382 



GENERAL STRUCTURE AND FUNCTIONS 



Fig. 128. 



and medulla oblongata. The anatomy of these parts, though some- 

 what complicated, can be readily understood if it be recollected 

 that they are simply a double series of nervous ganglia, connected with 

 each other and with the spinal cord ly transverse and longitudinal 

 commissures. The number and relative size of these ganglia, in 

 different kinds of animals, depend upon the perfection of the bodily 

 organization in general, and more especially on that of the intelli- 

 gence and the special senses. They are most readily described by 

 commencing with the simpler forms and terminating with the more 

 complex. 



The brain of the Alligator (Fig. 128) consists of five pair of 

 ganglia, ranged one behind the other in the interior of the cranium. 

 The first of these are two rounded masses (i), lying just above and 



behind the nasal cavities, which distri- 

 bute their nerves upon the Schneiderian 

 mucous membrane. These are the olfac- 

 tory ganglia. They are connected with 

 the rest of the brain by two long and 

 slender commissures, the " olfactory com- 

 missures." The next pair (-2) are some- 

 what larger and of a triangular shape, 

 when viewed from above downward. 

 They are termed the " cerebral ganglia," 

 or the hemispheres. Immediately follow- 

 ing 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 tuber- 

 cles ;" 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, we have a single triangular collection of nervous 

 matter (4), which is called the cerebellum. Finally, the upper por- 

 tion 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 respiratory 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 



BRAIN OF ALLIGATOR. 1. Ol- 

 factory ganglia. 2 Hemispheres. 3. 

 Optic tubercles. 4. Cerebellum. 5. 

 Medulla oblongata. 



