66 



ESSENTIALS OF PHYSIOLOGY. 



The condition lasts for a few days, at the end of which time the blood- 

 vessels and muscles have regained their tone, the sphincters of the anus 

 and bladder are contracted, defsecation and micturition occur reflexly, 

 and it is possible to evoke muscular reflexes. 



The condition of spinal shock is not due to the effect of the operation, 

 nor to the fall of blood pressure, for it only affects the region of the cord 

 posterior to the lesion. It is believed to result from the_cutting off of im- 

 pulsesjyhich a_re continually rea f p.hm<? t.T^p. nnr(] fro,m th^J^-j^ centres. 



SECTION V. 

 THE BRAIN. 



There are three main divisions of the brain, named respectively the 

 fore-brain, mid-brain, and hind-brain. The fore-brain consists of the 

 cerebral hemispheres and most of the structures bounding the third 

 2 ventricle, the mid-brain of the 



corpora quadrigemina and the 

 -8 7^ cerebral peduncles, and the hind- 



brain of the cerebellum, pons, 

 and medulla oblongata. 



THE MEDULLA OBLON- 

 GATA. 



The medulla oblongata may 

 be regarded structurally as an 

 upward continuation of the spinal 

 cord, in which certain conducting 

 tracts decussate, and the struc- 

 ture of which is further compli- 

 cated by the appearance of cell 

 stations on some of these tracts. 

 Owing to the decussations and 

 also to other modifications, the 

 upper part of the medulla oblon- 

 gata differs greatly in structure from the lower part. In the upper 

 part, the pyramidal tracts occupy a position anteriorly close to the 

 anterior median fissure, each on the side of the cerebral hemisphere 

 from which it is derived. The tracts are here known as the pyramids. 

 In the lower part of the medulla oblongata, the greater part of each 

 pyramid crosses to take up the position which it occupies in the 

 spinal cord as the crossed pyramidal tract (fig. 16). "The decussating 

 fibres separate the grey matter continuous with the anterior horn of 

 the spinal cord into two parts. One, continuous with the head of the 



FIG. 16. Section of the medulla oblongata 

 at the level of the decussation of the 

 pyramids. (Testut.) From Gray's 

 Anatomy. 



1, Anterior median fissure ; 2, posterior median 

 sulcus ; 3, motor roots ; 4, sensory roots ; 5, 

 base of the anterior horn, from which the 

 head (5') has been detached by the crossed 

 pyramidal tract ; 6, decussation of the pyra- 

 mids ; 7, posterior horns (in blue) ; 8, gracile 

 nucleus. 



