THE TUBER AISTNULARE. 499 



else than straightforward flexion and extension of the posterior limbs ; 

 and the cerebellum is exceedingly small, much inferior in size to that 

 of fishes, and forms only a thin narrow ribbon of nervous matter 

 stretched across the upper part of the fourth ventricle. In the che- 

 lonia, or turtles, the movements of the body are accomplished by the 

 consentaneous action of the anterior and posterior limbs, and those of 

 the head and neck are also much more varied than in the frog, while the 

 cerebellum exhibits a corresponding increase of development. In the 

 alligator and allied species, whose motions approximate more closely to 

 those of the quadrupeds than is the case with other reptiles, the cere- 

 bellum is also larger in proportion to the remaining parts of the brain. 

 In birds, in quadrupeds, and in man there is a very evident increase in 

 the size and convolutions of the cerebellum, corresponding with the 

 greater variety and delicacy of movements which they are capable of 

 performing. These facts are not decisive in determining the physio- 

 logical function of this portion of the brain, since other nervous endow- 

 ments also vary in their degree of development in different animals and 

 in man ; but they show that the assumption of a co-ordinating power in 

 the cerebellum is not at variance with the comparative anatomy of the 

 nervous system. 



Everything which we know with certainty, therefore, in regard to the 

 cerebellum, indicates its close connection with the power of co-ordination 

 for the movements of the body and limbs. It cannot be regarded as 

 exclusively presiding over this function ; since there is strong evidence 

 that the posterior columns of the spinal cord are in great measure 

 devoted to the same purpose, and their morbid alteration necessarily 

 induces, in man, the disease known as locomotor ataxia. But the pos- 

 terior columns of the cord form by their divergence, at the level of the 

 fourth ventricle, the inferior peduncles of the cerebellum. The cerebel- 

 lum accordingly is a highly developed and convoluted nervous centre, 

 placed at the upper extremity of the cord, and communicating, by tracts 

 of white substance, with its posterior columns. The spinal cord itself 

 is, of course, essential to the co-ordinated motions of the body, arms, 

 and legs, since its posterior columns are for them the direct agents of 

 control and communication ; but the cerebellum may also be regarded 

 as a focus or nervous centre of reflex action for all the more vigorous 

 and complicated movements of the trunk and limbs. 



The Tuber Annulate. 



The tuber annulare is an isthmus, which makes connection between 

 the remaining parts of the encephalon above, and, through the medulla 

 oblongata, with the spinal cord below. It may be described in general 

 terms as constituted, 1st, by longitudinal tracts of white substance, 

 the prolongation of the anterior pyramids of the medulla oblongata, 

 which pass through it in a nearly straight course, becoming continuous 

 above with the crura cerebri ; 2d, by transverse bundles of white sub- 

 stance coming from the two sides, and encircling it with the superficial 



