662 STRUCTURE OF THE CEREBRUM [CH. XLVI. 



nervous circles. In health, all these nervous circles are in action to 

 produce co-ordinated muscular impulses. In locomotor ataxy, which 

 is a degeneration of the cells of the ganglia on the posterior roots 

 and their branches, all these nervous circles are deranged, and the 

 result is loss of reflex action, and inco-ordination of muscular move- 

 ments. 



It should be noted that the pyramidal cells in the cortex, though 

 the largest in size, are the least numerous. Similarly the large 

 motor cells of the cord are relatively few in number. The innumer- 

 able smaller cells in both situations are association cells concerned 

 in the co-ordination of impulses. 



The Convolutions of the Cerebrum. 



The surface of the brain is marked by a great number of depres- 

 sions which are called fissures or sulci, and it is this folding of the 

 surface that enables a very large amount of the precious material 



B 



FIG. 483. 



A. Cerebral Hemisphere of adult Macacque monkey. 



B. Cerebral Hemisphere of child shortly before birth. 



The two brains aie very much alike, but the growth forwards of the frontal lobes even at this early 

 stage of development of the human brain is quite well seen. S, fissure of Sylvius ; B, fissure of 

 Rolando. 



called the grey matter of the cortex to be packed within the narrow 

 compass of the cranium. In the lowest vertebrates the surface of 

 the brain is smooth, but going higher in the animal scale the fissures 

 make their appearance, reaching their greatest degree of complexity 

 in the higher apes and in man. 



In an early embryonic stage of the human foetus the brain is also 

 smooth, but as development progresses the sulci appear, until the 

 climax is reached in the brain of the adult. 



The sulci, which make their appearance first, both in the animal 

 scale and in the development of the human foetus, are the same. 

 They remain in the adult as the deepest and best marked sulci ; they 

 are called the primary fissures or sulci, and they divide the brain into 

 lobes; the remaining sulci, called the secondary fissures or sulci, 

 further subdivide each lobe into convolutions or gyri. 



A first glance at an adult human brain reveals what appears to 

 be a hopeless puzzle; this, however, is reduced to order when one 

 studies the brain in different stages of development, or compares the 



