NERVOUS SYSTEM STRUCTURE. 359 



naturally led to the same conclusion with regard to the architecture 

 of the human brain, that we were led to establish as the principle 

 of developement in the inferior creatures that it is composed of 

 primitive cords, primitive ganglia upon those cords, commissures 

 to connect those ganglia, and developements from those ganglia. 



In the adult, the primitive longitudinal cords have become cement- 

 ed 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 of the hemispheres, which, the merest lamina in the fish, 

 has 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 varying in diameter, 

 according to Krause, from the 7 | 7 to the y^ of a .line. These 

 fibres are composed of a thin and transparent neurilemma, en- 

 closing a soft homogeneous nervous substance, and they possess a 

 remarkable tendency, when compressed, to assume a varicose ap- 

 pearance. The nervous fibres of the olfactory, optic, and auditory 

 nerves have the same disposition to become varicose on pressure. 

 The neurilemma of the primitive fibre, according to Fontana, con- 

 sists of two layers, of which the internal is thin and transparent, 

 and the external cellular and less transparent. 



The gray substance of the brain, according to Valentin, is com- 

 posed of spherical globules of considerable size, having a central 

 nucleus, and near the margin of the latter another smaller nucleus, 

 and frequently upon the surface of the globule, patches of pigment. 

 Numerous minute fibres have been observed by Remak to proceed 

 from the surface of these globules, and are supposed to maintain a 

 communication with surrounding globules. The various shades of 

 gray observed in different parts of the brain, depend upon the 

 greater or smaller number of globules existing in those parts. Two 

 kinds of gray substance are described by Rolando as existing in 

 the spinal cord ; the one (substantia cinerea spongiosa vasculosa) is 

 the ordinary gray matter of the cord, and the other (substantia 

 cinerea gelalinosd) forms part of the posterior cornua. The former 

 resembles the gray matter of the brain, consisting of globules, while 

 the latter is composed of small bodies resembling the blood corpus- 

 cules of the frog. 



The nerves are divisible into two great classes, those which 

 proceed directly from the cerebro-spinal axis, the cranial and 

 spinal nerves, and constitute the system of animal life; and those 

 which originate from a system of nervous centres, independent of 

 the cerebro-spinal axis, but closely associated with that centre by 



