THE BRAIN. 1055 



paralysis {polio-myelitis^, will lead to atrophy of the corresponding muscles. The 

 vasomotor centres are also in the anterior horns, probably in the intermedio-lateral 



tract. 



Sensory impulses pass to the posterior horns through the posterior roots, and 

 some of them soon cross to the opposite side of the cord, others ascending in the 

 posterior column. The lemniscus is probably the chief sensory tract in the medulla 

 oblongata, pons, and cerebral peduncles. 



Every segment of the spinal cord contains centres for certain groups of muscles, 

 and for reflex movements associated with them. A reflex begins in the stimulation 

 of a sensory nerve. The impulse thus created passes to a centre in the cord and 

 thence is transmitted to a motor nerve, thus producing a contraction of the muscle 

 supplied by that nerve. The complete path of this impulse is called a reflex arc. 

 The sensory impulse may be transmitted to different segments of the cord and thence 

 out through the corresponding motor roots. Thus a complicated reflex arc is 

 produced. It is to be assumed, however, that the impulse will take the shortest 

 route, so that simple reflexes will have their reflex arc chiefly in those segments of 

 the cord in which the posterior root enters. 



Each segment of the cord is connected with fibres from the brain to which must 

 be ascribed the function of reflex inhibition. If the inhibitory fibres are irritated, the 

 reflexes are impaired from stimulation of inhibition. If the conductivity of these 

 fibres is destroyed, the reflexes are increased; but if the reflex arc is broken at any 

 point, the reflexes are lost. Among the most important of these are the skin and 

 tendon reflexes. 



The centres for the bladder, rectum, and sexual apparatus, are located in the 

 sacral segment of the spinal cord at and below the third sacral segment. They 

 regulate the functions of these organs and are associated in some unknown way with 

 the brain. (See mechanics of urination, page 1914). 



Hcemato-rhacliis, or hemorrhage into the membranes of the cord (extramedullary 

 hemorrhage), may result from an injury to the spinal column, as a fracture or a severe 

 sprain. The bleeding may be from the plexus of veins between the dura and bony 

 wall of the canal (most frequent), or from the vessels between the dura and the cord. 

 In either case the symptoms will be much the same. There will be a sudden and 

 severe pain in the region of the spine, diffused some distance from the seat of the in- 

 jury, due to irritation of the meninges, and pain transferred along the distribution of 

 the sensory nerves coming from the affected segments of the cord, accompanied by 

 abnormal sensations, as tingling and hyperaesthesia. In the motor distribution there 

 will be muscular spasm, or sometimes a persistent contraction of the muscles. Gen- 

 eral convulsive movements, retention of urine, and, later, symptoms of paralysis may 

 appear, but as a rule the latter is not complete. 



HcBmato-myelia, or hemorrhage into the substance of the cord (intramedullary 

 hemorrhage) from traumatism, usually occurs between the fourth cervical segment 

 of the cord and the first dorsal (Thorburn), and is commonly due to forced flexion 

 of the spine, which is most marked in this region, as in falls on the head and neck. 

 The cord has been crushed in such accidents without fracture of the spine and with 

 only temporary dislocation. The hemorrhage is usually chiefly in the gray matter 

 and may be only punctate in size, or may be large enough to extend far into the 

 white matter, or even outside the cord into the subarachnoid space. The symptoms 

 usually appear immediately after the injury and are bilateral, suggesting a total 

 transverse lesion. There will be much pain in the back, occasionally extending along 

 the arms or around the thorax. Spasms, rigidity, and paralysis rapidly ensue, with 

 loss of the reflexes in the segment of the cord involved. There may be the same 

 dissociation of sensatiort as in syringomyelia when the hemorrhage is confined to the 

 centre of the cord. 



THE BRAIN. 



The brain, or the encephaloyi, is the part of the cerebro-spinal axis that lies within 

 the skull. It is produced by the differentiation of the cephalic segment of the neural 

 tube. Although the brain is often of great relative bulk and high complexity, as in 

 man and some other mammals, it must not be forgotten that the spinal cord is the 



