DEVELOPMENT OF THE BRAIN. 339 



the infundibulum in the mid-ventral line. The infundibulum terminates 

 in an expansion which is the posterior lobe of the hypophysis. This 

 body, together with the anterior lobe derived from the oral ectoderm but 

 later severed from it, is lodged in the sella turcica of the sphenoid bone. 

 The development of the brain is summarized in the following table 

 (after His). 



f Myelencephalon . . Medulla oblongata. 

 Hind-brain . Metencephalon . . { ^rebellum. 



| Isthmus Isthmus. 



f Cerebral peduncles. 



Mid-brain Mesencepnalon . . . \ ^ , . 



. Corpora quadngemma. 



Mammillary part of the hypothal- 



amus. 

 Diencephalon .... Thalamus< 



Pineal body. 



Fore-brain j f Optic part of the hypothalamus. 



Hypophysis (posterior lobe). 

 Hemisphere: 

 Pallium. 

 Telencephalon.. Rhinencephalon. 



Corpus striatum. 

 Corpus callosum. 



MEDULLA OBLONGATA. 



Before considering the medulla the student should review the arrange- 

 ment of fiber tracts in the spinal cord (Fig. 147, p. 121). The cerebro- 

 spinal fasciculi, both ventral and lateral, consist of the fibers which 

 descend from the hemispheres. These four fasciculi of the cord arise 

 from two in the medulla, which there produce a pair of ventral swellings 

 (pyramids} shown in Fig. 391, B. In the section, Fig. 395, it is seen that 

 the pyramids are in the position of the ventral cerebro-spinal tracts of the 

 cord. In the lower or posterior part of the medulla the greater number 

 of fibers in each pyramid crosses through the ventral commissure to the 

 opposite side; thence they proceed across the gray substance to the 

 lateral cerebro-spinal fasciculus, which they form (Fig. 394). The crossing 

 is called the decussation of the pyramids, or, since these fibers terminate 

 about motor cells, it is called the motor decussation. The relatively 

 small number of pyramidal fibers which do not decussate in the medulla, 

 form the ventral cerebro-spinal fasciculi of the spinal cord. 



The fibers from the spinal ganglia ascend to the medulla in the cuneate 

 and gracile fasciculi. Within the medulla their fibers terminate, but 

 their course toward the hemispheres is prolonged by a second group or 

 "relay" of nerve cells, the bodies of which form four nuclei. These nuclei 



