MEDULLA SPINALIS. 



101 



There is scarcely any evidence 

 of a disposition to refltix move- 

 ments of the limbs of the human 

 body during health, or in the 

 waking condition, these move- 

 ments being restrained by the 

 controlling influence of the brain. 

 But when the spinal cord is in a 

 state of unnatural excitability, as 

 in tetanus and hydrophobia, or 

 during the presence of strychnine 

 in the system, or when the com- 

 munication of cerebral influence 

 to the limbs is cut off* by disease 

 of the spinal cord or brain itself, 



spasms may be excited in the extremities often by the slightest touch. 

 It is to be understood in this explanation of the reflex system, that 

 the spinal cord has the power of reflecting the action of sensitive 

 nerves upon the motor nerves, without itself perceiving the impres- 

 sion, in other words, without possessing sensation, that being a func- 

 tion exclusively of the brain. 



It is probable that all convulsive movements are produced through 

 the agency of the spinal cord and medulla oblongata, either by irri- 

 tation seated in the cord itself, [centric irritation ;) or by causes 

 seated at a distance, the irritation of which is transmitted to the 

 spinal cord, {eccentric irritation.) 



The ganglionic system of the lower orders of animals does not 

 correspond with the great sympathetic or ganglio7iic of the higher, 

 (the latter system existing only in those of most perfect develop- 

 ment,) but with the reflex or true spinal. In the lower orders, 

 the ganglia are scattered over various parts of the body, in the 

 leech around the mouth ; in the higher they are collected into one 

 common centre, the spinal cord. 



In regard to \\\q, functions of the columns of the cord, nothing definite 

 is settled. In the opinion of some physiologists (Bellinger and Valentin), 

 besides being concerned in sensation and motion, when the posterior 

 columm is irritated, at the point where the nerves of either extremity are 

 given off*, that extremity is extended; and that when the anterior is irri- 

 tated, the extremity \^ flexed. According to others, (Todd and Bow- 

 man), if it could be proved that the anterior roots were exclusively 

 connected with the antero-lateral columns, and the posterior roots 



* Diagram of the origins and terminations of the different groups of nervous fibres : aa. Vesi- 

 cular substance of spinal cord. hh. Vesicular substance of brain, e. Vesicular substance 

 at commencement of afferent nerve, which consists of c i The sensory nerve passing to the 

 brain, and si The spinal division, or excitor nerve, which terminates in the vesicular sub- 

 stance of the spinal cord. On the other side is the efferent or motor nerve, consisting of 

 two divisions, c^ The cerebral portion conveying voluntary motion, ands^ The spinal 

 division conveying the reflex power. 



9* 



