BRAIN OF MAMMALIA. 7 



pair of which (with a few exceptions) are destined for organs of flight. 

 The body is covered with feathers. 



MAMMALIA. Vertebrate animals, with warm and red blood ; with true 

 lungs, free and divided : the heart consists of two auricles and two ven- 

 tricles, and, together with the lungs, is enclosed in a distinct chest, 

 divided from the abdominal cavity by a diaphragm, or muscular partition. 

 The jaws are furnished with teeth emerging from deep alveoli ; the limbs 

 are generally four, sometimes two ; the epiglottis is distinct. The mode 

 of reproduction is viviparous ; the young are suckled on the mother's milk, 

 secreted in mammary glands, which open by ducts, externally. The 

 brain is highly developed ; the hemispheres contain large ventricles, 

 and are usually convoluted ; the optic lobes are small, concealed, solid, 

 and divided by a transverse furrow ; the spinal chord is less, in propor- 

 tion to the cerebral mass, than in the inferior classes. In the lower Mam- 

 malia, as the Rodentia and Marsupialia, the brain approximates to that 

 of the bird, and is destitute of external convolutions; but in none do 

 the cerebral hemispheres cover those of the cerebellum, till, ascending in 

 the scale, we arrive at the higher Simiae, and these are eclipsed by Man 

 in all that constitutes the highest condition of cerebral development. 



Here it may be observed, that the cerebral system not only exhibits 

 an ascending series of advances, from the lowest fish up to the highest 

 mammal, but that, in the highest mammal, a parallel series of advances ob- 

 tains, from the fish-like condition of the brain, at an early foetal stage, up to 

 its complete development. In the human foetus, for example, the optic 

 lobes, as in the fish, are larger than the hemispheres, and are hollow within ; 

 and their cortical portion in like manner predominates. The cerebral 

 hemispheres are then, as also in fishes, particularly the osseous fishes, 

 destitute of internal ventricles and external convolutions, having a smooth 

 cortical surface ; a wide longitudinal canal runs down the centre of the 

 spinal chord, as in fishes, and the cerebellum appears in the form of a 

 simple lobe its permanent condition in the highest of those animals. 



As it is to the brain that all impressions produced by external 

 agents on our bodily senses are conveyed as it is here that the incite- 

 ments to bodily motion originate as it is in this mysterious labora- 

 tory that all mental operations are conducted moreover, as, according to 

 the degree of excellence of this organ, so is the grade of the animal in the 

 chain of being, we are called upon to enter into a few details respecting 

 its parts and composition. It has been already stated that, among the 

 vertebrate classes, we find the brain the most completely developed in 

 the Mammalia and, among the Mammalia, in Man : to the human brain, 

 therefore, it is that our observations have now especial reference. This 

 wonderful organ, which may be regarded as the central axis, to which 

 the whole nervous system converges, and which is, in fact, the expanded 



