282 



NORMAL HISTOLOGY. 



Cerebral 

 peduncle 



divergingly descend from the sides of the corpora quadrigemina to the cere- 

 bellum ; similar arms, the inferior cerebellar peduncles, also known as the 

 rest (form bodies, convergingly descend from the cerebellar hemispheres to 

 the posterior columns of the medulla and form the lower lateral boundaries 

 of the fourth ventricle. Along the floor of the fourth ventricle and of the 

 Sylvian aqueduct lies an important sheet of gray matter, continuous with 

 that surrounding the central canal of the spinal cord, while within the white 

 matter of each cerebral hemisphere are embedded two large gray masses, 

 the corpus striatum and the optic thalamus, often together termed the basal 

 ganglia. The optic thalamus is of especial importance as the station towards 

 which, in a general way, all afferent (sensory) impulses destined for the brain 

 converge. 



THE MEDULLA OBLONGATA. 



The medulla oblongata, sometimes called the bulb and usually desig- 

 nated by the convenient but indefinite name " medulla," is the lowest seg- 

 ment of the brain-stem and the direct upward prolongation of the spinal 



cord. It is regarded as beginning be- 

 low at the lower border of the foramen 

 magnum and extends upwards to the 

 lower margin of the pons, a distance of 

 about 2. 5 cm. Its general form is taper- 

 ing, increasing in breadth from the 

 transverse diameter of the cord (10 

 mm.) below, to about twice as much 

 (18 mm. ) above, and in the antero-pos- 

 terior dimension from 8 to 15 mm. 



In many respects the medulla ap- 

 pears to be the direct continuation of 

 the spinal cord. Thus, it is divided 

 superficially into symmetrical halves by 

 the anterior and posterior median fis- 

 sures; . each half is subdivided by a 

 Centra-lateral and a dorso-lateral line of 

 nerve-roots into tracts that seemingly 

 are continuations of the columns of the 

 cord. This correspondence, however, 

 is only superficial, cross-sections of the medulla revealing the presence of 

 considerable masses of gray matter and important tracts of nerve-fibres not 

 represented in the cord, as well as the rearrangement, modificatior or disap- 

 pearance of spinal components prolonged into the bulb. 



As just stated, each half of the medulla is subdivided into three longi- 

 tudinal tracts, the anterior, lateral and posterior areas, by two grooves 

 situated at some distance from the mid-line. Of these, the antero- lateral 

 furrow marks the emergence of the root-fibres of the hypoglossal nerve, 

 which are entirely motor and correspond to the ventral roots of the spinal 

 nerves, with which they are in series. The other groove, the postero- 

 lateral furrow, marks the attachment of the fibres of the ninth, tenth and 

 bulbar part of the eleventh cranial nerves. These fibres are both afferent and 

 efferent and do not correspond to the posterior roots of the spinal nerves. 



Seen in transverse sections, the medulla, even at its lower end, presents 

 new features, and towards its upper limit varies so greatly from the cord 

 that only slight resemblance to the latter is retained. The characteristic 



Spinal 

 cord 



FIG. 329. Ventral aspect of brain-stem ; the 

 lines lettered on right side indicate the levels of 

 succeeding cross-sections. 



