CH. XLIX.] REFLEX ACTION OF THE CORD 709 



is reached. It is impulses from the joints and muscles which are 

 specially important for the cerebellum to enable it to carry out its 

 functions of equilibration and coordination of muscular movements. 

 These impulses are non-sensory, but they are carried by fibres which 

 originate as collaterals from those which carry the true sensory 

 impulses of the same nature to the cerebrum. This group of fibres 

 therefore remains in the cord on the side of entry, in order to be in 

 the neighbourhood of the cerebellar tracts ; the impulses reach the 

 cerebellar tracts with the intermediation of a cell-station in Clarke's 

 column. When the tract conveying what we may term the motorial 

 sensations to the cerebrum has ministered in this way to the needs 

 of the cerebellum, there is nothing to prevent it following the 

 example of the other tracts, so when the spinal cord is passed and 

 the bulb reached, crossing of these fibres occurs in due course. 



To sum up the spinal cord is the seat of the transmutation of 

 most of the impulses of the first or peripheral level into those of 

 the secondary level of the afferent projection system. This recom- 

 bination takes place on the same side as that by which the impulses 

 enter the cord. The secondary paths for sensory impulses then cross 

 with greater or less rapidity, so that ultimately all except those 

 subserving the sense of position and movement and tactile dis- 

 crimination have passed to the opposite side within the limit of the 

 spinal cord and those which do not cross in the cord do so after 

 reaching the nuclei of the posterior columns. At the same time, 

 within the spinal cord afferent impulses become separated into sensory 

 and non-sensory, and the latter are exemplified by those which reach 

 the same side of the cerebellum by the cerebellar tracts. 



Reflex Action of the Spinal Cord. 



The reflex actions of the spinal cord may first be studied in the 

 brainless frog. In such a low type of animal, the interdependence of 

 cord and brain is not such a marked feature as it is in the higher 

 animals, and the spinal cord possesses within itself a great power of 

 controlling and coordinating very complex reflex actions. A study 

 of the reactions of the frog's spinal cord, moreover, illustrates most of 

 the fundamental facts in relation to reflex action generally. 



After destruction of the brain the shock of the operation 

 renders the animal for a variable time motionless and irresponsive 

 to stimuli, but later on it gradually assumes a position which differs 

 but little from that of a living conscious frog. If thrown into 

 water it will swim ; if placed on a slanting board it will crawl up it 

 (Goltz) ; if stroked on the flanks it will croak (Goltz) ; if it is laid on 

 its back, and a small piece of blotting-paper moistened with acid be 

 placed on the skin, it will generally succeed in kicking it off; if a 



