162 ANATOMY OF THE SPINAL MARROW, OF GANGLIA, 



furnish the laminae. The main trunk then penetrates the 

 coffin bone posteriorly with the principal division of the 

 plantar artery, to be, like that, distributed over the sensi- 

 tive surfaces of the pedal bone. 



THE NERVES OF THE HINDER EXTREMITIES. 



We have seen that some of the muscles of the haunch 

 and thigh are supplied with feeling and motion by the 

 sacro-sciatic nerve ; which nerve principally divides into the 

 anterior, the posterior tibial, and into popliteal twigs : the 

 posterior tibial and principal branch, passing between the 

 fleshy portions of the gastrocnemii, gains the hock, where 

 it travels to one side of the perforans tendon, and sepa- 

 rating, forms the two metatarsal nerves. The popliteal, 

 which consist of a great number of fine twigs, go princi- 

 pally to the muscles at the back of the stifle joint, and to 

 the joint itself. The anterior tibial proceeds towards the 

 extensor muscles in front of the thigh ; when, having reached 

 the artery bearing the same name, it travels with it to the 

 middle of the cannon ; when leaving it, and passing over the 

 fetlock joint, it spends itself on the surface of the pastern. 



The internal and external metatarsal nerves pursue a course 

 similar in the hind to those of the fore extremities, so that 

 it is unnecessary to proceed with them further. 



ANATOMY OF THE SPINAL MARROW, OF GANGLIA, AND OF 

 THE SYMPATHETIC NERVE. 



The spinal marrow has its length decided by the length 

 of the tail. Thus, in baboons, it only just emerges from 

 the dorsal region ; in the horse it advances to the begin- 

 ning of the sacrum ; and in the kangaroo, and some monkeys 

 with prehensile tails, it extends even to the coccygeal bones. 

 The order of the red and white matter is opposite in the 

 spinal marrow to that which is displayed in the brain ; that 

 is, in the brain, the red substance is the exterior of the two, 

 whereas, in the spinal marrow, the white is externally, and 

 the red substance is internally situated. Though the spinal 

 marrow is anatomically said to travel along the spinal canal : 

 neither the marrow nor the canal are of one size throughout, 

 but the cavity enlarges as its contents become more bulky. 

 This it does in proportion to that which it has to supply, 



