Comparative Account of Brain in Vertebrate
Brain of all vertebrates, from fish to man, is built in accordance with the same basic architectural plan. However, form of brain differs in different vertebrates in accordance with the habits and behaviour of the animals.
In amphioxus, brain does not consist of forebrain, midbrain and hindbrain. Instead, the so-called brain is made of an anterior prosencephalon or cerebral vesicle with a single enlarged ventricle. It is lined with cilia and long filamentous processes of ependymal cells as revealed by electron microscope. Anterior extension of notochord may suggest absence of a forebrain.
Brain is very primitive. Subdivisions are not well marked. Two olfactory lobes are prominent, but cerebral hemispheres are quite small. Cavities of cerebral hemispheres or lateral ventricles are rudimentary. Pineal apparatus and parapineal (=parietal) body are very well developed in Petromyzon, though they are vestigial in Eptatretus (=Bdellostoma) and absent in Myxine. Connected to pineal apparatus is epithalamus made of two habinulae ganglia. The two optic lobes are imperfectly differentiated. Medulla oblongata is very well developed while cerebellum is a small transverse dorsal band. A well defined infundibulum from hypothalamus of diencephalon bears a hypophysis or pituitary body.
Brain of fishes is more advanced than that of cyclostomes. However, subdivisions of brain are seen in their primitive relations.
In elasmobranch fishes (shark or dogfish), olfactory organs are enormous so that olfactory lobes of brain are correspondingly large, attached to cerebrum by short but stout olfactory tracts or peduncles. Optic lobes and pallium are relatively moderate in size. Midbrain cavity (III ventricle) is quite large and extends into optic lobes. A thin-walled vascular sensory organ, called saccus vasculosus, is attached to pituitary and connected by fibre-tracts with cerebellum. Pineal apparatus is well developed. Topographical features of hindbrain are least pronounced. Cerebellum is especially large due to active swimming habit. To assist cerebellum in the maintenance of equilibrium, ruffle-like restiform bodies are present at the antero-lateral angles of medulla.
In bony fishes, brain is more specialized than in elasmobranchs. In perch, olfactory lobes, cerebral hemispheres and diencephalon are smaller while optic lobes and cerebellum larger than in a shark. Some bony fishes have restiform bodies. In bottom-feeders, having scattered taste buds on body surface, the antero-lateral sides of medulla show unusual bulgings or vagal lobes. Parapineal body is absent in modern teleosts.
Brain of frog shows many contrasts from that of dogfish. Smaller olfactory lobes and larger optic lobes indicate a greater reliance on sight rather than smell. Corpus striatum or paleostriatum (floor of cerebrum) receives greater number of sensory fibres projected forward from thalamus than in fishes. Two cerebral hemispheres show greater development in accordance with more complex activities of locomotion, hibernation, breeding, etc. However, optic lobes are probably the dominant coordinating centres in amphibian brain. The walls of mid brain are thickened and reduce the lumen into a narrow passage called, aqueduct. Poor development of cerebellum, a mere transverse band, shows relative decrease in muscular activity. Medulla is also small. A small pineal body is present in all the modern amphibians.
Reptilians brain shows advancement in size and proportions over that of amphibians because of complete terrestrial mode of life. Telencephalon increases to become the largest region of brain. Two long olfactory lobes are connected to cerebral hemispheres which are larger than in amphibians because of greater thickness and enlargement of corpora striata. A fine vomeronasal nerve from the organ of Jacobson goes to the olfactory bulbs. Parapineal body, more often called the parietal eye, is still found in Sphenodon and some modern lizards, but is vestigial or absent in other reptiles. A pair of auditory lobes are found posterior to optic lobes which are not hollow. The III ventricle is reduced to a narrow cerebral aqueduct. Cerebellum is somewhat pear-shaped and larger than in amphibians.
Avian brain is proportionately larger than that of a reptile, and is short and broad. Olfactory lobes are small due to poor sense of smell. Two cerebral hemispheres are larger, smooth and project posteriorly over the diencephalon to meet the cerebellum. Pallium is thin but corpus striatum is greatly enlarged making lateral ventricle small and vertical. Third ventricle is also narrow due to great development of thalami. Optic lobes on mid-brain are conspicuously developed in correlation with keen sight, but they are somewhat laterally displaced. The cerebellum is greatly enlarged with several superficial folds (flocculi) due to many activities involving muscular coordination and equilibrium such as flight and perching.
Parts of vertebrates brain in linear arrangement become progressively enlarged from fishes onwards until they reach their peak in mammals. Brain is proportionately larger than in other vertebrates. Cerebral hemispheres of Prototheria are smaller and smooth, like those of reptiles. They are larger but smooth in Metatheria. In most higher mammals (Eutheria), cerebral hemispheres become greatly enlarged and divided into lobes, with thick cerebral cortex of gray matter. In mammals such as rabbit, the surface of cerebral hemispheres is relatively smooth with few fissures. In others, such as man and sheep, surface is immensely convoluted with a number of elevations (gyri) separated by furrows (sulci). This folding increases the surface cortex or gray matter containing nerve cells, resulting in greater intelligence without adding to the size of brain. The two hemispheres are joined internally by a transverse band of fibres, the corpus callosum, not found in other vertebrates or even in Prototheria and Matatheria. Olfactory lobes are relatively small but clearly defined and covered by the hemispheres. Diencephalon and midbrain are also completely covered by the cerebral hemispheres. Characteristic of mammals are 4 almost solid optic lobes, called corpora quadrigemina, on the roof of midbrain. The III ventricle or iter of midbrain is a laterally compressed vertical passage, called cerebral aqueduct. Cerebellum is also large, conspicuously folded and may overlie both midbrain and medulla. Usual folds are a median vermis, two lateral flocculi and their mushroom – like projections, the paraflocculi. The other chief topographical features of mammalian hindbrain include the pyramids carrying voluntary motor impulses from higher centres, the pons varoli with crossing or decussating fibres connecting opposite sides of cerebrum and cerebellum, and the trapezoid body of transverse fibres relaying impulses for sound. Hindbrain contains centres for the regulation of digestion, respiration and circulation.