314 NERVOUS SYSTEM. 



united together by a delicate cellular tissue : it is, in fact, the 

 immediate source from which the brain derives its blood, and, at 

 the same time, the medium through which the unexpended 

 blood is returned to the sinuses of the dura mater. 



THE ARACHNOID MEMBRANE (so called from being 

 resembled to the spider's web) is of a texture so fine, and of a 

 nature so perfectly transparent, that, under ordinary circum- 

 stances, it is hardly demonstrable from the pia mater underneath 

 it, to which it is everywhere closely and intimately applied, ex- 

 cepting that it does not (like that membrane) dip down between 

 the convolutions. With pains, it is occasionally demonstrable 

 in places about the base of the brain. Physiologically con- 

 sidered, we have no right to doubt of its being organized ; al- 

 though, hitherto, the most successful injections have not demon- 

 strated the existence of bloodvessels in it. In the human subject 

 (in whom these parts are more developed) it has been said to have 

 shewn vascularity in cases where the membranes have recently 

 undergone the effects of inflammation. Concerning its use, no 

 writer has been bold enough to speak ; physiologists are unable 

 to say for what purpose so delicate and transparent a structure is 

 here interposed. 



Vascula?- communication. — From the vascular connexion which 

 subsists between the scalp, upon the exterior of the skull, and 

 the dura mater, upon its interior, we have at once an explanation 

 of that apparent anomaly in pathology, viz. that external injuries 

 of the skull frequently induce symptoms of inflammation of the 

 brain, or its membranes : in the human subject, such wounds 

 are always considered, on this account, as dangerous, and, in- 

 deed, it not very unfrequently happens that they prove mortal. 

 Though we have never seen a case of this description in the 

 horse, there does not appear to be any good reason why we 

 should not be cautious how we make or treat wounds of such a 

 nature. 



Effusion. — A fluid, differing from serum in its properties, 

 though like it in appearance, is occasionally effused between the 

 dura mater and tunica arachnoides, or, more commonly, under- 

 neath the latter membrane (as well as within the substance of the 

 brain itself), constituting a disease, termed hydrocephalus : it 

 rarely happens in horses ; but in the human subject, and more 

 especially in children, it is by no means an unfrequent cause of 

 dissolution. 



BRAIN. — Having already given an outline of the situation 

 and division of this organ, I shall now make some general ob- 

 servations in regard to its structure. 



Structure. — If a vertical section is made of any part of the 



