108 



THE NERVOUS TISSUE. 



three pairs, as before, in succession ; and by unveiling this 

 single one, the two primitive pair of Ganglia are revealed, 

 which are now hid by an additional development. These 

 Ganglia, at first disjointed, become connected by transverse 

 fibres, called commissures. 



The order, then, of development in the lower amimals 

 seems clearly to be, first, primitive cords, second, Ganglia, 

 and third, commissures uniting these Ganglia and associa- 

 ting their functions. 



In the development of the nervous system of man, there 

 are also first seen two filaments or cords, placed side by side 

 longitudinally, and upon these cords at the upper ex- 

 tremity are five pairs of Ganglia. In an after period these 

 two filaments become united and form the spinal cord, 

 except at their superior end, where they continue apart in 

 the crura cerebri. The first pair of Ganglia are developed 

 into the Cerebellum. The second pair, in animals the optic 

 lobes, have in man become the Tubercula Quadrigemina. 

 The third pair form the Optic Thalami, and the fourth the 

 Corpora Striata,the basis of theHem- 

 ispheres, while the fifth, very large 

 in the lower animals, are small in 

 man and form the olfactory bulbs. 

 Thus the same order of development 

 in the nervous tissue is established 

 in man as in the lower animals : 1. 

 The primitive fibres or cords. 2. 

 Ganglia upon these cords. 3. Com- 

 missures connecting these Ganglia and finally, develop- 

 ment from these Ganglia into the mature and perfect Brain. 



The Nervous Tissue wherever examined, is seen to be 



FIG. 13 represents the minute structure of nerve. 



a Loop termination of nerve fibres one of the loops is convoluted and 

 three are simple. 6 Represents the varicose appearance of a white nerve 

 fibre from the brain, which is made so by pressure or traction, c A white 

 nerve fibre enlarged to exhibit its structure, which is seen to be tubular, and 

 containing a substance called neurine. d A nerve cell, showing its granular 

 contents, e Nucleus and nucleolus. / Processes given off from a nerve cell. 

 g Nerve granules. 



