576 PHYSIOLOGY OF THE NERVE CELL AND THE SPINAL CORD 



of escape that the segmentation in' the normal organism is not apparent. 

 That it does exist, however, has been demonstrated by Sherrington's investi- 

 gations on the posterior root reflexes in the monkey. It is unfortunate that 

 space will not permit us to discuss these results in detail. The following 

 concrete illustration of segmental reflexes in the higher vertebrates must 

 suffice. 



Goltz and Ewald isolated the spinal cord of the dog from the higher parts 

 of the nervous system by a section in the lower cervical or upper thoracic region 

 and then in a second operation removed the posterior end of the cord. There 

 remained only the upper part of the thoracic cord controlling what they called 

 the " middle animal." When the hand was rubbed over the right side of the 

 thorax of such an animal, that portion of the vertebral column containing the 

 remnant of the spinal cord was bent strongly to the right. If a very gentle 

 stimulus was given distortion of the skin only was observed. Wetting the thorax 

 with water caused the " middle animal " to tremble all over. 



Sherrington has found that reflexes spread most readily to the motor fibers 

 of the same pair of spinal nerves, and more readily to anterior roots near 

 the afferent path than to those far distant. They can also pass centrifugally 

 as well as centripetally in the cord, for stimulation of the fore leg will produce 

 a reflex contraction in the hind leg. Motor fibers springing from the same 

 segment of the spinal cord are not all roused to action with equal facility 

 e. g., in reflex excitation of the hind leg the flexors of the same side and the 

 extensors of the opposite side are called into play much more easily than 

 the extensors of the same and the flexors of the opposite side (cf. below, 

 page 587). 



B. GENERAL FEATURES OF REFLEXES 



Although reflexes may be radiated very widely, as a rule, they have a 

 rather limited distribution, certain efferent nerves being set in action by 

 definite afferent nerves, whence the so-called regulative reflexes. Thus stimu- 

 lation of the nerves of taste produces a reflex secretion of saliva and of 

 gastric juice; the afferent nerves of the lungs influence reflexly the respira- 

 tory muscles; the afferent nerves of the heart act reflexly upon the efferent 

 nerves of the heart, and upon the vasomotor nerves; the heat nerves and 

 cold nerves produce reflex alterations in the secretion of sweat and in the 

 supply of blood. to the skin, etc. 



From these examples, which represent but a small number of such reflex 

 processes, we may deduce the general rule : that the reflexes serve to regulate 

 various functions of the body, and to adapt them to their appropriate ends. 



How extremely useful to the body this purely machinelike regulation is 

 will be readily appreciated, if we but recall the importance of the examples 

 just mentioned and try to picture to our minds what would happen in the 

 event of their failure. How important it is, too, that this regulation should 

 go on independently of our own wills! It has only been by long and toil- 

 some investigation that scientists have learned the little we know about the 

 reflex processes in our bodies. If now our bodily functions could only be 

 carried out after mastering all the details, how should we ever learn them 



