intestines, while the horse has a voluminous large intestine. Animals utilize different diets. Accordingly, their digestive tracts are different.  The digestive system includes the alimentary canal (a tube that extends from the mouth to the anus), oral structures (lips, teeth, tongue) and accessory digestive glands (salivary gland, liver, and pancreas).  In addition to oral structures, the principal parts of the digestive system are pharynx, esophagus, stomach, small intestines, and large intestines.  Many of these structures have evolved very little since the earliest fishes.    

The major function of the digestive system is to provide nutrients to the body tissues.  This involves: Ingestion of food; chewing; swallowing ; secretion of enzymes, acids, buffers, and bile), movement and mixing of food; absorption, and getting rid of the waste products


Mechanical and chemical digestion ...

Digestion involves mechanical and chemical activities.  The mechanical part of digestion involves tearing, grinding and mixing food with enzymes.  Chemical digestion is done by various digestive enzymes.


Oral Cavity

The oral cavity is responsible for the grasping and ingestion of food, grinding of food, moistening and lubrication, and formation of food boluses that are then plunged into the pharynx for swallowing.  The structures of the oral cavity include lips, cheeks, teeth, tongue, palate, and salivary glands.

~Secondary Palate

The evolution of the secondary palate allows for dual mastication and breathing.  The hard palate separates the nasal passages from the mouth.  This first appears in reptiles and further evolves into the mammalian secondary palate.  


During mammalian evolution, tongue became more prominent in the process of digestion.  It became more mobile and muscular.  In turn, the tongue could be used to manipulate food while in the oral cavity.  In addition to a means of digestion, the tongue also became the primary focus of taste receptors.  Another specialization of the tongue, as seen in the cat, is the development of tongue papillae.  These spiky structures are used as rasping devices when feeding, as well as grooming devices.        


The specialization of teeth was a significant aspect of mammalian evolution.  The teeth of a mammal are specialized to meet the needs of that animals diet.  Teeth aid in tearing, cutting, and mastication of food.  For instance, the cat, whose diet is mainly carnivorous possess teeth that resemble large blades with serrated edges for slicing or scissor-like cutting edges.      

~Salivary Glands~
The major salivary glands are:

bullet parotid
bullet sublingual
bullet mandibular salivary glands.

Salivary glands are an important aspect of mammalian evolution because they produce saliva, which facilitate mastication and swallowing of food.  It provides buffered fluid in ruminants to neutralize the acidity in the rumen produced as a result of fermentation.  Some animals secrete salivary amylase which converts starch into maltose. 


The Upper Digestive System


The esophagus is a muscular tube that moves boluses from the pharynx to the stomach. There are two muscle layers in the esophagus: circular and longitudinal. These muscle layers, when they contract and relax, help in the movement of bolus of food by peristalsis. Peristalsis consists of relaxation followed by contraction.

~The Stomach~

In mammals, the caudal end of the esophagus is closed off by the cardiac sphincter.  This structure allows for the passage of food from the esophagus to the stomach.  When the bolus reaches the stomach, the first portion of the stomach is the cardiac portion of the stomach.  In this region, mucus is secreted.  The next region of the stomach is the fundus.  This portion of the stomach secretes digestive enzymes that are responsible for the chemical breakdown of the ingested food.  The last portion of the stomach is the pyloric region of the stomach.  It, like the cardiac region, secretes mucus.  Food in then passed through the pyloric sphincter into the duodenum.  Other specializations in the stomach are apparent in mammals with other types of diets. 


The Lower Digestive System           

~Small Intestines~

The small intestines are the longest part of the alimentary canal and the site of most enzymatic digestion and virtually all absorption of nutrients.  Most of the digestive enzymes are secreted by the pancreas.  The small intestine runs from the pyloric sphincter through three subdivisions.  The subdivisions include:

bullet The Duodenum
bullet The Jejunum
bullet The Ilium

Because of its carnivorous diet, the cat has evolved a relatively short small intestine and a reduced cecum.  The cecum contains bacteria that break down plant material.  In mammals, the cecum varies considerably in size, depending on the animals diet. 

~Large Intestines and Anus~

The large intestines include:

bullet Ascending Colon
bullet Transverse Colon
bullet Descending Colon
bullet Rectum

For mammals and other tetrapods, the entire large intestines is an important instrument in the conservation of water.  Another function of the large intestine is to consolidate the feces.  The rectum temporarily stores the feces until it is released through the anus.


Accessory Organs

~Liver~    secretes digestive solutions containing bile salts.

~Gall Bladder~    stores the bile secreted by the liver. 

~Pancreas~   secretes digestive enzymes.


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