500 THE BRAIN. 



band of arched fibres, known as the pons Yarolii, or great commissure of 

 the cerebellum ; and 3d, by a deposit of gray substance contained in its 

 anterior, and more or less mingled with its other portions. The tuber 

 annulare is, therefore, like the other central masses of the encephalon, 

 at the same time a channel of communication between the interior and 

 the exterior, and a nervous centre with special endowments of its own. 



In the most superlicial portions of the pons Yarolii, the transverse 

 bundles of nerve fibres are closely packed together, without any percep- 

 tible admixture of nerve cells. The deposit of gray substance commences, 

 however, according to Henle* at a short distance below the surface, occu- 

 pying minute spaces between the transverse bundles, and containing 

 stellate nerve cells. Beneath the superficial transverse bundles of the 

 pons come the longitudinal tracts of pyramidal nerve fibres, and be- 

 neath these again a deeper layer of transverse commissural fibres. The 

 deposit of gray matter in the pons is still more abundant in its deep 

 than in its superficial layer ; often alternating, according to Henle, 

 with the transverse bundles, in interspaces of J millimetre in thickness. 

 It also fills a space about 2 millimetres wide, on each side the median 

 line, between the two pyramidal tracts of white substance. 



Physiological Properties of the Tuber Annulare. In the tuber 

 annulare the phenomena both of excitability and sensibility, under 

 artificial irritation, become much more marked than in the great centres 

 of the cerebrum and cerebellum. According to Longet, a galvanic 

 stimulus, when the electrodes are passed into the substance of this 

 organ, produces distinct convulsive movements even in recently killed 

 animals, although its external surface does not appear to be excitable by 

 similar means either in front or behind. Yery slight irritation of its 

 posterior surface has been found, by both Longet and Yulpian, to give 

 rise in the living animal to indications of pain ; but this effect may be 

 partly due to the contiguity of the sensitive nerve-roots which traverse 

 the nervous substance in this situation. Excitability and sensibility 

 are also manifested on irritating the crura cerebri, between the tuber 

 annulare and the cerebral ganglia. 



The nature of the physiological actions taking place in the tuber 

 annulare, as a nervous centre, can only be studied by observing the 

 effects produced by its injury or removal, in comparison with other 

 parts of the encephalic mass. It is seen that the cerebrum and cere- 

 bellum may be taken away, either together or separately, without de- 

 stroying the evidences of sensibility or the power of motion. According 

 to the experiments of both Longet and Yulpian, the cerebral hemi- 

 spheres, the cerebellum, the corpora striata, the optic thalami, and the 

 tubercula quadrigemina may all be removed, in dogs and rabbits, and 

 yet the signs of sensibility and the power of motion in the limbs con- 

 tinue to exist ; and, if the cerebellum remain, the normal attitude of 

 the body and limbs, and even the power of progression, may still be 

 maintained. 



The manifestations of these nervous functions, however, are so much 



