i8 



Journal of Agriculture. 



[lo Dec, 1908. 



outflow is concerned with tlie muscles of the trunk limbs and tail. The 

 thoracic autonomic outflow produces constriction O'f arteries throughout 

 the body, acceleration of the heart, erection of hairs over the whole skin, 

 dilation of the iris, inhibition of gut movements, &c. The sacral autono- 

 mics produce contraction of the colon, rectum, bladder and uterus, and 

 special movements of the genital apparatus in both sexes. 



If the spinal cord be cut across in the lower neck region a mammal 

 continues to breathe by its diaphragm and can live many months. In this 

 case the whole of the central nervous system posterior to the cut wall be 

 isolated from the higher nerve centres, but it will continue, after a brief 

 period of shock, to act normally. As already pointed out in Chapter III. 

 the function of the central nervous system is to di\-ert afferent impulses 

 along particular paths and lead them eventually to the exit nerves. 

 (Fig. 37.) In this wav reflexes are produced. For instance in a dog 

 operated on as described, the afferent impulses, started by movement of a 

 few hairs on the back of the animal, will enter the spinal cord below the 



Fig, 60. Different views of a portion of the spinal cord with the roots of 

 nerves. — i, ventral fissure; 2, dorsal fissure; 3, origin of ventral rootlets ; 4, tip 

 of dorsal horn with entry of dorsal root; 5, ventral root; 5^ ventral root divided; 

 6, dorsal root; 61, ganglion on dorsal root; 7 and 7I, roots combined to form mixed 

 nerve which divides at once into two. (After Allen Thomson.) 



cut, wall traverse definite channels, and will finally leave by the motor 

 nerves which innervate certain muscles in a rhythmic way and thus 

 produce a movement of the hind limbs which we call scratching. _ The 

 foot will te applied fairly close to the region tickled though the animal, 

 through the operation, is unconscious of the stimulus and even of its own 

 response. A decapitated frog will scrape off wdth wonderful accuracy a 

 piece of paper soaked in acid which has been applied to its flank. _ As 

 iinother instance may be given the reflexes due to afferent impressions 

 arising from the presence of faeces in the rectum : these enter the spinal 

 cord, are grouped within the grey matter, and finally leave the cord by 

 the nerves, motor and preganglionic, which are concerned with that com- 

 bined action of muscles, skeletal and smooth we call defiecation. It .so 



