C74 



A TEXTBOOK OF PHYSIOLOGY 



paralyzed, and 



thetic and tactile sensations are markedly impaired, while those of 

 pain and temperature, which cross soon after entering the cord, are 

 but little affected (Fig. 396). Degeneration takes place on the side 

 of the lesion in the motor tracts e.g.. the pyramidal, rubro-spinal. 



In complete section for example, in the thoracic area there is 

 paralysis below the site of the lesion of sensation and of all the 

 voluntary muscles, and of some of the involuntary, on both sides of 

 the body. This leads to a total loss of movement, a lowering of the 

 blood-pressure, and loss of control of the various sphincters. If the 

 section ho made high up in the cervical region, the muscles of 



respiration, including the dia- 

 phragm, are 

 death results. 



In the spinal cord there takes 

 place an upward degeneration in 

 the ascending tracts, and a down- 

 ward degeneration in the descend- 

 ing tracts. 



The Cord as a Reflex Centre. 

 In addition to conducting im- 

 pulses to and from the higher 

 parts of the central nervous 

 system, the spinal cord acts as a 

 reflex centre. Afferent messages 

 entering the cord are received, 

 and diverted into appropriate 

 efferent channels. Such reflexes 

 are regulated by a certain code, 

 which is applicable to all the 

 reflex reactions carried out by 



PIG. 399. SHOWING REFLKX ACTION OF 

 COKB AFTKB .REMOVAL OK _EN"TIRE 

 BRAIN, 



The beaker contains weak sulphuric 

 aeid. 



means of the central nervous 

 system. The system has been 

 most conveniently worked out 



iii the ease of the " spinal animal " that is to say, an animal in 

 which the spinal cord is separated from the higher parts of the central 

 nervous system, which may, or may not, be destroyed. 



Such an operation at first results in " spinal shock/' After the 

 familiar operation of pithing a frog without destroying the spinal 

 cord, the animal lies in a state of flaccidity, irresponsive to any form 

 of stimulus. After a time it recovers movement of the limbs, and 

 assumes a nearly normal sitting position. In response to a pinch, the 

 hir.d-limb is drawn away; if a piece of paper moistened with" acid be 

 placed on the side of the belly, the animal attempts to sweep away the 

 irritant by movements of the hind-limb of the same side (Fig. 399), 

 or, if the stimulation be sufficiently intense, by movements of tho 

 limbs of both sides. Placed in water, the animal will perform swimming 

 movements, swimming, however, somewhat more deeply than usual, 

 owing to the head being held in a lower position than normal. 



With higher animals e.g., the dog the effects of such an operation 



