PAET II. 



THE FORM AND STRUCTURE 



GRAY SUBSTANCE OF THE TRAPEZIUM, 

 MAMMALIAN. 



CHAPTER I. 



MORPHOLOGICAL CHANGES IN THE TRAPEZIUM OP THE MAMMALIA. 



(1.) The most striking feature in the upper part of the medviUa oblongata, is the 

 development of the auditory nucleus, which is conspicuously presented in two dis- 

 tinct masses, an inner or anterior portion formed from the remains of the uriderlyi)ig 

 nuclei, together with the new pyramidal mass, which, as we have seen in the foie- 

 going chapters, is formed partly from the vagal and partly from the post-pyramidal 

 nucleus, and an outer or posterior portion, consisting of a remarkable network inclos- 

 ing in its meshes numerous longitudinal bundles, formed from the outer part of the 

 post-pyramidal and inner portion of the restiform nucleus. The outer network is 

 closely connected with the entire mass of the restiform nucleus, by numerous bundles 

 of transverse fibres radiating from the network and traversing the dark mass of 

 longitudinal bundles constituting the restiform body, which now forms the outer 

 border of the medulla ; a part of these radiating fibres are connected with the pos- 

 terior auditory root, while another portion join the arciform plexus. 



The glossopharyngeal nucleus is still somewhat distinct, and is situated in a 

 depression near the apex of the anterior portion of the auditory nucleus, where 

 it is entered by numerous bundles of nerve-roots, traversing the caput cornu. In 

 the cat the roots of the glossopharyngeal are very distinct (Plate IX, Fig. 33), 

 passing into the small, transparent nucleus, which is crowded with quite large cells, 

 especially numerous near the posterior part. A portion of the glossopharyngeal 

 roots do not enter at the apex, but pass along the px)sterior margin of the nucleus, 

 entering farther back, spreading out both anteriorly and posteriorly in the posterior 

 part of the nucleus. 



The inner portion of the restiform nucleus contains many large multipolar cells, 

 and is closely connected with the network formmg the outer portion of the auditory 



( 43) 



