7 86 A MANUAL OF PHYSIOLOGY 



terminations of the axons and their collaterals, and, therefore, 

 the greater is the number of possible paths between one nerve- 

 cell and another. 



II. Functions of the Central Nervous System. 



Much of our knowledge of the functions of the central nervous 

 system and of its divisions has been gained by the removal or 

 destruction of more or less extensive tracts of nervous substance, 

 or the cutting off of connection between one part and another. 

 But it is well to warn the reader at the very outset that in no 

 other part of physiology is such caution required in making 

 deductions as to the working of the intact mechanism from the 

 phenomena which manifest themselves after such lesions. 



In the first place, every operation of any magnitude on the brain 

 or cord is immediately followed by a depression of the functional 

 power of the nervous tissue distal to the lesion, a depression which 

 may extend far from the actual seat of injury and manifest itself 

 by various phenomena, which are grouped together under the name 

 of ' shock,' better termed spinal or cerebro- spinal shock, to dis- 

 tinguish it from the cardiovascular or surgical shock already de- 

 scribed (p. 175). Thus, when the spinal cord of a dog is divided, 

 e.g., in the dorsal region, all power, all vitality, one might almost 

 say, seems to be for ever gone from the portion of the body below 

 the level of the section. The legs hang limp and useless. Pinching 

 or tickling them calls forth no reflex movements. The vaso-motor 

 tone is destroyed, and the vessels gorged with blood. The urine 

 accumulates, overfills the paralyzed bladder, and continually 

 dribbles away from it. The sphincter of the anus has lost its tone, 

 and the faeces escape involuntarily. And if we were to continue our 

 observations only for a short time, a few hours or days, we should 

 be apt to appraise at a very low value the functions of that part of 

 the cord which still remains in connection with the paralyzed 

 extremities. But these symptoms are essentially temporary. They 

 are the immediate results of the section ; they are not permanent 

 ' deficiency ' phenomena. And if we wait for a time, we shall find 

 that this torpor of the lower dorsal and lumbar cord is far from 

 giving a true picture of its potentialities ; that, cut off as it is from 

 the influence of the brain, it is still endowed with marvellous powers. 

 If we wait long enough, we shall see that, although voluntary 

 motion never returns, reflex movements of the hind-limbs, complex 

 and co-ordinated to a high degree, are readily induced. Vaso-motor 

 tone comes back. The functions of defsecation and micturition are 

 normally performed. Erection of the penis and ejaculation of the 

 semen take place in a dog. A man with complete paralysis below the 

 loins and destitute of all sensation in the paralyzed region has been 

 known to become a father (Brachet) . Pregnancy carried on to labour 

 at full term has been observed in a bitch whose cord was completely 

 divided above the lumbar enlargement. The severity and duration 

 of spinal shock are greater in the monkey than in the dog, in man 

 than in the monkey, and in the whole mammalian group than in the 

 lower vertebrates. The mechanism of its production has been 

 much discussed, and will be referred to on another page (p. 808). 



