152 COMPARATIVE MORPHOLOGY OF VERTEBRATES 



ated in the adult, but in amniotes they are increasingly developed 

 and persist throughout Hfe. 



Brain Stem. — In the higher vertebrates the parts of the brain 

 already enumerated, cerebrum and cerebellum excepted, but usually 

 including the corpora striata, are called the brain stem, because they 

 are the part from which cerebrum and cerebellum project as well as 

 the place where all of the fibre tracts are situated. 



Medulla Oblongata. — Of all parts of the brain the medulla or 

 myelencephalon most closely resembles the spinal cord. Viewed 

 from above it is triangular in outline, the dorsal parts of its side walls 

 being wedged apart, and then forced to turn abruptly toward the me- 

 dian line by the growth of the cerebellum. This brings about the 

 great expansion of the roof plate of the fourth ventricle already men- 



FiG. 159. — Median section of brain of pig 15.5 mm. long, showing flexures of the 

 brain C, principal flexure; cs, corpus striatum; CP, chorioid plexus of fourth ventricle; 

 k, hypophysis; i, infundibulum; M, mid-brain; N, nuchal flexure; nc, notochord and 

 vertebrae; P, pontal flexure; RO, optic recess; T, 'twixt- brain. 



tioned. The floor plate has lost its non-nervous character by the 

 invasion of several nerve centres in the ventral side, these serving as 

 places where nerve impulses, brought by efferent fibres, are redistrib- 

 uted to other neurons. 



As a result of the expansion of the anterior end of the medulla 

 oblongata, the fibre tracts of the dorsal and the dorsal part of the 

 lateral funiculi are carried to the sides of the cerebellum, to enter 

 which they must bend abruptly inward, thus forming a cord, the 

 corpus restiforme (inferior cerebellar peduncle) on either side. The 

 other fibre tracts form a pair of bands, the pyramids, on the ventral 

 surface of the myelencephalon which are continued forward beneath 

 the mid-brain as the cerebral pedtmcles (crura cerebri), the con- 



