642 



allied structures. Inasmuch as it is thus placed in a position to in- 

 fluence the respiratory, cardiac and vasomotor activities, it must also 

 dominate in an indirect way, the function of the cerebral centers. 



Lastly, the medulla must be considered as an organ of conduction, 

 because it occupies a position directly in the path of the cerebro- 

 spinal tracts. It also gives origin to several of the cranial nerves which 

 in this way are enabled to gain access to the higher centers. All in 

 all, therefore, the medulla is one of the most widely connected struc- 

 tures of the nervous system. 



, .. . -^Ala cinerea 



Nuc. dorsahs vagis^ ^*^ _ . 



^s^ zr^xZi ^---Tngonum hypoglossi 

 Nuc. faso. solitari 



Fasc. 



Nuc. 



cuneatus 



X root' 

 Nuc. ambiguus- _ 



^Tr. spino-cereb. 



ventralis 

 \Lemniscus spinalis 



s Tr. tectospinalis 

 Inferior olive-"^ ^^ /\ / ^\^T :_. . medialis 



XII root"^ "^^i- -"-^ ^Pyramidal tract 



FIG. 317. DIAGRAMMATIC CROSS-SECTION THROUGH THE HUMAN MEDULLA OBLONGATA 

 AT THE LEVEL op THE VAGUS NERVE, ILLUSTRATING DETAILS OF FUNCTIONAL LOCALIZATION. 



CHAPTER LIII 

 THE CRANIAL NERVES 



The Functional System of the Cranial Nerves. We have seen 

 above that the spinal nerves enter the cord by a series of roots 

 arranged in strict agreement with segmentalism. The sensory fibers 

 and corresponding gray matter occupy the dorsal realm of this struc- 

 ture, while the motor fibers with their gray matter are situated ante- 

 riorly. The cranial nerves show a similar functional arrangement, 

 because the sensory centers are situated dorsally to the motor, but the 

 segmentalism observed in the case of the spinal fibers has here given 

 way to a perfectly definite grouping of the different units. This enables 

 all impulses of like character to become closely associated. In general, 

 therefore, it may be said that the twelve pairs of cranial nerves repre- 

 sent twelve pairs of interlocking systems, regulating one or several 

 independent functions, irrespective of their anatomical location. This 

 fact shows that the grouping of the components of the cranial nerves 



