436 



GENERAL ARRANGEMENT OF 



Fig. 149. 



each of these divisions, various deep-seated deposits of gray substance, 

 which are concealed by the overlying parts. 



The construction of the brain, as a whole, may therefore be represented 

 by considering it as a double series of nervous centres or deposits of 

 gray substance, of varying size and position, connected with each other 

 and with the spinal cord by transverse and longitudinal tracts of white 

 substance. The number and relative importance of these nervous 

 centres, in different kinds of animals, depend upon the perfection of the 

 bodily organization in general, and more especially upon the develop- 

 ment of the functions and capacities connected with particular parts of 

 the nervous system. In the inferior classes, as fish and reptiles, the 

 brain is more simple in its anatomical characters ; while it becomes suc- 

 cessively more complicated in birds, quadrupeds, and man. 



In fish and reptiles the different nervous centres of the brain are so 

 distinctly separated, and of such moderate size, that they are frequently 

 designated as " ganglia." In the brain of the alligator (Fig. 149) there 

 are five pairs of these ganglia, arranged one behind the other in the 

 cavity of the cranium. The first of these are two rounded masses ( a ), 



lying just above and behind the nasal cavi- 

 ties, which distribute their nerves upon the 

 olfactory membrane. These are the olfac- 

 tory ganglia. They are connected with the 

 rest of the brain by two long and slender 

 commissures, the " olfactory commissures." 

 The next pair( 3 ) are somewhat larger and 

 of a triangular shape, when viewed from 

 above downward. They are termed the 

 u hemispherical ganglia," or the hemispheres, 

 and correspond to the " cerebrum" in the 

 higher classes. Immediately following them 

 are two quadrangular masses ( 3 ) which give 

 origin to the optic nerves, and are therefore 

 called the optic ganglia. They are termed 

 also the " optic tubercles ;" and in some of 

 the higher animals, where they present an 

 imperfect division into four nearly equal 

 parts, they are known as the "tubercula 

 quadrigemina." Behind them is a single tri- 

 angular collection of nervous matter ( 4 ), the 

 cerebellum. Finally, the upper portion of the cord, just behind and 

 beneath the cerebellum, is seen to be enlarged and spread out laterally, 

 so as to form a broad oblong mass( 5 ), the medulla oblongata. It is 

 from this latter portion of the brain that the pneumogastric or respira- 

 tory nerves originate, and its ganglia are therefore sometimes termed 

 the " pneumogastric" or " respiratory" ganglia. 



It will be seen that the posterior columns of the cord, as they diverge 

 laterally, to form the medulla oblongata, leave between them an open 



BRAIN OF 

 1. Olfactory ganglia. 2. Hemi- 

 spheres. 3. Optic tubercles. 4. 

 Cerebellum. 5. Medulla oblon- 

 gata. 



