THE HORSE. 



171 



THE BRAIN AND ITS APPEN- 

 DAGES. 



The cerebrum, cerebellum, and medulla 

 oblongata, and medulla spinalis are invested 

 with three membranes : the dura mater, pia 

 mater, and tunica arachnoides. Of these 

 the exterior is the dura mater ; which, 

 though called a membrane, is of a dense, 

 tough, and inelastic texture. It is so firmly 

 adherent, by means of numerous little 

 prominences, to the sutures of the cranium, 

 that it is difficult to separate them; this 

 membrane is to the internal cranium what 

 the pericranium is externally. The inner 

 surface of the dura mater is lubricated by 

 a fluid furnished by its own blood-ves- 

 sels. 



The pia mater is that membrane which 

 closely envelopes the substance of the 

 brain, and dips down between its convolu- 

 tions, and adheres to its surface by num- 

 berless minute blood-vessels. It differs in 

 its appearance and texture from the dura 

 mater; presenting a smooth surface exte- 

 riorly, but a rough and villous one next to 

 the brain, and being composed of a beauti- 

 ful network of blood-vessels united together 

 by a delicate cellular tissue. 



The third membrane has been compared 

 to a spider's web, in allusion to which the 

 name of mcmbrana arachnoides has been 

 given to it. 



The arteries which supply the brain with 

 blood are the two vertebrals, besides two 

 other branches called the internal carotids. 

 Its blood is returned from the sinuses of 

 the dura mater by the vertebral and jugular 

 veins. 



It is on the supply of the vertebral ar- 

 teries, however, that the brain mainly de- 

 pends for its supply, for, if ligatures are 

 placed on these arteries, the animal dies; 

 whereas, both the carotids may be tied 

 without occasioning any apparent ill effects. 

 If our memory serves us. Dr. J. C. Warren, 

 of this city, has performed the latter 

 operation on the human subject with suc- 

 cess. 



In raising the bony covering of the brain, 



we meet with two processes, called the 

 falx, or longitudinal process, and the ten- 

 torium, or transverse process. The former 

 resembles the blade of a scythe, hence its 

 name. These processes are formed from 

 duplicatures of the dura mater; the first 

 descends for a short distance between the 

 lobes of the cerebrum. It takes its rise 

 from the crista galii, and terminates on the 

 OS occipitis. 



The tentorium is extended from the inner 

 plate of the os occipitis along the sides 

 of the cranium to its base, whence it may 

 be traced to the sphenoid bone, and is 

 lost in the common covering of the dura 

 mater. It is composed of two laminae : one 

 is continuous with the falx : the other forms 

 that portion of the membrane which covers 

 the cerebellum. The tentorium is equally 

 divided by the falx into two lateral por- 

 tions. 



The sinuses. — The superior, or longitu- 

 dinal sinus, runs within the duplication of 

 the falx, along its superior border. 



The two lateral sinuses are formed within 

 the duplicature of that part of the tento- 

 rium which is attached to the temporal and 

 occipital bones ; one extending to the right, 

 and the other to the left. They receive 

 veins both from the cerebrum and cerebel- 

 lum. 



The cavernous sinuses, so named from the 

 cavernous appearance of their interior ; 

 they receive some important nervous trunks 

 in their passage from the brain, and for 

 lodging the terminations of the internal 

 carotid arteries. They commonly commu- 

 nicate with the sub-occipital sinus ; these 

 are also of membranous formation, and are 

 found upon the cuneiform process of the os 

 occipitis, running longitudinally to the fora- 

 men magnum. They receive veins from 

 the cerebellum and posterior parts of the 

 cerebrum. 



Cerebrum. — The largest portion of the 

 cerebral mass, and that which presents it- 

 self to our view in raising the skull, is the 

 cerebrum. It is equally divided by a longi- 

 tudinal fissure along its middle, into which 

 the fabi cerebri descends ; and its divisions. 



