46 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



spinal cord, or, conversely, the spinal cord as an extra- 

 cranial continuation of the basal part of the brain. 

 But this extracranial part is so important that it must be 

 treated separately. 



Envelopes. — Like the brain, it is incased and pro- 

 tected by a bony cover; but in this case the bony cover 

 must be flexible, and is therefore jointed. This is the 

 jointed backbone so characteristic of vertebrate ani- 

 mals, and giving name to the department. Like the 

 brain, also, it is invested by membranes — an outer tough 

 fibrous and a thin vascular one. As in the brain, too, 

 it is the inflammation of the membranes which gives rise 

 to the acuter forms of disease, such as cerebro-spinal 

 meningitis. 



Description. — The spinal cord is a nearly cylindrical 

 white cord, about half an inch in diameter and eighteen 

 inches long. Like the brain, it is a double organ, divided 

 almost into two semicylinders by a cleft down the dorsal 

 and the ventral side. Thus it consists of two semicylin- 

 ders joined along the axis. Therefore, viewed from the 

 ventral side, we have two anterior columns, and, from the 

 dorsal side, two posterior columns. The posterior col- 

 umns carry sensory fibers; the anterior, motor fibers. 



Spinal Nerves. — From the spinal cord proper there 

 go off thirty-one pairs of nerves; from the intercranial 

 continuation of the same there go off, in addition, twelve 

 pairs— making, in all, forty-three pairs of axial nerves. 

 The spinal or extracranial (but not the intercranial) 

 nerves have each two roots, which quickly unite to form 

 one nerve. One of these roots is connected with the 

 posterior or sensory column, and one with the anterior 

 or motor column of the cord. The posterior root has on 

 it a knot or ganglion (Fig. 30, a, b, and c). These nerves 

 pass out between the joints of the backbone and go to 

 be distributed to all parts of the body. This is the case 



