1056 



HUMAN ANATOMY. 



fundamental and essential part of the nervous axis and that the degree to which the 

 brain is developed is, in a sense, accidental and dependent upon the necessities of the 

 animal in relation to the exercise of the higher nervous functions. In the lowest 

 vertebrates, the fishes, in which association of the impressions received from the 

 outer world is only feebly exercised, those parts of the brain rendering such functions 

 possible, as the cerebral hemispheres, are \ery imperfectly represented. On the 

 other hand, in man, in whom the capacity for the exercise of the higher nervous 

 functions involving association is conspicuous, the antero-superior parts of the brain, 

 the pallium, as the regions particularly concerned are called, are so enormously 

 developed that the human brain is thereby distinguished from all others. Whether of 

 low or high development, all brains are e\'olved from certain fundamental parts, the 

 braiii-vesicles, differentiated in the head-end of the embryonic neural canal ; the 

 underlying conception of the brain, therefore, is that of a tube, bent and modified to a 

 variable degree by the thickening, unequal growth and expansion of its walls. Even 

 when most complex, as in man, the adult organ exhibits unmistakable evidences of 

 subdivision corresponding more or less closely with the primary brain-vesicles, and 

 contains spaces, the ventricles, that represent the modified lumen of these segments. 



Fig: 909. 



Orbital surface of 

 frontal lobe 



Optic commissure 

 Optic tract 



Cerebral peduncle 

 Interpeduncular space 



Medulla 

 Cerebellum 



Olfactory tract 



Stalk of pituitary body 



Tuber cinereuni 

 Mammillary bodies 

 Cerebral peduncle 

 Temporal lobe 

 Pons 



Cerebellum 



Occipital lobe 



Spinal cord 



Simplified drawing of brain as seen from below, showing relations of brain-stem to spinal cord and cerebrum. 



Preparatory to entering upon a description of the fully formed brain, it is desirable 

 to consider briefly the broad plan according to which the organ is laid down and 

 the general lines along which its evolution proceeds. Before doing so, however, it 

 will be necessar) to take a general survey of the relations of the several divisions 

 composing the brain. * 



Denuded of its investing membranes and the attached cranial nerves, and viewed 

 from below (Fig. 909), the encephalon is seen to consist of a median brain-stem, that 

 inferiorly is directly continuous with the spinal cord through the foramen magnum 

 and above divides into two diverging arms that disappear within the large overhang- 

 ing mass of the cerebrum. The brain-stem includes three divisions, the inferior of 

 which, the medulla oblongata, is the uninterrupted upward prolongation of the spinal 

 cord and above is limited by the projecting lower border of the quadrilateral mass 



