NERVOUS SYSTEM. STRUCTURE. 



401 



the inferior creatures, namely, that it is composed of primitive cords, 

 primitive ganglia upon those cords, commissures to connect those 

 ganglia, and developments from those ganglia. 



In the adult, the primitive longitudinal cords have become cemented 

 together, to form the spinal cord. But, at the upper extremity, they 

 separate from each other under the name of crura cerebri. The first 

 pair of ganglia developed from the primitive cords, have grown into 

 the cerebellum ; the second pair (the optic lobes of animals) have 

 become the corpora quadrigemina of man. The third pair, the optic 

 thalami, and the fourth, the corpora striata, are the basis oftlve, 'hemi- 

 spheres, which, the merest lamina in the fish, have become the largest 

 portion of the brain in man. And the fifth pair (olfactory lobes), 

 so large in the lowest forms, have dwindled into the olfactory bulbs 

 of man. 



The white substance of the brain and spinal cord when examined 

 with the microscope, is found to consist of fibres embedded in granular 

 matter. The fibres vary in diameter from g ' 00 to l4 ^ 00 of an 

 inch ; the former is the measurement of the fibres where the white 

 substance is accumulated in any quantity ; the latter, where the fibres 

 enter the grey substance, and between these admeasurements every 

 intermediate size occurs. The fibres are composed of a thin, trans- 

 parent, and structureless neurilemma, containing in the living body a 

 pellucid, colourless, oil-like fluid (neurine). After death the nervous 

 fluid coagulates, and then presents the appearance of a white, opake, 

 curd-like matter, which aggregates in masses when the fibres are 

 pressed or stretched, and gives to the nervous fibre a varicose form. 

 The disposition of the nervous 



Fig. 138.* 



fibre to take on the varicose 

 form bears a close relation to 

 the thinness of the neurilemma ; 

 hence in the fibres of the brain 

 and spinal cord, and also in 

 those of the olfactory, optic, 

 and auditory nerves, where the 

 neurilemma is very thin, this 

 character is most remarkable. 

 The nervous fibres of the brain 

 terminate in the grey substance 

 by forming loops, as do the 

 peripheral nerves in nearly all 

 parts of the body. 



The grey substance of the 

 brain is composed of globular 



* Minute structure of nerve. 1. The mode of termination of nerve-fibres m 

 loops ; three of these loops are simple, the fourth is convoluted. The latter is 

 found in situations where an exalted degree of sensation exists. 2. A nerve- 

 fibre from the brain, shewing the varicose appearance produced by traction or 

 pressure. 3. A nerve-fibre enlarged to shew its structure, namely, a tubular 



2 D 



