Central nervous system (display 89)

The encephalon of tetrapods differs from that of fish in terms of its complexity, even though the cerebral regions remain the exact same.

The rhombencephalon and the mesencephalon of reptiles are more developed than those of fish. They control balance, vital functions (breathing, heartbeat) and locomotion.

The brain of ectotherms (cold blooded animals) can withstand considerable variations in temperature thanks to its capacity for physiological adaptation; this gives rise to behavioural responses that differ according to climate.

Crocodilians, which have been around for some 220 million years, are amongst the most evolved reptiles. Their brain more closely resembles the brain of birds than that of other reptiles. As with fish, the growth of a crocodile’s brain and body continues throughout its life but slows down with age.

The bird brain has evolved from a brain very close to that of primitive crocodilians. Compared to reptiles, birds have a more developed encephalon. This distinction applies not only to the cerebellum and mesencephalon, but also to the cerebral hemispheres, which can resemble those of certain primates. Moreover, a high neuronal density results in complex cognitive abilities that are comparable to those of mammals.

Large parrots and crows have the largest numbers of neurons. The blue-and-yellow macaw has about 3.14 billion neurons, more than 70% of which are in the telencephalon; far more than those present in certain primates or rodents with a comparable cerebral volume. Species belonging to other lines such as the emu, chicken or pigeon have neuron densities that are comparable to those observed in some mammals.

Over millions of years vertebrates have deployed various methods to increase their cerebral capacity. For birds, an efficient brain that is also light in weight is an advantage for flight.

The central nervous system of mammals is characterised by a very high number of neurons and glial cells as well as a high degree of specialisation and vascularisation.

The expansion of the cerebral cortex is the most distinctive morphological characteristic of the mammal brain and is at the origin of the development of the neocortex. This portion of the cortex is characterised by six layers and occupies the largest part of the cerebral hemispheres. The neocortex is responsible for many important functions such as memory, language, information processing, reasoning, perception and consciousness.

As for all vertebrates, mammals have a brain that varies greatly in absolute size, weight, neuronal density and morphology of its constituent parts; it's the result of a long evolutionary adaptation of the animal to its environment.

Most of the objects exhibited in these display cases date back to the 19^th century, a historically important period for the Professorship of Comparative anatomy of the Muséum National d’Histoire Naturelle. While the initial exhibition can be attributed to the specimen preparer Henri Neuville (1872-1946), numerous scientists and preparers have added to the collection over the years. 

The display cases 90 to 83 contain jars, casts and dry anatomical pieces. The jars vary in size and shape depending on the dimensions of the specimens and desired effect. More than 320 specimens are immersed in a preserving liquid. The 247 casts are of two types: either cranial endocasts (casts taken from the inside of the cranium) or lifecasts (casts taken from the brain itself). The latter type reveals more details of the morphology of the brain than the cranial endocasts which principally detect the imprints left by the brain on the walls of the cranial cavity.