**12. The large intestine**

Caecum, colon, and rectum are the three portions of the large intestine. This portion also has all four tunics present in the small intestine that works together to achieve the organs' function. The mucosa is lined by simple columnar epithelium with the handsome number of goblet cells. The lamina propria have rich glandular regions and lymphoid tissues. The muscularis layer is of smooth muscles. There are solitary lymphoid follicles present in the colon over which the dome epithelium is lining on the luminal surface consisting of columnar enterocytes and M cells. Nutrient absorption occurs in the gut *via* the lining epithelium. The submucosa, muscularis, and serosa tunics are the same as that of the small intestine. Muscular contractions mix the intraluminal chyme with microbiota and forward the chyme further caudally. The caecum is a tubular structure located at the beginning of the large intestine also known as the blind gut. The opening of ileo-cecal valve allows the movement of chyme to enter the cecum where the enlarged space permit further mixing of the partially digested material with bacteria. The chyme enters the colon (ascending, transverse, descending, and sigmoid) and rectum through peristaltic muscular contractions [62, 67]. The undigested/partially digested and unabsorbed feed materials take entry in these compartments and further digestion and absorption of salts, water, and important nutrients take place in the large intestine. The more digestion of these materials occurs through microorganisms which is also the function of the large intestine.

Rectum is the final part of the large intestine located dorsally to the urogenital tract and ends on anus. The recto-anal Junction marks the termination of the lamina muscularis and longitudinal layer of the tunica muscularis, which forms the internal muscular anal sphincter. The anal sphincter is mostly comprised of skeletal muscle. The transition of epithelium columnar to stratified squamous non-keratinized occurs at the junction. The leftover material is getting rid of this portion.

Some differences can be are as follows:


### **13. The liver and pancreas**

Liver is red-brown in color and is the largest gland of the goat's body, positioned on the right side and just caudal to the diaphragm within the abdominal cavity. This organ has parietal and visceral surfaces. The liver is covered by an FCT Glisson's capsule from which the septa emerge downward and divide the parenchyma into partially completed hexagonal lobules. The central vein is present in the center of the hepatic lobule. The polygonal-shaped hepatocytes are radiated from the central vein area in the form of cords. Sinusoids are present in between the cords where the endothelium and fenestrations are visible. Prominent CT is present in the hepatic triad area at the corner of classical hepatic lobules where hepatic artery, portal vein, and bile ducts are present [68–70].

The liver and pancreas play a pivotal role in digestion. The liver performs metabolic and immunologic functions. It produces bile, which is also stored and then secreted by gallbladder helping the emulsification of fats for digestion. Proteins, fats, and carbohydrates are digested with the help of enzymes secreted by the pancreas in the small intestine. A goat liver also performs different functions like the destruction of hemoglobin, storehouse of glycogen and further converting it into glucose, transformed area into uric acid, detoxification, metabolizing drugs, synthesizing phospholipids and cholesterol, storehouse of vitamins and iron, and the production of almost 10% of the erythropoietin in adult animals [71].

The gall bladder of goat is a pear-shaped sac, yellowish-white in color, which remains filled with green-colored bile juice located at the level of the ninth rib. This structure is lined internally by tall columnar epithelium with occasional goblet cells. Thick muscles at the neck region are thought to be the sphincter of the gall bladder [61].

Pancreas has both the exocrine and endocrine portions and is present in the abdominal cavity. Its head lies in the loop formed by the duodenum and its tail is headed toward the spleen. The major portion is the exocrine comprised of the acini lined internally by the pyramidal acinar cells forming lobules having zymogen granules at their apical portion. The granules are precursors of several digestive enzymes which are secreted into the duodenum *via* duct. The secretion drains from the acini through the intercalated duct, which merges to form an intralobular duct. The later

duct joins to form larger inter-lobular ducts, which enter into the main duct of the pancreas [68, 72].

### **14. Health and clinical issues that can occur with goat alimentary system**

In goats, a range of diseases affecting the gastro-intestinal tract is found, chiefly including bacterial, viral, parasitic, and fungal infections. Oral cavity affections caused by bacterial pathogens are stomatitis caused by a number of bacteria such as Staphylococci, Streptococci, Fusobacterium necrophorum, Sphaerophorus necrophorus, and Actinobacillus lignieresii (Wooden Tongue). The common viral infections include PPR (PPR Virus, mainly Lineage IV), Bluetongue (Orbi virus), ORF (Parapoxvirus), Goat Pox (Goat Pox virus), Foot and Mouth Disease (Aptho virus), etc. Fungal infections are frequently caused by Monilia spp., Candidia spp., Fusarium spp., etc. Pharyngitis and esophagitis are caused by bacteria such as Actinobacilli and Fusobacterium necrophorum besides the pathogens invading down from the oral cavity to pharynx and esophagus. Ovine Herpes virus-2 is the viral agent causing esophagitis in goats [73, 74].

Rumenitis in goats accounts for a significant subclinical disease in survivors of acute episodes, favoring it by serving as a portal for the entry of fungi in this pivotal organ of digestion in goats. Fusobacterium necrophorum may cause secondary infections of abomasum. In Abomasum, a number of parasites belonging to the genera Haemonchus and Mecistocirrus have been recorded. They are large abomasal bloodsucking Trichostrongyles, capable of causing severe anemia and hypoproteinemia. In addition, Ostertagia spp. and related genera, such as Camelostrongylus, Teladorsagia, Marshallagia, and Trichostrongylus axei, are also found in various ruminants, causing chronic abomasitis with mucous metaplasia, achlorhydria, diarrhea, and plasma protein loss [75, 76].

Intestinal diseases affecting goats include Johne's disease generally associated with wasting of body condition, but often not diarrhea mostly targeting the small intestine. The large bowel may be involved in a minority of cases, but the ileum is consistently affected. Acute undifferentiated diarrhea in neonatal goats is frequently associated with enterotoxigenic E. coli, rotavirus, coronaviruses, and Cryptosporidium parvum. Coccidiosis (a protozoan disease) is a multifaceted ailment caused by Eimeria spp. in caprine kids and may occur in animals as young as 3 weeks to 5 months of age. Raised white plaques of coccidia-infected proliferative epithelial cells are found in the terminal ileum; there may be some degree of hemorrhage in severe cases. Some parasites use the peritoneal cavity as their final habitat. Setaria spp. are Onchocercid filarioid nematodes inhabiting the peritoneal cavity of many wild and domestic ungulates, including goats [26, 74, 77].

Gastroenteritis has been recorded in goats >3 weeks of age. The major causes of diarrhea and ill-thrift in goats at pasture are parasitic. The main helminth endoparasite species causing this syndrome are Ostertagia, Nematodirus, and Trichostrongylus. Diarrhea and enterocolitis may also be associated with Clostridium perfringens type D enterotoxemia in goats, although; animals may die quite suddenly without showing any premonitory signs such as diarrhea. C. perfringens type D Enterotoxemia ("pulpy kidney" disease also known as "overeating" disease) is an important disease of goats worldwide resulting in significant mortality. Mycoplasma mycoides may cause acute fibrinous peritonitis in goats, although acute death from septicemia, or arthritis and mastitis are more common. Paratuberculosis caused by

Mycobacterium avium subspecies paratuberculosis frequently produces nodular granulomatous lymphangitis in the mesentery and sometimes caseous or mineralized lymphadenitis [10, 26, 74, 78–80].
