2-iO SPECIAL PHYSIOLOGY. 



ing out the Sjlvian fissure, a fifth lobe is seen, named the central lobe 

 or island of Reil. Of these lobes in the perfect human brain, the 

 frontal is the largest, the temporal and parietal are next in size, then 

 the occipital, and, lastly, the central lobe is the smallest. Each of 

 these lobes has its surface moulded into numerous tortuous and corn- 

 Fig. 59. 



Fig. 59. Longitudinal median section through the human cerebrum, cerebellum, cerebral peduncles, pons 

 Yarolii, medulla oblongata, and part of spinal cord, a, inner surface of the right cerebral hemisphere, 

 showing its sulci and convolutions; the fissure, running upwards and a little backwards, above the cere- 

 bellum, marks off the occipital lobe from the parietal, c, is the section through the corpus callosum, the 

 large white commissure, which connects the two hemispheres ; below this, are the Tentricles of the brain, 

 and thtn the cut surface of the right peduncle ; the section of the pons Varolii, is the part level with the 

 cerebellum, rf, the section through the cerebellum, showing the branched white substance, penetrating 

 the gray matter, and forming the so-called arbor vita, e, section of the medulla oblongata; /, the same of 

 a part of the spinal cord. 



plicated elevations of the cerebral substance, which have been named 

 the gyri or convolutions; these are marked off from each other by sec- 

 ondary winding fissures called sulci. Most of these convolutions have 

 long since been separately described and named by Flourens and 

 others, but a more systematic account of them has been recently given 

 by Gratiolet, in their respective groups of frontal, parietal, temporal, 

 occipital, and central. The convolutions of each lobe are connected 

 together at the bottom of the sulci, and also more or less at their ends. 

 So also the different lobes of the hemispheres are not distinctly severed 

 from each other, but are united at the bottom of the divisional fissures, 

 and also round the ends of these fissures. On the outer surface of 

 each hemisphere in the human brain, owing to the absence of the ex- 

 ternal part of the so-called perpendicular fissure which exists more or 

 less marked in the Quadrumana generally, the line of distinction be- 

 tween the parietal and temporal areas or lobes in front, and the occip- 

 ital lobe behind, is obliterated by many sinuous bridges of nervous 

 substance, called the connecting convolutions. The general plan of 

 the convolutions in the two hemispheres is the same; but, in point of 

 detail, there is a want of exact symmetry, a character which seems to 

 be associated with a higher development of any given type of cere- 

 brum. 



