NERVOUS SYSTEM. 311 



subject ; though both organs, in regard to the bones of the 

 cranium, are similarly lodged : e. g. the cerebrum, which forms 

 the upper and anterior portion in the human subject, constitutes 

 the lower and anterior in the horse ; while the cerebellum, which 

 in the former is placed below and behind the cerebrum, in the 

 horse is placed above and behind it. This difference, however, 

 is but imaginary, being entirely referable to the position of the 

 head ; for if we place the horse's head upon a table, so that it 

 rest upon the branches of the lower jaw, we shall find no difference 

 whatever in the relative situation of these parts. 



Coverings. — The brain has three coverings, called its mem- 

 hranes or meninges ; the dura mater, the pia mater, and the tunica 

 arachnoides. Of these, the exterior is the 



DURA MATER. — Though called a membrane, this outer 

 covering is a substance, dense, tough, and inelastic in its texture, 

 its component material being chiefly tendinous fibre. It is 

 firmly adherent to the interior of the cranium, offering consider- 

 able resistance to the elevation of the skull-cap, even after the 

 bone has been completely divided ; which union is the result of 

 many little processes shooting from the membrane in between the 

 teeth of the sutures of the cranium, as well as of numerous small 

 bloodvessels passing from it into the pores of the inner table of 

 the bone, to which it supplies the place of internal periosteum. 



Surfaces. — The external exhibits a smooth, dry, opaque, red- 

 dish aspect, rendered uneven by the subjacent prominences of 

 the brain. The internal is lighter in shade, shiny and slippery, 

 and moist from the exudation of a serous secretion. It has no 

 connexion with the membrane underneath ; at least, no other 

 than it has with the brain itself — by means of the vessels passing 

 between them. 



Organization. — The bloodvessels of the dura mater are not 

 numerous, but of sufficient magnitude to admit of coarse injec- 

 tion. The membrane itself aflbrds a good example of the truth 

 of the observation, " that the capillaries are not abundant in 

 fibrous texture." By some, nerves are denied to exist in the 

 dura mater: others conceive that they have discovered nervous 

 filaments in its texture. In a sound state, the membrane ap- 

 pears to possess very little if any sensibility ; for it may be cut 

 or irritated in various ways without seeming to cause the animal 

 any pain whatever. 



Processes. — The dura mater sends off detached portions or 

 processes, which are extended across the cavity of the cranium, 

 for the purpose (it would seem) of more steadily and effectually 

 supporting the brain, and guarding against the pressure of one 

 part or division upon another. These processes are two in num- 



