Anatomy of the Corpus Callosum
When studying the brain, it is impossible to ignore the corpus callosum (corpus callosum). This structure is the largest commissure of the telencephalon. Its fibers pass from one hemisphere to the other, ensuring their reliable functional and anatomical unity.
Traditionally, the anatomical structure of the corpus callosum is divided into four consecutive parts:
- The rostrum (rostrum) — the narrowed anteroinferior part;
- The genu (genu) — a pronounced anterior bend;
- The body (truncus) — the longest central part;
- The splenium (splenium) — the thickened posterior end.
The fibers making up the corpus callosum do not just cross the midline, but fan out into the substance of each hemisphere. In the frontal lobes, these fibers form the so-called forceps minor — forceps frontalis (minor). In the posterior part of the brain, they similarly form forceps major — forceps occipitalis (major).
The surface of the corpus callosum is not "naked". Above it lies the indusium griseum (indusium griseum), a very thin but important layer of gray matter. Within the structure of this indusium griseum, anatomists distinguish the medial and lateral longitudinal striae (striae longitudinales mediales and striae longitudinales laterales).
Fornix: white matter tracts
The fornix (fornix) is a massive, arch-shaped tract. It consists entirely of longitudinally running nerve fibers and topographically belongs to the white matter of the hemispheres, while functionally belonging to the ancient rhinencephalon.
For ease of study, the fornix is divided into three main parts:
- The columns of the fornix (columnae fornicis) — directed forward and downward;
- The body of the fornix (corpus fornicis) — the central, most elevated part;
- The crura of the fornix (crura fornicis) — diverging posteriorly and inferiorly.
A transverse connection exists between the right and left fornices. It is called the commissure of the fornix (commissura fornicis) and ensures communication between symmetrical structures. Descending inferiorly, the crura of the fornix smoothly continue into a new structure — the fimbria of the hippocampus (fimbria hippocampi).
Hippocampus and its topography
The hippocampus (hippocampus) is a massive structure visualized as a prominent, curved elevation. The length of this elevation is about 3 centimeters. Topographically, it is located within the cavity of the inferior horn of the lateral ventricle, forming a characteristic bulge there.
The surface of the hippocampus facing the ventricular cavity is lined with a thin layer of gray matter called the alveus hippocampi (alveus hippocampi).
Connection with other structures
Looking medial to the hippocampus itself, one can find the fimbria of the hippocampus (fimbria hippocampi). Visually, it appears as a white band. As noted earlier, this fimbria is a direct anatomical continuation of the crus of the fornix (crus fornicis).
The anterior end of the hippocampus has a characteristic thickening and bend, through which it smoothly transitions into the uncus (uncus). This uncus, in turn, is considered a component of the parahippocampal gyrus.
Briefly: answers to topical questions
Where do the columns of the fornix end?
The columns of the fornix (columnae fornicis) end in the mammillary bodies at the base of the brain. These structures represent the anterior diverging parts of the fornix. Passing through the gray matter of the hypothalamus (hypothalamus), they descend through its lateral wall on both sides and terminate in the mammillary bodies (corpora mamillaria). In addition, the columns of the fornix bound the interventricular foramina, which connect the third ventricle with the lateral ventricles.
Specifically, what structures does the commissure of the fornix connect?
The commissure of the fornix connects the right and left fornices, specifically their posterior diverging parts known as the crura of the fornix. This structure consists of transverse bundles of nerve fibers. It is located between the crura of the fornix (crura fornicis) and extends directly beneath the splenium of the corpus callosum (splenium corporis callosi).
What neural pathways pass within the fimbria of the hippocampus?
Efferent neural pathways pass within the fimbria of the hippocampus (fimbria hippocampi). They include the following structures:
- Fibers from hippocampal neurons — converge on its ventricular surface to form the alveus, which then continues into the fimbria.
- Efferent fibers of the subiculum (subiculum) — also participate in the formation of the fimbria.
Subsequently, the fimbria of the hippocampus continues into the crus of the fornix (crus fornicis) and the fornix itself (fornix), which acts as the main efferent output of the hippocampus.
Class and exam questions
What anatomical parts make up the corpus callosum?
The corpus callosum features the rostrum, genu, trunk (body), and the thickened posterior end — the splenium.
What are the forceps minor and forceps major?
These are nerve fiber bundles of the corpus callosum. Fanning out into the hemispheres, they form the forceps frontalis anteriorly and the forceps occipitalis posteriorly.
Where exactly is the hippocampus located?
The hippocampus is a curved elevation about 3 cm long that prominently bulges into the cavity of the inferior horn of the lateral ventricle.
How are the crura of the fornix and the hippocampus connected?
Legs of the fornix (crura fornicis), descending downward, continue as a white strip — the fimbria of hippocampus (fimbria hippocampi), which is located medially from the hippocampus itself.
What else to study on this topic
- Formation of the forceps minor and forceps major
- Structure of the indusium griseum and longitudinal striae
- Anatomical division of the fornix into columns, body, and crura
- Topography of the commissure of the fornix (commissura fornicis)
- Transition of the anterior end of the hippocampus into the uncus of the parahippocampal gyrus
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