170 PHYSIOLOGY OF CENTRAL NERVOUS SYSTEM. 



portant exceptions, and histologically there is proof that some of 

 the fibers in each pyramid (2 in Fig. 74) continue into and termi- 

 nate in the cord on the same side. The pyramidal system varies, 

 in an interesting way, in the extent of its development among the 

 different vertebrates. It reaches its highest development in man 

 and the anthropoid apes. In the other mammalia it is relatively 

 less important and the direct tract in the anterior columns is lack- 

 ing altogether. In the birds what represents the same system is 

 found in the anterior columns (Sandmeyer), while in the frog the 

 system does not exist at all. 



The relative importance of the system in the different mammalia 

 is indicated in the accompanying table taken from Lenhossek,* 

 in which the area of the pyramidal system is given in percentage 

 of the total cross-area of the cord: 



Mouse 1.14 per cent. 



Guinea pig 3.0 



Rabbit 5.3 



Cat 7.76 



Man 11.87 



Evidently, therefore, the importance of the pyramidal system 

 varies in different animals, and it is necessary to bear this fact 

 in mind in applying the results of experiments on the lower animals 

 to man. In the lowest vertebrates there are undoubtedly motor 

 paths between the brain and cord through which so-called voluntary 

 movements are effected, but these are probably short paths in- 

 volving a number of neurons. The higher the position of the 

 animal in the phylogenetic scale, the more complete is the develop- 

 ment of the long pyramidal system; but even in the higher mam- 

 mals it is probable that motor paths, other than the pyramidal 

 system, connect the cortex and subcortical centers with the motor 

 nuclei in the cord. In the dog, for example, section of the pyramids 

 is not followed by complete paralysis, and, indeed, after such sections 

 stimulation of the motor areas of the cortex still causes definite 

 muscular movements.! One such indirect motor path is referred 

 to below in connection with the rubrospinal tract (Monakow's 

 bundle). 



Less Weil-Known Tracts in the Cord. In addition to the 

 tracts just described there are a number of others mainly, descend- 

 ing tracts concerning which our anatomical knowledge is less com- 

 plete, and the physiological value of which is entirely unknown or 

 at best is a matter of inference from the anatomical relations. { 



* Lenhossek, " Bau des Nervensystems," second edition, 1895. 



t Rothmann, "Zeitschrift f. klin. med.," vol. xlviii., 1903. 



j Collier and Buzzard, "Brain," 1901, 177; and Fraser, "Journal of 

 Physiology," 28, 366, 1902. For summary and literature consult Van Ge- 

 huchten, "Anatomic du systeme nerveux de 1'homme," 4th ed., 1906. 



