500 OF THE NERVOUS SYSTEM AND ITS ACTIONS. 



plane, and are connected by a transverse tract of gray matter. The 

 remainder of the cord is made up of white or tubular substance, the 

 course of whose fibres is for the most part longitudinal. The posterior 

 peak of the crescentic patch of gray matter approaches very closely 

 to the bottom of the posterior furrow ; whilst the anterior peak does 

 not come into nearly the same degree of proximity with the bottom of 

 the anterior furrow. Hence it is considered by some, that the lateral 

 or middle columns of the cord, being much less completely isolated 

 from the anterior columns than they are from the posterior, should be 

 associated with the former, under the name of antero-lateral columns. 



882. Upon tracing the roots of the nerves into the substance of the 

 Cord, the connexion of a part of their fibres with its gray or vesicular 

 substance is easily made evident. Of these fibres, therefore, it serves 

 as the proper ganglionic centre. There is reason to believe, both from 

 anatomical investigation, and from physiological phenomena, that, as 

 in the Articulata ( 857), a part of the afferent or excitor fibres, after 

 traversing the gray substance, pass out on the same side as the efferent 

 or motor ; whilst another portion crosses to the opposite side, and forms 

 part of its efferent trunks. It is pretty certain that other fibres of the 

 roots become continuous with the longitudinal fibres that form the white 

 strands of the Spinal Cord ; but it is by no means certain, on that 

 account, that they pass on to the Brain ; and, in fact, there is adequate 

 evidence that if any of the fibres thus establish a direct communication 

 between the Encephalic centres and the spinal nerve-trunks, their pro- 

 portion must be very small, the chief part of the longitudinal strands of 

 the Cord being apparently made up of commissural fibres which esta- 

 blish an intimate connexion between its different segments, as in Insects. 

 This will appear from the facts to be next stated. The thickness of the 

 Spinal Cord differs considerably at its different parts. Thus in the 

 cervical region, there is an enlargement corresponding with the origins 

 of the nerves that form the brachial plexus ; this enlargement is partly 

 caused by an increase in the amount of gray matter ; but the amount of 

 fibrous structure also, is much greater than at the upper part of the 

 cervical region. On the other hand, there is a still greater enlarge- 

 ment of the cord in the lumbar region, at the part whence the nerves 

 of the lower extremities arise; and this enlargement is caused by the 

 great increase in the amount both of the gray matter and of the white 

 at that point. It may be easily shown by direct measurement, that 

 the fibrous strands of the upper cervical region would not by any means 

 serve to carry onwards to the brain those of the lumbar region alone, 

 much less with the addition of other fibres proceeding from all the in- 

 termediate nerves. Further, if the fibrous strands were for the most 

 part (as formerly supposed) directly continuous between the brain and 

 the roots of the spinal nerves, the white portion of the Spinal Cord, in 

 such animals as Serpents, in which it has no ganglionic enlargements, 

 should progressively diminish in diameter with every pair of nerves 

 into which it sends fibres, from its cephalic to its caudal extremity ; 

 this, however, is by no means the case, the Spinal cord of Serpents 

 being remarkable for its uniform diameter throughout. It is obvious, 

 then, that if any of the longitudinal fibres of the cord should thus pass 



