THE BRAIN. ' 141 



single bundle of nerve fibres, which suddenly bends at the periphery 

 of the cord in order to descend vertically through the white matter 

 towards the dorsal horn; the fibres divide into three sets, one to 

 the dorsal commissure, a second to the upper horn itself, and the 

 third helps to form the dorsal columns. 



Ventral roots of the spinal nerves. Each ventral root consists 

 of three or four delicate threads, which may be traced to the ven- 

 tral cornua, which they reach after a vertical or very slightly 

 oblique course through the white matter. . Other fibres arise from 

 the ventral columns, but these oblique fibres never extend to the 

 median plane. 



Pigment is found distributed irregularly through a section from 

 any part of the spinal cord ; it is increased in amount in those parts 

 in which there is an increased amount of grey matter. The pigmen- 

 tation is always found more marked in the lower parts of the 

 ventral horns than in other parts. 



B. THE BEAIX (Figs. 98, 102, 103, and 105). 



1. General description. From behind forwards, the dorsal 

 surface of the brain presents the following parts for examination : 

 the medulla oUongata, the cerebel 'I 'urn, the optic lobes, the thalamen- 

 cephalon, the cerebral hemispheres, and the olfactory lobes. 



The medulla ollongata is a direct continuation of the spinal cord ; it 

 is wider anteriorly than posteriorly, and is separated in front from the 

 optic lobes by a vertical plate of nervous matter, the cerebellum. The 

 optic lobes are two symmetrical ovoid bodies touching each other in 

 the median plane, and together forming the widest part of the brain. 

 In front of the optic lobes is the thalamencephalon, with a thick 

 vascular membrane, the choroid plexus, lying on its upper surface, 

 and connected with the pineal gland ; the thalameneephalon extends 

 forwards between the posterior ends of the cerebral hemispheres. 

 The cerebral hemispheres are two symmetrical ovoid bodies, narrow 

 in front, wider and slightly diverging behind : each hemisphere is 

 prolonged forwards to form an olfactory lobe. 



The ventral surface has in front the olfactory lobes, then the 

 cerebral hemispheres, behind these the lamina terminalis, the tuber 

 cinereum, the optic chiasma, the pituitary body, the cru.ra ctrelri, 

 and lastly the medulla oltongata, in the order here given. 



The various cranial nerves (Figs. 102 and 103) may be seen to 

 arise as follows : The olfactory nerve (7, /, L.ol') arises directly 



