664 OF THE FUNCTIONS OF THE NERVOUS SYSTEM. 



ganglionic enlargements of the spinal cord; and in Man it is the posterior one 

 (or rather the inferior), which contains the largest quantity of gray matter. 



699. No doubt is now entertained amongst Physiologists that the Spinal 

 Cord is to be regarded under a double aspect; on the one hand, as an inde- 

 pendent centre of nervous power, on which excitor impressions operate to pro- 

 duce reflex movements; and, on the other, as the channel of communication 

 between the roots of the spinal nerves and the encephalic centres, whereby 

 sensory impressions are transmitted upwards to the sensorium, and motor influ- 

 ences originating in the brain are transmitted downwards to the efferent nerves. 

 But with regard to what may be termed the mechanism by which this is effected, 

 there is at present a considerable diversity of opinion ; which arises out of the 

 difliculty of determining, on the one hand, whether any of the root-fibres actually 

 terminate in the gray matter of the Cord, and, on the other, whether the longi- 

 tudinal fibres of the white columns are actually continuous from the Medulla 

 Oblongata to the roots of the Spinal nerves. Three distinct notions of this 

 mechanism at present have their several advocates amongst Anatomists of dis- 

 tinction. 



I. According to the first view, which may be regarded as a modification of 

 the doctrine which was current before the independent power of the Spinal 

 Cord had been distinctly recognized, all the root-fibres of the Spinal nerves are 

 brought by means of its longitudinal columns into direct continuity with the 

 Encephalic centres; so that sensory impressions are transmitted upwards to the 

 sensorium, and motor impulses are transmitted downwards from the seats of 

 volition, emotion, &c., without any interruption. But these fibres pass through 

 the gray substance of the Spinal Cord, in their transit from the white columns 

 to the nerve roots, and are thus subjected to the influence of its vesicular matter, 

 which is capable of exerting an independent operation through them, especially 

 when their continuity with the Brain is interrupted. 



II. According to the second view, the Spinal Cord is the real ganglionic 

 centre of all the root-fibres of the nerves issuing from it; each pair of nerves 

 being thus directly connected only with its own segment of the cord, or with 

 other segments a little above or below this ; and the function of the white or 

 longitudinal columns being to establish a commissural connection between the 

 several segments of the Cord, and to bring them into connection also with the 

 Encephalic centres. On this hypothesis, no sensory impressions pass directly 

 from the Spinal nerves to the Sensorium, and no motor impulse is directly 

 transmitted from any part of the Encephalon to these nerves, the vesicular 

 matter of the Spinal Cord being in each case the immediate recipient of the 

 change, and serving (so to speak) as a fresh starting-point for the nerve-force ; 

 whilst, if the connection of any segment with the Encephalon be interrupted, 

 that segment reacts upon impressions transmitted to it, in virtue of the direct 

 connection of the nerve-roots with its own ganglionic centre. 



in. The third view is a combination of both the preceding doctrines; for it 

 is considered by those who advocate it, that certain of the root-fibres, passing 

 continuously along the longitudinal columns of the Cord, establish a direct con- 

 nection, for sensory and motor purposes, between the Spinal nerves and the 

 Encephalic centres ; while certain other fibres have their central termination in 

 the gray matter of the Cord itself. It is considered, on this hypothesis, that the 

 fibres which minister to sensory impressions do so in virtue of their direct and 

 continuous passage from the peripheral surface to the sensorium; whilst the 

 fibres which transmit downwards from the Encephalic centres the motor im- 

 pulses originating in them, pass continuously to the muscles which they call into 

 play. On the other hand, each segment of the Cord is considered to minister 

 to itsown reflex action; the different segments, however, possessing such a 

 commissural connection with each other, that an impression made upon one of 



