148 MAMMALIA. 



point downwards, the remaining dorsal and two first lumbar upwards, in passing to the 

 ganglia at the notches leading from the spinal canal, and thus are directed each way 

 towards the most yielding portion of the spinal cord ; they are thereby prevented 

 from being stretched when the animal bends the back much, and particularly in 

 springing forwards. The third lumbar ganglion begins to incline downwards, and 

 the rest, as well as the sacral, then take an almost perpendicular direction. 



The enlargements of the spinal cord are more or less extensive, according to the 

 number and size of the nerves furnished by it in a given space, and the motion to 

 which the different parts must be subjected, the thickest portions being placed where 

 there is the least motion. It need not correspond with the extent of the spinal canal, 

 but for more or less of its length have nerves substituted for it. In the hedgehog it is 

 short and thick, and terminates very high up ; its point reaches to about the sixth 

 dorsal vertebra. In man its extreme point reaches to the second lumbar vertebra ; 

 but the thick portion, forming the principal part of the centre of the cauda equina, is 

 placed much higher. In simian it descends a little lower, the thick portion extending 

 to the bottom of the second lumbar vertebra, and its point reaching to the inferior part 

 of the fourth ; in a very small monkey its point reached to the bottom of the fifth ; its 

 thick portion extends to about the bottom of the sixth lumbar vertebra in the calf, 

 goat, dog, jaguar, pig, and other animals, and its point into the canal of the sacrum. 

 In the porpoise it terminates in a point about the sixth lumbar vertebra ; the nerves 

 of the cauda equina are then continued in the spinal canal for a considerable distance, 

 and afterwards the canal is occupied by a posterior caudal artery ; below the cervical 

 portion the spinal cord diminishes, and about the last rib enlarges again for giving 

 origin to the numerous and large caudal nerves, and it is not much less at this part 

 than in animals having lower extremities. When, therefore, it is shorter, the nerves 

 of the cauda equina occupy a greater length of the spinal canal ; so that they are 

 longest in the hedgehog, next in man and some animals, whilst in others they are very 

 short, with the exception of several slender ones continued through the canal to some 

 distance into the tail. 



The spinal cord is continued from the oblong medulla in a canal formed by the 



