Functions of various parts of the digestive tract: Mouth, Oesophagus

Mouth:
The primary function of the mouth include prehension (grasping), mastication (chewing), and mixing of feed with saliva as preparatory steps for subsequent digestion in the remaining parts of the alimentary canal. The lips, tongue, and teeth are used to pick and hold feed while it is in the mouth. The teeth grind the feed, and the tongue mixes it with saliva. The lips of sheep, goats, and horses are flexible, while those of cattle and buffaloes are relatively stiff and immobile. In horses, the upper lip is very sensitive and mobile and is used to feel the feed and direct it towards the moouth. Horses also have both upper and lower incisors which cut herbage like scissors and help in prehension. The mastication of feed with grinding molars is also very efficient in horses. In cattle, buffaloes, and sheep, the upper incisors are absent. Instead, they have a hard pad called the dental pad. During grazing, the tongue is used to guide feed into the mouth, and plants are grasped between the dental pad and lower incisors and torn away.
Saliva is a watery solution containing small amounts of mucin, inorganic salts and enzymes. It is secreted into the mouth from three pairs of salivary glands (paratoid, sub-maxillary, and sub-lingual). Secretion of saliva increases during mastication of feed. Cattle and buffaloes produce 130 - 200 litres, sheep and goats 2 - 3 litres, and horses 10 - 12 litres of saliva per day. Saliva lubricates the ingested feed and thus facilitates swallowing. It also helps in maintaining optimum pH in the rumen and provides a mechanism for recycling urea in ruminants.
Oesophagus:
This is a muscular tube extending from the back of the mouth (pharynx) to the stomach. Ingested feed is forced through the oesophagus by its wavelike muscular contractions.

(Note: In next Post, find about Function of stomach in digestive system of farm animals)

Parts of digestive tract

The digestive tract, also known as the alimentary canal, is a tube like structure extending from the mouth to anus. The different parts of the digestive tracts of ruminants and non-ruminants are illustrated in figures. The alimentary canal is comprised of the following parts,
  1. Mouth (lips, tongue, gums and teeth) and pharynx
  2. Oesophagus
  3. Stomach (In ruminants, the stomach includes the rumen, reticulum, omasum, and abomasum)
  4. Small intestine (duodenum, jejunum, and ileum)
  5. Large intestine (caecum, colon, and rectum)
  6. Anus
In addition to the above, the following accessory organs/glands also take part in the process of digestion: salivary glands, liver, gall bladder, and pancreas.
Among domestic animals, the proportion of the digestive tract to total body weight is higher in ruminants than in non-ruminants, as shown in table. Horses, for example, have a large caecum (15.6% of the total alimentary tract) where extensive microbial fermentation of plant materials takes place. Similarly, rabbits also have a well developed caecum. In ruminants, the stomach compartments (rumen, reticulum, omasum, and abomasum) occupy the major part (62%) of the digestive tract. Large variations in the proportion of different parts of the digestive tract in farm animals reflect differences in their feeding habits. These variations have evolved over time, and the animals have gradually adapted to various feed resources.



Cattle
Buffaloes
Sheep/Goats
Horses
Dogs
Body Weight (Kg)
450
600
50
450
20


Volume in liters, and proportion of body weight (percent)
Stomach









Reticulo-rumen
97.6
(48)
130.3
(48)
10.7
(56.7)
-
-
Omasum
15.8
(7.6)
20.9
(7.6)
0.6
(3.3)
-
-
Abomasum
11.8
(5.8)
15.7
(5.8)
1.3
(6.7)
8.0
(8.9)
1.0
(63)
Small Intestine
50.9
(25)
67.7
(25)
3.8
(20)
27.0 (30.0)
0.37 (23.0)
Caecum
7.7
(3.8)
10.3
(3.8)
0.6
(3.3)
14.0 (15.6)
0.02
(1.0)
Large Intestine
19.7
(9.5)
26.0
(9.5)
1.9
(10.0)
41.0 (45.6)
0.21 (13.0)
Total
203.5
270.9
18.9
90.0
1.6

Introduction to Digestion in Farm Animals

Digestion in animals involves mechanical, enzymatic, and microbial processes in the gastrointestinal tract, which convert large feed particles to a size which can be absorbed, transported, and used by the animals. The digestive systems of all farm animals are not the same, rendering them less competitive and able to adapt to a wide range of available feed resource niches. Based on the structure and functions of the digestive tract, farm animals are divided into two major groups:  ruminants or polygastrics, e.g. cattle, buffaloes, sheep, and goats; and non-ruminants or monogastrics, e.g. horses, donkeys, rabbits, dogs and cats. The key difference between these two groups of animals lies in the structure of the stomach. Ruminants, as opposed to non ruminants, have three additional stomach compartments where digestion takes place in different environments. In general, for farm animals the processes associated with digestion include prehension, ingestion, grinding or mastication, digestion of feed, absorption of nutrients, and excretion of waste products.