(a) Describe in detail the comparative histology of liver among domestic animals. (20 marks) (b) Explain the anatomical…

(a) Comparative histology of liver among domestic animals

Common plan: a thin connective-tissue (Glisson's) capsule and trabeculae divide the parenchyma into hexagonal lobules. Each lobule has a central vein, from which plates of polygonal hepatocytes radiate to the periphery, separated by sinusoids. Sinusoids are lined by fenestrated endothelium and phagocytic Kupffer cells, with a perisinusoidal space of Disse holding stellate (Ito) fat-storing cells. Portal triads (branches of portal vein, hepatic artery and bile ductule, with lymphatics) lie at lobule corners. Bile flows in canaliculi between hepatocytes from the centre towards the portal ductules, while blood flows the opposite way, from portal area to central vein. Functionally the classic lobule is supplemented by the portal lobule (bile drainage) and the hepatic acinus (zones 1–3 by oxygenation).

Species differences:

• Pig: interlobular connective tissue is abundant, so lobules are sharply outlined, and this is the species that best shows the classic lobule.
• Cattle, sheep and goat: interlobular connective tissue is scanty, so lobule boundaries are indistinct and identified by portal areas; binucleate hepatocytes are common.
• Horse: lobulation is also indistinct, and a gallbladder is absent, so bile ducts are large.
• Dog and cat: lobules are indistinct, hepatocyte plates are narrow, and in the cat hepatocytes commonly contain fat vacuoles.
• Bile storage: gallbladder present in cattle, sheep, pig, dog and cat, but absent in horse.

(b) Anatomical adaptations for flight in birds

Flight adaptations reduce weight, give power and rigidity, and supply oxygen at a high rate.

• Skeleton: light pneumatic bones, hollow and air-filled, connected to air sacs (humerus, femur, sternum, some vertebrae); fusion of bones for rigidity, as in the synsacrum, pygostyle, carpometacarpus and furcula (wishbone); a large keeled sternum to anchor flight muscles; uncinate processes strengthening ribs.
• Muscles: pectoralis depresses the wing and supracoracoideus raises it by a tendon through the triosseal canal; red fibres give endurance.
• Weight reduction: no teeth (a muscular gizzard lies near the centre of gravity), no urinary bladder, a short gut, and in most females only the left ovary and oviduct.
• Respiration: lungs with parabronchi and nine air sacs give one-way airflow and double respiratory cycle for efficient gas exchange.
• Circulation: a four-chambered heart of large size with high heart rate.
• Feathers: flight feathers with interlocking barbules give an air-proof surface, and a tail for steering; wings are modified forelimbs.

(c) Indications and sites of nerve blocks in the head region of cattle

• Cornual nerve block: indicated for dehorning and horn surgery; the cornual branch of the zygomaticotemporal nerve is blocked midway between the lateral canthus and the horn base, below the temporal line (frontal crest). In adult cattle a ring block around the horn base is added for accessory supply.
• Auriculopalpebral block: indicated for eyelid akinesia in eye examination and surgery; the site is over the zygomatic arch, just behind the supraorbital process.
• Peterson (retrobulbar) and four-point blocks: indicated for enucleation and eyeball surgery; the needle is placed at the notch between the supraorbital process and the zygomatic arch to reach the nerves in the orbital cone.
• Supraorbital (frontal) nerve block: anaesthetises the forehead and upper eyelid; the site is the supraorbital foramen.
• Infraorbital nerve block: used for nose-ring insertion and nasal or upper-lip surgery, at the infraorbital foramen.
• Mental and inferior alveolar blocks: used for lower lip and tooth or jaw surgery, at the mental foramen and the mandibular foramen.