Anatomically, stomach is divided into four parts: cardia, fundus, body, and pyloric part. Cardia is the upper part surrounding the entrance of the esophagus and is lined by the mucus-secreting epithelium. The epithelium of the fundus and the body of the stomach is composed of different cell types including: (i) mucus-secreting cells which protect gastric mucosa from self-digestion by forming an overlying thick layer of mucus, (ii) parietal cells which secrete hydrochloric acid and intrinsic factor, and (iii) peptic cells or chief cells which secrete the proteolytic enzyme pepsinogen. Pyloric part is divided into pyloric antrum and pyloric canal. It is lined by mucus-secreting cells and gastrin-secreting neuroendocrine cells (G cells) (Figure 859.1).
Figure 859.1 Parts of stomach and their lining cells
Figure 859.1 Parts of stomach and their lining cells 
In the stomach, ingested food is mechanically and chemically broken down to form semi-digested liquid called chyme. Following relaxation of pyloric sphincter, chyme passes into the duodenum.
There are three phases of gastric acid secretion: cephalic, gastric, and intestinal.
  • Cephalic or neurogenic phase: This phase is initiated by the sight, smell, taste, or thought of food that causes stimulation of vagal nuclei in the brain. Vagus nerve directly stimulates parietal cells to secrete acid; in addition, it also stimulates antral G cells to secrete gastrin in blood (which is also a potent stimulus for gastric acid secretion) (Figure 859.2). Cephalic phase is abolished by vagotomy.
  • Gastric phase: Entry of swallowed food into the stomach causes gastric distension and induces gastric phase. Distension of antrum and increase in pH due to neutralization of acid by food stimulate antral G cells to secrete gastrin into the circulation. Gastrin, in turn, causes release of hydrochloric acid from parietal cells.
  • Intestinal phase: Entry of digested proteins into the duodenum causes an increase in acid output from the stomach. It is thought that certain hormones and absorbed amino acids stimulate parietal cells to secrete acid.
The secretion from the stomach is called as gastric juice. The chief constituents of the gastric juice are:
  • Hydrochloric acid (HCl): This is secreted by the parietal cells of the fundus and the body of the stomach. HCl provides the high acidic pH necessary for activation of pepsinogen to pepsin. Gastric acid secretion is stimulated by histamine, acetylcholine, and gastrin (Figure 859.2). HCl kills most microorganisms entering the stomach and also denatures proteins (breaks hydrogen bonds making polypeptide chains to unfold). Its secretion is inhibited by somatostatin (secreted by D cells in pancreas and by mucosa of intestine), gastric inhibitory peptide (secreted by K cells in duodenum and jejunum), prostaglandin, and secretin (secreted by S cells in duodenum).
  • Pepsin: Pepsin is secreted by chief cells in stomach. Pepsin causes partial digestion of proteins leading to the formation of large polypeptide molecules (optimal function at pH 1.0 to 3.0). Its secretion is enhanced by vagal stimulation.
  • Mucus
  • Intrinsic factor (IF): IF is necessary for absorption of vitamin B12 in the terminal ileum. It is secreted by parietal cells of stomach.
Figure 859.2 Stimulation of gastric acid secretion
Figure 859.2 Stimulation of gastric acid secretion. Three receptors on parietal cells stimulate acid secretion: histamine (H2) receptor, acetylcholine or cholinergic receptor, and gastrin/CCK-B receptor. Histamine is released by enterochromaffin-like cells in lamina propria. Acetylcholine is released from nerve endings. Gastrin is released from G cells in antrum (in response to amino acids in food, antral distention, and gastrin-releasing peptide). After binding to receptors, H+ is secreted in exchange for K+ by proton pump
Previously we discussed about various exocrine glands in and foramen in animals. This topic covers about various principle ducts and canals present in animal body and their location and their functios.
Location: Rectum of Mammals
Function: It extends from the pelvic floor downward and posteriorly to the anus.
Location: Frog’s kidney sperms.
Function: It connects testis to kidney and conducts
Location: Vertebrates
Function: It runs from gall bladder to duodenum through which bile flows.
 bile duct11
Location: Mammalian eye
Function: It lies near the junction ofcornea and sclera, this canal passes circularly around the cornea, and drains the aqueous humour.
Location: Spinal cord
Function: It is situated in the centre of gray matter and extends entire length of the cord. Cerbrospinal fluid flows in it.
Location: Uterus
Function: Canal of cervix of uterus
Location: Internal ear of mammals
Function: It is middle canal in the cochlear duct filled with peri lymph
Location: Kidney of mammals
Function: These are many and collect urine.
Location: part of bile duct in vertebrates,
Function:  which leads from gall bladder receives branches from various part of the liver and it eventually forms the common bile duct.
Location: Salivary glands of mammals
Function: Ducts of sublingual glands (Sublingual ducts) lie just beneath the floor of the mouth and open into the floor qf the mouth.
Location: Internal ear of mammals
Function: It is a small short duct connected with saccule and end blindly in the endo lymphatic sac.
Location: Middle ear of all land vertebrates
Function: It connects middle ear with pharynx.
Location: Bones
Function: These are longitudinal canals found characteristically in long bones.
Haversian canals14
Location: Liver of vertebrates
Function: It originates from liver cells and unites with the cystic duct from the gall bladder to form the common bile duct.
hepatic duct9
Location: Eye of mammals
Function: It extends between optic disc and the lens.
Location: Reproductive organs of male mammal.
Function: It connects abdominal cavity to scrotal cavity.
inguinal canal6
Location: Eyes of man
Function: These are two (superior and inferior) connect lacrimal gland with lacrimal sac.
Location: Mammary glands of placentals
Function: These carry milk from mammary glands to the tip of
Location: Reproductive organs of liver fluke
Function: It is temporary copulatory canal which opens on the dorsal body surface.
Location: Thorax of vertebrates
Function: It is a short duct receives lymph from the right side of the head, the right upper extremity and the right side of the thorax.
Location: Lower jaw of mammals
Function: It originates from mandibular foramen for in- ferior alveolar vessels and nerve and lies on the inner surface of each ramus.
Location: Nose
Function: It connects lacrimal sac with inferior nasal meatus.
Location: Vertebrate embryo
Function: It is a canal joining the primitive gut with the cavity of neural tube.
Location: Pancreas
Function: The duct which conducts enzymes from the pancreas into the duodenum.
Location: Kidney of mammals (part of renal tubule)
Function: These open on the tip of renal papilla and pour their contents into a minor calyx of the renal pelvis.
Location: Salivary gland of mammals
Function: It passes anteriorly to pour into the yes- tibule of the mouth, opposite the upper second molar tooth.
Location: Internal ear of vertebrates
Function: These are filled with endolymph and concerned with equilibrium.
Location: Salivary gland of mammals
Function: It extends from the anterior end of each submaxillary gland and opens into the floor of the mouth on each side of the frenulum.
Location: Thorax and Abdomen of mammals
Function: The chief collecting duct of lymphatic system receive lymph from lacteals. It lead towards left subclavian
thoracic duct7
Location: On the floor of pharynx in mammals
Function: An embryonic duct between thyroid and pharynx. it disappears in adults.
Location: Cervical vertebrae of mammals, birds and reptiles
Function: These canals are found at the base of transverse processes of cervical vertebrae for the passage of cervical blood vessels and nerves.
Location: Long bones of mammals
Function: These are transverse canals, connecting Haversian canals. These carry blood.
Volkmanns canal8
Elphidium is a marine form. It is found creeping on Sea weeds to a depth of 300fathoms
Phylum : Protozoa
Class : Rhizopoda
Order : Foraminifera.
Elphidium is also called 'Polystomella is a 'dimorphic rhizopod'. It is a unicellular microscopic protozoan, and" 1 mm in diameter It is pale yellow in colour. It lives in marine water. The body is covered by a shell. The shell is biconvex. The first formed chamber is proloculum. The shell contains spirally arranged V shaped chambers. Hence it is called "polythalamus or multilocular They overlap one another. These chambers show 'openings' hence it is perforate Through these openings cytoplasm will come out. The cytoplasm is produced into a number reticulopodia. which will form a network. From hinder end of each chamber cytoplasmic processes will develop. They are directed backwards. They are called retral processes'.
The chambers are filled with the cytoplasm.
  1. The cytoplasm contains one or many nuclei.
  2. Contractile vacuole is absent.
  3. Mouth is absent.
  4. Cytoplasm contains food vacuoles. They take up the process of digestion.
Dimorphism: Polystomella exhibits dimorphism. The individual occurs in two distinct forms.
1. Megalosperic form.
2. Microspheric form.
  1. Megalospheric form: Its proloculum is big in size. A single large nucleus is present in one of the chambers. It takes up sexual reproduction.
  2. Microspheric form: Its proloculum is small in size. Many nuclei are present in the cytoplasm. This form reproduces by asexual reproduction.
Locomotion and Nutrition:
Polystomella show slow creeping movements with the help of reticulopodia. It is a holozoic feeder. These Pseudopodia will capture the prey. When it comes in contact with the prey, it kills the prey by secreting toxic substance. The prey is digested in the food vacuole the digested food is absorbed by the cytoplasm.
In Shark 3 pair of internal ears are present. They are organs of equilibrium and hearing. The middle and external ears are absent. The internal ear is a complex structure. It is called the membranous labyrinth. It is present in the auditory capsule of the cranium. It is covered by cartilaginous labyrinth. A space is present between the membranous and cartilaginous labyrinths. It is called peri lymphatic space. It contains peri lymph. It protects and transmits vibrations to the membranous labyrinth. Connective tissue extends from the Cartilaginous to the membranous labyrinth.
The membranous labyrinth has laterally compressed vestibule which is divided into two chambers,
a) The upper narrow utricles,
b) Lower wide sacculus.
The two chambers are connected with each other by a wide passage, the sacculoutricular duct. The sacculus gives off a conical projection from its lower side, called lagena. A narrow canalarisesfrom the dorsal side of the sacculus called ductus endolymphaticus. It pierces through the roof of the cranium and open out by a minute pore. Before it opens out the duct dilates into saccus endolymphaticus. Three semicircular ducts are present. They are called the external, anterior and posterior semicircular canals. . The external canal is horizontal and the other two are vertical in position. All the semicircular anals open into the utriculus at both of their ends. One end of each canal is dilated into oval ampulla. The ampullae of the anterior and the external canals lie close together towards the anterior end of the ear.
The cavity in the membranous labyrinth is filled with a fluid Endo lymph. Six sensory patches are present in the wall of the internal ear.The sensory patches' of the ampullae are called the "cristae". The sensory patches of the utriculus, sacculus and lagena are called maculae acousticae They contain neurosensory and supporting cells. Each neurosensory cell bears a sensory hair at the free end, and passes into a nerve fiber at the other end. The nerve fibres join the auditory nerve. The sensory hair are much longer cristae than in the maculae. They are enclosed in a gelatinous substance. The gelatinous substance is called the cupule. In Maculae the otolithic membrane has in its outer part minute crystalline bodies called otoliths.
They are formed of calcium carbonate.
  1. The cristae detect turning movements of the head.
  2. The maculae in the utriculus and sacculus detect changes in the position of the body at rest and in the forward movement. With a change in the position of the body, the otoliths change their position due to gravity and bring pressure on the underlying sensory hairs and this stimulates the neurosensory cells. The body position is corrected. Thus equilibrium is brought forward.
  3. The maculae & lagena will detect under water sound vibrations to some extent.
Shark two eyes are present on the head. They are located in the orbits and are laterally directed.
The eye is large in size and elliptical body. It is a hollow structure. It is covered by three coats.
1) The fibrous outer coat is thick. It protects the eyeball and maintains its form. It has large posterior scierotic and anterior transparent cornea The sclerotic coat is is hidden in the orbit. A small portion is visible externally, called the white of the eye. The cornea is composed of connective tissue. It is covered by a transparent epithelium called conjunctiva. The cornea permits the light to pass into the eye.
2) The middle coat has three distinct regions.
   a) The choroidis composed of a soft connective tissue with pigment cells and blood vessels
   b) The choroid is adjacent to the retina and contain light reflecting crystals called tapetum lucidium.
   c) At the junction of the sclerotic and the cornea the middle coat, abruptly bends into the cavity of the eye as "iris", it has slit called pupil. Behind the periphery of the iris, the middle coat is thicker, and less pigmented and is called the ciliary body. It has no muscles and plays no role in accommodation.
3) The retina is very delicate and lines the whole of the middle coat, it has three regions.
   a) optic part is in contact with the choroid. It is sensitive to light,
   b) The ciliary and
   c) Iridial parts are non sensory parts.
The point of the retina from where the optic nerve leaves the eye is called the blind spot. This region has no receptors. It is not sensitive the light. The retina contains rods. The cones are absent. The fish is colour blind. Its eye is adapted to "dim light" vision.
A solid transparent crystalline lens, lies just behind the pupil. It is held in place by a gelatinous suspensory ligament. It extends from the lens capsule to the ciliary processes. The lens divides the cavity of the eye into two chambers. The anterior smaller aqueous chamber and the posterior larger vitreous chamber. The aqueous chamber is partly divided into anterior and posterior parts by iris. It has aqueous humour. The vitreous chamber is filled with the vitreous humour. The humours keep the eye inflated and aid in focusing Sight rays on the retina  of shark.
The eye is protected by three lids. The upper end lower lids are folds of skin are immovable. The third eyelid, the nictineting membrane is well developed. It covers the eye fully. It is a growth of the anterior region of the lower eyelid. Lacrimal gland is absent.
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