THE HIND-BRAIN OR RHOMBENCEPHALON 775 



ateral aspect of the fasciculus cuneatus, and is separated from the surface of the 

 medulla oblongata by a band of nerve fibers which form the spinal tract (spinal 

 root) of the trigeminal nerve. Narrow below, this elevation gradually expands 

 above, and ends, about 1.25 cm. below the pons, in a tubercle, the tubercle of 

 Rolando (tuber cinereum). 



The upper part of the posterior district of the medulla oblongata is occupied 

 iy the inferior peduncle, a thick rope-like strand situated between the lower part 

 of the fourth ventricle and the roots of the glossopharyngeal and vagus nerves. 

 The inferior peduncles connect the medulla spinalis and medulla oblongata with 

 the cerebellum, and are sometimes named the restiform bodies. As they pass 

 upward, they diverge from each other, and assist in forming the lower part of the 

 lateral boundaries of the fourth ventricle; higher up, they are directed backward, 

 each passing to the corresponding cerebellar hemisphere. Near their entrance, 

 into the cerebellum they are crossed by several strands of fibers, which run 

 to the median sulcus of the rhomboid fossa, and are named the striae medullares. 

 The inferior peduncle appears to be the upward continuation of the fasciculus gra- 

 cilis and fasciculus cuneatus; this, however, is not so, as the fibers of these fasciculi 

 end in the gracile and cuneate nuclei. The constitution of the inferior peduncle 

 will be subsequently discussed. 



Caudal to the striae medullares the inferior peduncle is partly covered by the 

 corpus pontobulbare (Essick 1 ), a thin mass of cells and fibers extending from the 

 pons between the origin of the VII and VIII cranial nerves. 



Internal Structure of the Medulla Oblongata. Although the external form of the 

 medulla oblongata bears a certain resemblance to that of the upper part of the 

 medulla spinalis, its internal structure differs widely from that of the latter, and 

 this for the following principal reasons: (1) certain fasciculi which extend from the 

 medulla spinalis to the brain, and vice versa, undergo a rearrangement in their 

 passage through the medulla oblongata; (2) others which exist in the medulla spin- 

 alis end in the medulla oblongata; (3) new fasciculi originate in the gray substance 

 of the medulla oblongata and pass to different parts of the brain; (4) the gray 

 substance, which in the medulla spinalis forms a continuous H-shaped column, 

 becomes greatly modified and subdivided in the medulla oblongata, where also 

 new masses of gray substance are added ; (5) on account of the opening out of the 

 central canal of the medulla spinalis, certain parts of the gray substance, which 

 in the medulla spinalis were more or less centrally situated, are displayed in the 

 rhomboid fossa; (6) the medulla oblongata is intimately associated with many 

 of the cranial nerves, some arising form, and others ending in, nuclei within its 

 substance. 



The Cerebrospinal Fasciculi. The downward course of these fasciculi from the 

 pyramids of the medulla oblongata and their partial decussation have already 

 been described (page 761). In crossing to reach the lateral funiculus of the oppo- 

 site side, the fibers of the lateral cerebrospinal fasciculi extend backward through 

 the anterior columns, and separate the head of each of these columns from its 

 base (Figs. 687, 688). The base retains its position in relation to the ventral 

 aspect of the central canal, and, when the latter opens into the fourth ventricle, 

 appears in the rhomboid fossa close to the middle line, where it forms the nuclei 

 of the hypoglossal and abducent nerves; while above the level of the ventricle it 

 exists as the nuclei of the trochlear and oculomotor nerves in relation to the floor 

 of the cerebral aqueduct. The head of the column is pushed lateralward and forms 

 the nucleus ambiguus, which gives origin from below upward to the cranial part 

 of the accessory and the motor fibers of the vagus and glossopharyngeal, and still 

 higher to the motor fibers of the facial and trigeminal nerves. 



1 Essick, Am. Jour. Anat., 1907. 



