The Cerebrum

- Formed from two cerebral hemispheres connected together by the corpus callosum.

- Each cerebral hemisphere has 3 poles, 3 surfaces, 5 borders and 4 lobes.

- Poles: three poles; frontal, temporal and occipital.

The frontal and temporal poles are directed anteriorly.

The frontal pole lies above and the temporal pole below.

The occipital pole directed backwards.

- Surfaces of the cerebral hemisphere: It has three surfaces:

1- Superiolateral surface: convex. It is directed upwards and laterally.

2- Medial surface: contains the corpus callosum which connects the two cerebral hemispheres.

3- Inferior surface: divided by a deep groove called the lateral sulcus into two parts:

a. Orbital part: lies on the roof of the orbit.

b. Tentorial part: lies on the tentorium cerebelli separating it from the cerebellum.

- Borders of the cerebral hemisphere:

1- Superomedial border: extends from the frontal pole to the occipital pole. It lies between the medial and the superiolateral surfaces

2- Inferolateral border: extends from the temporal pole to the occipital pole. It lies between superiolateral and the inferior surfaces

3- Medial orbital border: extends from the frontal pole to the medial end of the lateral sulcus (Fig. 88). It lies between the medial and the inferior surfaces.

4- Medial occipital border: extends from the temporal pole to the occipital pole. It lies between the inferior and the medial surfaces

5- Superciliary border: extends from the frontal pole to the lateral end of the stem of the lateral sulcus.

It separates the superiolateral surface from the orbital part of the tentorial surface.

N.B.: all the borders of the cerebral hemisphere can be seen on the inferior surface except the superiomedial border.

- Lobes of the cerebral hemisphere

- The cerebral hemisphere is divided into four lobes. These are frontal, temporal, parietal and occipital lobes.

- To outline these lobes four landmarks are used:

1- The central sulcus.

2- The parietooccipital sulcus.

3- The posterior ramus of the lateral sulcus.

4- The preoccipital notch which present on the inferolateral border infront of the occipital pole.

A- Lobes of the cerebral hemisphere on the superiolateral surface:

Draw a line between the upper end of the parietooccipital sulcus and the preoccipital notch.

Connect the posterior ramus of the lateral sulcus to the previous line.

The lobes are:

1. The frontal lobe: in front of the central sulcus and above the lateral sulcus.

2. Parietal lobe:

a. Behind the central sulcus

b. Above the posterior ramus of the lateral sulcus.

c. Infront of the line between the preoccipital notch and the parietooccipital sulcus.

3. Temporal lobe:

a. Below the lateral sulcus.

b. Infront of the line between the parietooccipital sulcus and the preoccipital notch.

4. Occipital lobe: behind the line connecting the parietooccipital sulcus and the preoccipital notch.

N.B.: A circle is drawn around the C.H. passing through the preoccipital notch and parietooccipital sulcus.

B- Lobes of the cerebral hemisphere on the medial surface (:

1. Frontal lobe: in front the central sulcus and above the corpus callosum.

2. Parietal lobe: behind the central sulcus, above the corpus callosum and in front the parietooccipital sulcus.

3. Occipital lobe: behind the parietooccipital sulcus.

4. Temporal lobe: below the corpus callosum and infront of a line between the preoccipital notch and the precalcarine sulcus.

- Lobes of cerebral hemisphere on inferior surface:

- The area behind this circle is the occipital lobe.

- The rest of the tentorial part is the temporal lobe.

- The orbital part is formed by the frontal lobe.

- The parietal lobe is not seen on the inferior surface.

- Sulci and gyri of the cerebral hemisphere:

The surface of the cerebral hemisphere is formed from many raised areas called gyri (gyrus = area), and depressed grooves called sulci (sulcus = groove).

The sulci and gyri increase the surface area of the cortex of the cerebral hemisphere to accommodate more neurons.

- The most important sulci and gyri ONLY are discussed.

- Features of the superio-lateral surface:

A- Sulci:

2. Central sulcus (Sulcus of Rolando):

It is located on the superiomedial border about 1cm behind the midpoint between the frontal and occipital poles.

It runs downwards and forwards. It ends just above the lateral sulcus separated from it by narrow bridge of tissue.

It extends on the upper one inch of the medial surface

3. Precentral sulcus: in front of and parallel to the central sulcus.

4. Postcentral sulcus: behind and parallel to the central sulcus.

5. Lateral sulcus: extends upwards and backwards. It has three rami; (Fig. 91) transverse, ascending and posterior ramus.

6. Superior and inferior frontal sulci: in front of the precentral sulcus and above and parallel to the lateral sulcus.

7. Superior and inferior temporal sulci: below and parallel to the lateral sulcus.

8. Intraparietal sulcus: extends backwards from just behind the middle of the postcentral sulcus.

9. Parieto-occipital sulcus: extends on the upper one inch of the superiolateral surface. It cuts the superiomedial border about 5 cm infront of the occipital pole. Its main part is present mainly on the medial surface.

- Gyri on the superiolateral surface:

1. Precentral gyrus: between the central and precentral sulci.

2. Postcentral gyrus: between the central and the postcentral sulci.

3. Superior frontal gyrus: above the superior frontal sulcus.

4. Middle frontal gyrus: between the superior and the inferior frontal sulci.

5. Inferior frontal gyrus: lies below the inferior frontal sulcus and above the posterior ramus of the lateral sulcus.

It is subdivided by the horizontal and the ascending rami of the lateral sulcus into three parts. From infront backwards these are:

a. Orbital part: infront and below the horizontal ramus.

b. Triangular part: between the two rami.

c. Opercular part: behind the ascending ramus.

6. Superior temporal gyrus: lies above the superior temporal sulcus and below the posterior ramus of the lateral sulcus.

7. Middle temporal gyrus: between the superior and the inferior temporal sulci.

8. Inferior temporal gyrus: below the inferior temporal sulcus.

9. Superior parietal lobule: lies above the intraparietal sulcus.

10. Inferior parietal lobule: lies below the intraparietal sulcus.

11. Marginal gyrus: it is a part of the inferior parietal lobule. It surrounds the up-turned posterior end of the lateral sulcus.

12. Angular gyrus: surrounds the posterior end of the superior temporal sulcus.

- Sulci and gyri of the medial surface of the cerebral hemisphere:

A- Sulci:

1. Central sulcus: in the upper one inch of the medial surface.

2. Callosal sulcus: lies above the corpus callosum.

3. Cingulate sulcus: lies above and parallel to the callosal sulcus. Its posterior end is divided into two limbs. One limb turns upwards infront of the parietooccipital sulcus. The second limb turns downwards and forwards behind the spleneum of corpus callosum.

4. Medial frontal sulcus: infront of the central sulcus.

5. Parieto-occipital sulcus: begins at the upper border of the cerebral hemisphere about 5 cm infront of the occipital pole.

It descends downwards and forwards to join the junction of the two limbs of the calcarine sulcus.

6. Calcarine sulcus: formed from two limbs; anterior and posterior.

The anterior limb (called precalcarine sulcus) extends downwards and forwards below the splenium of corpus callosum.

The posterior limb (called postcalcarine sulcus) extends downwards and backwards towards the occipital pole.

B- Gyri:

1. Paracentral lobule: on the upper part of the medial surface on the sides of the central sulcus. It continues with the precentral and postcentral gyri on the superiolateral surface.

2. Cingulate gyrus: between the callosal and the cingulate sulci.

3. Medial frontal gyrus: infront of the paracentral lobule and above the cingulate sulcus.

4. Cuneus: behind the parieto-occipital.

5. Precuneus: infront of the parieto-occipital sulcus.

6. Lingual gyrus: between the pre and postalarine sulci and the collateral s

- Sulci and gyri on the inferior surface of the cerebral hemisphere:

A- Sulci:

1. The stem of the lateral sulcus: it divides the inferior surface into orbital part in front and tentorial part behind.

2. Olfactory sulcus: lodges the olfactory bulb and tract.

3. Orbital sulci: these are many sulci which form H-shaped sulcus on the orbital part lateral to the olfactory sulcus.

4. Collateral sulcus: present on the tentorial part lateral and parallel to the precalcarine sulcus.

5. Precalcarine sulcus: extends on the posterior part of the inferior surface.

6. Rhinal sulcus: present at the anterior end of the collateral sulcus. It surrounds the uncus.

7. Occipitotemporal sulcus: present on the tentorial surface lateral and parallel to the collateral sulcus.

- Gyri on the inferior surface of the cerebral hemisphere:

1- Gyrus rectus: medial to the olfactory sulcus.

2- Orbital gyri: between the orbital sulci.

3- Parahippocampal gyrus: medial to collateral sulcus. Its anterior end expanded to form the uncus.

4- Isthmus of the limbic system: narrow gyrus behind the splenium of the corpus callosum. It continues above with the cingulate gyrus and continues below with the parahippoampal gyrus (Fig. 93).

5- Lingual gyrus: lies between the posterior end of the collateral sulcus and the precalcarine and postcalcarine sulcui.

6- Occipito-temporal gyrus: lies between the collateral sulcus and the occipitotemporal sulcus.

7- Inferior temporal gyrus: near the inferiolateral border.