CERTAIN SETS OF NERVES 91 



The functions of the impulses passing in the anterior roots are various 

 but not yet definitely understood. Most conspicuous is their function 

 of controlling the cross-striated muscles of the body, these impulses being 

 perhaps of two phases, actuating and inhibitory. The starting-point of 

 these influences is probably in the cortex of the hemispheres anterior to 

 the Roland ic fissure. These impulses, then, are productive of voluntary 

 or deliberate movements. It is possible that many of these impulses 

 start in the cerebellar cortex. In the case of reflexions, the incitement 

 of the motor neurones comes from other neurones interlacing with them 

 and thereby usually from the periphery. As we have just seen, each 

 muscle is innervated by the fibers of several roots and so controls in part 

 a whole functional group of muscles, especially in the hands and feet. 

 Smooth muscles in the hollow viscera are in part actuated through the 

 anterior roots, notably those of the uterus, urinary bladder, and the skin. 

 Another sort of influence passing outward in the ventral roots is that of vaso- 

 motor action and tone, the probably two-phased influence which so essen- 

 tially adapts the size of the arteries to the body's requirements both local 

 and general. One phase is vaso-constrictor while the other is vaso-dilator. 

 Whether the latter is an inhibitory influence on the former or a contractile 

 effect is not yet certain. Impulses which bring about the secretion of 

 sweat pass out of the spinal cord in the anterior roots. This stimulus is 

 either secretory in the proper sense of the term or only an effect of vaso- 

 dilatation. Which, it is still undecided, largely because of the slight his- 

 tological doubt as to whether nerve-fibers enter epithelial cells in all 

 cases. Similar uncertainty sometimes exists about a supposed "trophic" 

 (chemic?) influence exerted by the central nerve-system. Whatever the 

 nature of this undoubted control (proved by a host of pathological cases 

 as well as by the phenomena following nerve-suturing), the .ventral, 

 anterior, roots are the paths of its conveyance outward. 



The posterior spinal roots (so far as their nerve-cells at present imply) 

 have their origin in ganglia, more or less ellipsoidal in shape, to be found 

 one on each dorsal root close to its junction with the ventral root. The 

 accompanying diagram (Fig. 46) indicates the probable finer structure of 

 these ganglia and the relations of the ventral and dorsal fibers to them. 

 The afferent, "sensory/' nerve-cells for the most part have two axones. 

 Each of these divides into two medullated branches, one having grown 

 from the cells of the "neural crests" into the posterior horn of the cord's 

 gray matter, and the other centrifugally into the body's tissues and 

 periphery. Some of the ganglion-cells are multipolar, like those of the 

 sympathetic, while others are bipolar, but with short branching axones 

 which communicate (Dogiel) with the other sort of bipolar cells. Fibers, 

 not numerous, from the sympathetic ganglia communicate freely with 

 the cells of these spinal ganglia. In the cord's gray matter the den- 

 drites of these neurones associate freely with hosts of nerve-cells and 

 neurones, motor and sensory, as well as with many of a purely 

 associative function and position. Fibers from the ventral ("sensory") 

 root pass round through the ganglion and centripetally into the anterior 



