V- Telencephalon • It forms , most rostral part of brain. • Cerebrum is separated caudally from cerebellum by transverse fissure. • Cerebrum is divided into two cerebral hemispheres by longitudinal fissure. Each hemisphere : 1-Contains lateral ventricle. 2-Consists of five topographical areas: a-frontal lobe rostrally b-occipital lobe caudally c-parietal lobe dorsally d-temporal lobe ventrolaterally e-olfactory lobe. 3-Presents 2 ends and 3 surfaces. A) two ends; rostral and caudal B) three surfaces; dorsolateral, medial, basilar.

Cerebrum is formed of: • Cerebral cortex (gray matter). • Cerebral white matter. • Rhinencephalon. A) Cerebral cortex: • Outer most gray matter of cerebrum, covers hemispheres , named pallium. • It is folded so has grooves and elevations as gyri and sulci , typical for species. B) Cerebral white matter: • Consists of two types of fiber system: 1) Corticocortical fibers. 2) Projection fibers. 1) Corticocortical fibers: • Originate and terminate in cerebral cortex. • Subdivided into; association and commissural fibers. A-Association fibers : Connect different cortical areas in same cerebral hemisphere. B-Commissural fibers: Extend between similar areas on opposite sides of cerebrum. It consists of corpus callosum, rostral commissure, dorsal and ventral commissures of fornix.

Corpus callosum : • largest commissure. • It consists of: 1-Genu rostrally, 2-Body in the middle. 3-Splenium caudally. • In the center, corpus callosum forms roof of lateral ventricle.

2) Projection fibers: • They are either originate or terminate in cerebral cortex. • Their other ends are found within lower centers of brainstem or spinal cord. • consist of ascending and descending fibers. C) Rhinencephalon: • Includes all parts of brain associated with olfaction. • It includes: 1-Olfactory bulb. 2-Olfactory tract. 3-Lateral and medial olfactory striae. 4-Rostral perforated substance. 5-Piriform lobe. 6-Hippocampus( lies in floor of lateral ventricle). 7-Fornix (connects hippocampus with mamillary body). Brain ventricles Number: 1) (right and left). 2) . 3) Mesencephalic (cerebral) aqueduct 4) . Development: developed as cavity of rostral part of neural tube Content: filled with . Communication: 1-With subarachnoid space through: A- Two lateral apertures (foramina of Luschka) B- (foramen of Magendie) 2-With central canal of spinal cord.

1. Lateral ventricles: • Two, situated within cerebral hemispheres.

• It may be subdivided into:

A- Central part.

B- Rostral horn.

C- Temporal horn extends caudally. 2. Third ventricle -Ring-shaped space situated within diencephalon around interthalamic adhesion. -Communicates with each lateral ventricle through interventricular foramen (of Monoro). Third ventricle extends: 1-dorsal to pineal body to form . 2-into pineal body to form pineal recess. 3- rostral to optic chiasma to form optic recess. 4-into infundibulum to form infundibular recess.

4. Fourth ventricle: -large cavity, situated between medulla oblongata and pons ventrally and cerebellum dorsally. -Continues rostrally with mesencephalic aqueduct and caudally with central canal of spinal cord. 3. Mesencephalic aqueduct: - Mesencephalic (cerebral) aqueduct or aqueduct of sylvius narrow duct connects 4th ventricle to 3rd ventricle. - Runs through mesencephalon.

Choroid plexus: - It consists of blood capillary forms finger-like processes project into lumen of ventricle, covered by layer of ependymal cells, derived from lining membrane of ventricle.

- It produces CSF which fills ventricles of brain and subarachnoid cavity.

Cerebrospinal fluid (CSF) Definition: • clear, colourless fluid similar to plasma. Occurrence: • It presents within: - Brain ventricles. - Central canal of spinal cord. - Subarachnoid space of brain and spinal cord. Formation: It is derived from: - mainly from choroid plexuses of brain ventricles. - subarachnoid space. - ependyma and blood vessels of pia mater. Function: A) Mechanical function: • protective cushion around CNS. • It bathes brain and spinal cord . B) Nutrional function: • It provides some nutrients (with high content of potassium and calcium) to CNS. • It flushes away waste product. Circulation: • From lateral venricles which fluid flows through interventricular foramina (of Monoro) to third ventricle. • Choroid plexus of third ventricle contributes small amount of fluid which passes through to fourth ventricle where its choroid plexus adds more CSF. • From fourth ventricle CSF passes : (1) Small amount to central canal of spinal cord. (2) Large amount through two lateral foramina (of luschka) to subarachnoid space. Within subarachnoid space CSF forms water jacket protects brain and spinal cord. CSF leaves subarachnoid space through arachnoid villi to venous sinuses of brain. From the sinuses blood flows to veins of general circulation (return to the vascular system).

Lateral ventricles ↓ Interventricular foramen of Monoro ↓ Third ventricle ↓ Mesencephalic aqueduct ↓ Fourth ventricle ↓ ↓ Large amount to subarachnoid Small amount to central space via foramina of Luschka. canal of spinal cord. ↓ Venous sinuses via arachnoid villi ↓ Return to vascular system

Clinical points: 1- Blockage foramen of fourth ventricle or cerebral aqueduct results in accumulation of CSF within ventricles (internal hydrocephalus) which leads to: * increased pressure internally. * dilate the ventricles. * press the brain tissue. 2-If fluid accumulates in subarachnoid space causes external hydrocephalus which leads to: *press the brain externally. Cisternal puncture: to obtain sample of CSF which is either: A) Normal  colorless and contains no blood corpuscles. B) Contains W.B.Cs  indicate infection. C) Contains R.B.Cs  indicate haemorrhage. Spinal cord Part of CNS , presents within vertebral canal. Its cranial end continues with medulla oblongata at foramen magnum. Its caudal tapered end terminates at middle of sacrum. Length of spinal cord: 190 – 195 cm in horse. Its diameter is greatest in cervical and lumbar enlargements.

General features: 1-Dorsal surface: A) Dorsal median sulcus:  Along length of spinal cord.  From it dorsal median septum extends ventrally to gray matter. B) Dorsal lateral sulcus:  shallow groove lateral to dorsal median sulcus.  through which dorsal root of spinal nerve enters spinal cord. C) Dorsal funiculus: - part of white matter between dorsal median sulcus and septum as well as dorsal lateral sulcus.

II-Ventral surface: A) Ventral median fissure: • Along length of spinal cord. • Extends dorsally till gray matter. B) Ventral lateral sulcus: • Sometimes present , where ventral root of spinal nerve exits from spinal cord. C ) Ventral funiculus: • Part of white matter between ventral median fissure and ventral lateral sulcus. III-lateral surface: • Represented by lateral funiculus which is part of white matter between dorsal and ventral roots of spinal nerve.

Enlargements of spinal cord: Definiton: • Two enlargements where spinal cord increases in diameters due to increase in nerve cells and fibers. Cervical enlargement: • Region of origin of brachial plexus. • At level of last three cervical and first two thoracic spinal cord segments. lumbar enlargement: • Region of origin of lumbosacral plexus. • At level of last three lumbar and first two sacral segments Special terms of spinal cord: Conus medullaris: • Tapered end of spinal cord caudal to lumbar segment