378 THE BRAIN OF MAMMALS, BY TH. MEYNERT. 



ation of the transitional region where the cerebral structure 

 passes into that of the spinal cord. 



1. THE CEREBRAL HEMISPHERES. 



The cerebral hemispheres weigh in Man from 1,100 to 1,400 

 grammes (16,984 21,616 grains). Their great preponderance 

 in weight may be attributed to the circumstance that, consti- 

 tuting the essential seat of revived sensations or ideas, they 

 embrace simultaneously a share of the conditions of excitation 

 of the whole existence, whilst the functional activity of other 

 parts of the brain is constituted only by momentary conditions 

 of excitation. 



The grey substance of the first category, the cortical grey 

 matter of the cerebrum, invests the medulla of the hemi- 

 spheres, and gives to them their external form. It presents 

 variations in texture in different parts, on which account it 

 is important to recollect their general form in order that 

 these variations may be referred to their proper localities. 



The cerebral hemispheres originate in the form of two late- 

 rally situated, lenticular, and hollow processes, which are budded 

 off from the anterior cerebral vesicle. The entire superficies 

 develops into cortical substance. The external surface of the 

 lens is convex, like a shield ; the internal surface, turned towards 

 the constricted base or peduncle, is annular, and the lumen of 

 the ring forms the aperture of communication between the 

 first or anterior cerebral vesicle and the vesicle of the 

 hemispheres. The perforation of the vesicles by the trabecular 

 .(callosal) fibres divides off a portion of the upper periphery 

 of the ring of the median surface as the septum pellucidum. 

 Besides this, the ring of the internal surface of the hemi- 

 spheres is divisible into an anterior smaller and a posterior 

 larger segment (or, as they may be termed, two semi-circles). 

 The posterior semi-circle forms the gyrus fornicatus, which 

 curves round the corpus callosum ; the anterior, presenting an 

 angle opening posteriorly, constitutes the olfactory lobe. The 

 apex of the angle dilates to form the bulbus olfactorius ; the 

 internal and at the same time upper limb runs as the internal 

 olfactory convolution into the frontal extremity of the gyrus 



