782 THE NERVOUS SYSTEM 



The location of the development of the various parts of the encephalon may be 

 determined, and their elaboration and changes in shape and position may be traced 

 by comparing the accompanying figs. 577, 578, and 579. The reference numbers in 



FIG. 579. MEDIAN SAGITTAL SECTION OF ADULT HUMAN BRAIN. (Drawing of model by His.) 

 (Reference numerals same as in figs. 577 and 578.) 



OLFACTORY BULB 



OPTIC CHIASMA 

 IXFUNDia UL UM 



the figures correspond with the like numbers after the names of the parts on p. 780 

 in the outline of the divisions of the encephalon. The more detailed subdivisions 

 of the parts will be met with in their individual descriptions. 



THE RHOMBENCEPHALON 



THE MEDULLA OBLONGATA 



The medulla oblongata is the upward continuation of the spinal cord. It is 

 only about 25 mm. long, extending from just above the first cervical nerve (begin- 

 ning of the first cervical segment of the spinal cord) to the inferior border of the pons. 

 It lies almost wholly within the cranial cavity, resting upon the superior surface of 

 the basal portion of the occipital bone, with its lower extremity in the foramen 

 magnum. Its weight is from 6 to 7 gm. or about one-half of one per cent, of the 

 whole cerebro-spinal axis. It is a continuation of the spinal cord and more it con- 

 tains structures continuous with and homologous to the structures of the spinal cord, 

 and in addition it contains structures which have no homologues in the spinal cord. 

 Due in part to these additional structures, the medulla, as it approaches the pons, 

 rapidly expands in both its dorso-ventral and especially in its lateral diameters. 

 With it are associated seven of the twelve pairs of cranial nerves. 



On its anterior or ventral aspect the anterior median fissure of the spinal cord 

 becomes broader and deeper because of the great height attained by the pyramids. 

 At the level at which the pyramids emerge from the pons, the region in which they 

 are largest, the fissure terminates in a triangular recess so deep as to merit the name 

 foramen caecum. The pyramids are the great descending cerebral or motor funi- 

 culi. In the medulla oblongata they decrease in bulk in passing towards the spinal 

 cord, for the reason that many of the pyramidal axones are contributed to struc- 

 tures of the medulla. At the lower end of the medulla occurs the decussation of 

 the pyramids, by which the anterior median fissure is almost obliterated for about 



