The endocranium in

comparative anatomy Comparative anatomy is the study of similarities and differences in the anatomy of different species. It is closely related to evolutionary biology and phylogeny (the evolution of species). The science began in the classical era, continuing in ...

is a part of the skull base in

vertebrates Vertebrates () comprise all animal taxa within the subphylum Vertebrata () ( chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, wi ...

and it represents the basal, inner part of the

cranium The skull is a bone protective cavity for the brain. The skull is composed of four types of bone i.e., cranial bones, facial bones, ear ossicles and hyoid bone. However two parts are more prominent: the cranium and the mandible. In humans, ...

. The term is also applied to the outer layer of the

dura mater In neuroanatomy, dura mater is a thick membrane made of dense irregular connective tissue that surrounds the brain and spinal cord. It is the outermost of the three layers of membrane called the meninges that protect the central nervous syste ...

human anatomy The human body is the structure of a human being. It is composed of many different types of cells that together create tissues and subsequently organ systems. They ensure homeostasis and the viability of the human body. It comprises a head ...

Structure

Structurally, the endocranium consists of a boxlike shape, open at the top. The posterior margin exhibit the '' foramen magnum'', an opening for the

spinal cord The spinal cord is a long, thin, tubular structure made up of nervous tissue, which extends from the medulla oblongata in the brainstem to the lumbar region of the vertebral column (backbone). The backbone encloses the central canal of the spin ...

. The floor of the endocranium has several paired openings for the

cranial nerves Cranial nerves are the nerves that emerge directly from the brain (including the brainstem), of which there are conventionally considered twelve pairs. Cranial nerves relay information between the brain and parts of the body, primarily to and ...

, and the anterior margin holds a spongy construction, allowing for the

external nasal nerve The external nasal nerve is a terminal branch of the anterior ethmoidal nerve. The trigeminal nerve (CN V) branches into three nerves, one of which is the ophthalmic nerve, which itself has three branches, one of which is the nasociliary nerve. One ...

s to pass through. Romer, A.S. & T.S. Parsons. 1977. ''The Vertebrate Body.'' 5th ed. Saunders, Philadelphia. (6th ed. 1985) All bones of the structure derive from the cranial neural crest during

fetal development Prenatal development () includes the development of the embryo and of the fetus during a viviparous animal's gestation. Prenatal development starts with fertilization, in the germinal stage of embryonic development, and continues in fetal devel ...

Endocranial elements in humans

In humans and other mammals, the endocranium forms during

as a cartilaginous

neurocranium In human anatomy, the neurocranium, also known as the braincase, brainpan, or brain-pan is the upper and back part of the skull, which forms a protective case around the brain. In the human skull, the neurocranium includes the calvaria or sk ...

, that ossifies from several centers. Several of these bones merge, and in the adult

primate Primates are a diverse order (biology), order of mammals. They are divided into the Strepsirrhini, strepsirrhines, which include the lemurs, galagos, and lorisids, and the Haplorhini, haplorhines, which include the Tarsiiformes, tarsiers and ...

s (including humans), the endocranium is composed of only five bony elements (from front to back):'' Colbert, E.H. & Morales, M. (2001): Colbert's Evolution of the Vertebrates: A History of the Backboned Animals Through Time''. 4th edition. John Wiley & Sons, Inc, New York - . * The ethmoid bone, lying behind the nose. * The sphenoid bone, underlying the forward portion of the brain * Paired petrous part of the temporal bones, containing the

inner ear The inner ear (internal ear, auris interna) is the innermost part of the vertebrate ear. In vertebrates, the inner ear is mainly responsible for sound detection and balance. In mammals, it consists of the bony labyrinth, a hollow cavity in t ...

structures * Most of the

occipital bone The occipital bone () is a cranial dermal bone and the main bone of the occiput (back and lower part of the skull). It is trapezoidal in shape and curved on itself like a shallow dish. The occipital bone overlies the occipital lobes of the c ...

, surrounding the ''foramen magnum''

Other animals

Endocranial components in other tetrapods

The endocranium in mammals is much reduced in relative size and number of bones compared to the condition in the ancestral land vertebrates, though the

occur as one or more stout bony elements in several mammal groups. The occipital bone is also found as several bony elements in

birds Birds are a group of warm-blooded vertebrates constituting the class Aves (), characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweig ...

and

reptiles Reptiles, as most commonly defined are the animals in the class Reptilia ( ), a paraphyletic grouping comprising all sauropsids except birds. Living reptiles comprise turtles, crocodilians, squamates (lizards and snakes) and rhynchocephali ...

, while the skull of modern amphibians is generally reduced with a simplified endocranium. The skull of early labyrinthodonts were rather complex, and contained in addition to the bones mentioned above several small cartilaginous components that are fused to temporal and occipital bones in mammals: * Paired prootic and opisthotic bones above each '' fenestra ovalis'', fused to the petrous part of the temporal bones in mammals. * Paired exoccipital bones medially and a single

basioccipital bone The basilar part of the occipital bone (also basioccipital) extends forward and upward from the foramen magnum, and presents in front an area more or less quadrilateral in outline. In the young skull this area is rough and uneven, and is joined t ...

below the ''foramen magnum'', part of the occipital bone in mammals.

The endocranium in fish

While the endocranium is an integral part of the skull in mammals,

and

, its connection to the roofing parts of the skull is more loose in the

lower vertebrates The anamniotes are an informal group of craniates comprising all fishes and amphibians, which lay their eggs in aquatic environments. They are distinguished from the amniotes (reptiles, birds and mammals), which can reproduce on dry land either by ...

. In

Agnathans Agnatha (, Ancient Greek 'without jaws') is an infraphylum of jawless fish in the phylum Chordata, subphylum Vertebrata, consisting of both present (Cyclostomata, cyclostomes) and extinct (conodonts and ostracoderms) species. Among recent animal ...

and

Chondrichthyes Chondrichthyes (; ) is a class that contains the cartilaginous fishes that have skeletons primarily composed of cartilage. They can be contrasted with the Osteichthyes or ''bony fishes'', which have skeletons primarily composed of bone tissu ...

, the skull lacks the

skull roof The skull roof, or the roofing bones of the skull, are a set of bones covering the brain, eyes and nostrils in bony fishes and all land-living vertebrates. The bones are derived from dermal bone and are part of the dermatocranium. In comparati ...

dermal elements, their whole cranium being composed of the endocranium, properly called a

chondrocranium The chondrocranium (or ''cartilaginous neurocranium'') is the primitive cartilaginous skeletal structure of the fetal skull that grows to envelop the rapidly growing embryonic brain.Salentijn, L. ''Biology of Mineralized Tissues: Prenatal Skull Dev ...

. In most

Osteichthyes Osteichthyes (), popularly referred to as the bony fish, is a diverse superclass of fish that have skeletons primarily composed of bone tissue. They can be contrasted with the Chondrichthyes, which have skeletons primarily composed of cartila ...

, the skull is only loosely joined, and the endocranial elements do not form a unit with the skull roof.

Fossilization

An endocast or endocranial cast is a cast made of the mold formed by the impression the

brain The brain is an organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. It consists of nervous tissue and is typically located in the head ( cephalization), usually near organs for special ...

makes on the inside of the

(braincase), providing a replica of the brain with most of the details of its outer surface. Endocasts can also form naturally, when sediments fill the empty

skull The skull is a bone protective cavity for the brain. The skull is composed of four types of bone i.e., cranial bones, facial bones, ear ossicles and hyoid bone. However two parts are more prominent: the cranium and the mandible. In humans, t ...

, after which the skull is destroyed and the cast

fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...

ized. Scientists are increasingly utilizing

computerized tomography A computed tomography scan (CT scan; formerly called computed axial tomography scan or CAT scan) is a medical imaging technique used to obtain detailed internal images of the body. The personnel that perform CT scans are called radiographers ...

scanning technology to create digital endocasts without damaging valuable specimens. This gives a 3D representation of the brain. Brain size and complexity can then be determined. Endocasts were used for looking at the brains of ''Homo sapiens'' to find hemispheric specialization.

References

{{Reflist Human anatomy Vertebrate anatomy