MEDULLA OBLONGATA. SPINAL CORD. 129 



the bulb behind, and in front to the peduncles of the cere- 

 brum, and on the sides to the peduncles of the cerebellum. 

 Its inferior surface, which rests on the basilar apophysis of 

 the occiput, shows fibres running transversely. It is grooved 

 along the median line, and is perfectly symmetrical. 



On its upper face there are four mammillar projections; 

 these are the corpora quadrigemina. Behind these, and 

 between the superior peduncles of the cerebellum, stretches 

 a thin layer of nerve substance, which has been called the 

 valve of Vieussens, and which forms part of the boundaries of 

 the fourth ventricle. 



The bulb of the spinal cord or medulla oblongata. This is 

 the term applied to the enlargement of the upper extremity 

 of the spinal cord. Pointing upward and forward, its anterior 

 surface corresponds to the basilar groove of the occiput; pos- 

 teriorly it rests in a depression of the cerebellum. Although 

 it is within the cranium, the bulb should be studied at the 

 same time as the spinal marrow, of which it forms a part. 



Spinal cord. This name is applied to the spinal portion 

 of the nervous centre. It is a nervous stem, white, cylindrical, 

 and symmetrical, and lies in, but does not completely fill, the 

 vertebral canal; it is held in place by the denticulated ligament 

 at each side. It is united with the encephalon by the med- 

 ulla oblongata. Terminating in a point at its lower extremity, 

 it rapidly increases in diameter, and forms the lumbar enlarge- 

 ment, so called from the region which it occupies; in the 

 dorsal region it diminishes in size; augmenting anew as it 

 approaches the neck, so as to form the cervical enlargement, 

 it shrinks again about the middle of the cervical region, and 

 then enlarges a third time at its superior extremity, and forms 

 the medulla oblongata. The spinal marrow is marked in front 

 and behind, throughout its entire length, by a fissure or median 

 furrow, which divides it into two distinct halves, excepting a 

 layer of white substance, which unites both the two fissures 

 and the medullary fascicles to right and left. This layer, 

 sprinkled with holes, designed to give passage to vessels, is 

 the perforated commissure. 



The anterior median furrow is covered at the top by the 

 interlacing of the nervous fascicles which run obliquely from 



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