356 PHYSIOLOGY 



the posterior horn of the opposite side, though a few fibres are derived from 

 cells of the same side. The tract consists of fine fibres which pass upwards 

 in the peripheral margin of the lateral column, extending from the direct 

 cerebellar tract behind to the level of the anterior roots in front ; it passes 

 upwards through the cord, the medulla, and the pons, then turns round to 

 enter the cerebellum through the superior cerebellar peduncle, ending chiefly 

 in the ventral portion of the superior vermis. 



(3) THE SPINO-THALAMIC AND SPINO-TECTAL TRACTS. These fibres form 

 a scattered bundle lying internally to the anterior cerebellar tract, and are 

 practically part of Gowers' tract. They may be traced through the cord, 

 medulla, and pons and end, partly in the anterior corpora quadrigemina of 

 both sides, but to a greater extent in the optic thalamus of the same side. 



(c) ANTERIOR COLUMNS. A number of scattered fibres pass up the 

 anterior columns, mingled with the descending fibres of the tract of Marie 

 in the angle of the anterior fissure. Others pass up partly to end in the 

 olivary body, partly to run on with the mesial fillet towards the thalamic 

 region. 



The white matter of the cord can thus be regarded as made up of short 

 and of long tracts, which maintain direct connection between the following 

 parts of the central nervous system : 



(1) Different levels of the cord itself by means of the proprio-spinal 

 fibres. 



(2) Hind-brain and spinal cord, by the anterior and posterior cerebellar 

 tracts, the posterior columns, and the spino-olivary fibres among the ascend- 

 ing tracts, and the vestibulo-spinal and olivo-spinal among the descending 

 tracts. 



(3) The mid-brain and cord connections are represented by the spino- 

 tectal tracts in the lateral columns as a direct ascending path, and by the 

 rubro-spinal tract which furnishes a direct efferent connection between 

 mid-brain and cord. 



(4) The fore-brain, viz. the thalamus, receives the spino-thalamic fibres, 

 which, though scattered, are of considerable importance. They run chiefly in 

 the lateral and anterior columns. Its efferent fibres cannot be traced below 

 the lower cervical region. 



(5) The cerebral cortex, the master tissue of the body, receives no fibres 

 directly from the cord or periphery of the body, but by the pyramidal tracts 

 is able to influence directly the activities of the motor mechanisms at every 

 level of the cord. These fibres, so far as is known, exist only in mammals, 

 and show a great increase in relative extent when traced from lower to higher 

 types. While in the rabbit the pyramidal tract is hardly perceptible, in the 

 monkey it is the best marked of all the tracts, and in man is still more highly 

 developed. This relative increase, which is probably associated with the 

 shunting of more and more of the reactions of the body from the region of 

 the unconditioned reflex to that of the educatable reaction, is shown not 

 merely by the tract occupying a larger proportion of the transverse area 

 of the cord, but by its fibres being more densely set within that area. 



