MEDULLA OBLONGATA 



799 



third, the pontine flexure, in the region of the future fourth ventricle. Both the cervical and 

 pontine flexures, while having a significance in the growth processes, are almost entirely ob- 

 literated in the later growth of the encephalon. 



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. 626, 627, 628. The reference numbers in 



Fig. 628. 



-Median Sagittal Section op Adult Human Brain. (Drawing of model by His.) 

 (Reference numerals same as in figs. 626 and 627.) 



Olfactory bulb ' 



Optic chiasma > 



Infundibulum 



the last three figures correspond with the like numerals after the names of the 

 parts on p. 797 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 



1. THE MEDULLA OBLONGATA 



The medulla oblongata [myelencephalon] is the upward continuation of the 

 spinal cord. It is only about 25 mm. long, extending from just above the first 

 cervical nerve (beginning 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 contains 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 struc- 

 tures, the medulla, as it approaches the pons, rapidly expands in both its dorso- 

 ventral and especially in its lateral diameters. With it are associated nine of the 

 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 

 funiculi. In the medulla oblongata they decrease in bulk in passing toward the 

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

 structures of the medulla, chiefly after crossing the mid-line. At the lower end of 

 the medulla occurs the decussation of the pyramids, by which the anterior median 

 fissure is almost obliterated for about 6 mm., and which, upon removal of the pia 

 mater, may be easily observed as bundles of fibres interdigitating obliquely across 

 the mid-line. 



Not all the pyi-amidal fibres cross to the opposite side at this level in man, but a portion 

 of those coursing in the lateral portion of the pyramid maintain their ventro-mesial position 



