182 INTRODUCTION TO NEUROLOGY 



In very young amphibian embryos Coghill 1 finds a still simpler condition 

 which is probably also more primitive. In the spinal cords of these larvae 

 the individual neurons of the motor tegmentum give rise both to fibers of the 

 longitudinal conduction tract of motor coordination (fasciculus proprius 

 ventralis) and to peripheral fibers of the ventral roots, the latter arising as 

 collaterals of the longitudinal axons. In older larvae separate neurons have 

 been differentiated for these two functions of peripheral conduction and 

 longitudinal conduction. The steps in the embryologic development and 

 probable evolution of the more complex centers of adjustment have been 

 briefly reviewed by Herrick and Coghill (see p. 66). 



Summary. The old clinical concept "general sensibility" 

 has recently been analyzed into a number of components, the 

 most fundamental division being the distinction between a 

 group of exteroceptive and a group of proprioceptive systems. 

 The exteroceptive systems are transmitted from the spinal 

 cord to the brain through a complex tract, the spinal lemniscus, 

 within which there are separate pathways for the three qualities 

 of sensation, touch, temperature, and pain. These sensation 

 qualities come into consciousness with a distinct peripheral or 

 external reference. The proprioceptive systems (muscle sense 

 and allied types) are transmitted to the brain through the dorsal 

 funiculus of the same side of the cord, the medial lemniscus of 

 the opposite side, the thalamus, and the somesthetic radiations 

 to the cerebral cortex; and also through the spino-cerebellar 

 tracts to the cerebellar cortex. Most of these reactions of 

 spatial adjustment do not come into consciousness at all, but 

 some appear subjectively as sensations of posture, bodily move- 

 ment, and spatial discrimination. The cerebellum is the great 

 clearing house for these and all other afferent systems which 

 are concerned in the proprioceptive functions, so far as these are 

 unconsciously performed. 



1 COGHILL, G. E. 1913. The Primary Ventral Roots and Somatic Motor 

 Column of Amblystoma, Jour. Comp. Neur., vol. xxiii, pp. 121-144. 



. 1914. Correlated Anatomical and Physiological Studies of the 

 Growth of the Nervous System of Amphibia, Jour. Comp. Neur.. vol. xxiv. 

 pp. 161-233. 



