SPECIAL ANATOMY. 155 



the dura mater ; the visceral portion invests the pia mater. 

 Between these layers is a serous fluid, the arachnoid fluid ; 

 and between it and the pia mater is the subarachnoid 

 fluid. Posteriorly this membrane forms the posterior part 

 of the roof of the fourth ventricle, arachnoidean valve; it 

 becomes reflected from the different prominences of the 

 brain, and does not dip between into the sulci. Subarach- 

 noid" spaces are most marked in front of and behind the 

 optic decussation and below the posterior lobes of the 

 cerebrum in the transverse fissure. The pia mater is the 

 vascular membrane of the brain, the surface of which it 

 closely invests. It is prolonged into the cavities of the 

 brain in several i^laces ; thus, in passing through the trans- 

 verse fissure, it runs forwards under the posterior part of 

 the cerebrum and becomes thicker, forming the velum 

 interpositum, which is the roof of the third ventricle, and 

 which anteriorly bifurcates, forming the plexus choroides of 

 the lateral ventricles, which, passing through the foramina of 

 Munro (anterior communicating foramina) and between 

 the hippocampi and thalami optici, extend into the poste- 

 rior cornua of their respective ventricles. Underneath the 

 posterior part of the cerebellum it is similarly reflected 

 into the fourth ventricle, forming the plexus choroides of 

 the fourth ventricle. 



The brain is primarily divided into cerebrum, cerebellum, 

 200ns Varolii, and medulla oblongata. The cerebrum is the 

 largest part, and is that ovoid mass situated in the ante- 

 rior part of the cranium. It presents on its external sur- 

 face a number of convolutions, gijri, separated by grooves, 

 sulci, which in the human subject are capable of division 

 into sets, but in the horse present no regularity, except 

 that at the middle line we may observe the " marginal 

 convolution." On each side of the superior longitudinal 

 fissure, and on opening the fissure, we may see a " gyrus 

 fornicatus " on each side of the bottom of it. The cerebrum 

 is separated from the cerebellum by the great transverse 

 flssure, which extends on either side, and also in a forward 

 direction, underneath the posterior lobes of the cerebrum, 

 where it contains the vellum interpositum. The cerebrum 

 is divided into two parts, hemispheres, by the superior lon- 

 gitudinal flssure, which runs forward from the transverse 

 fissure and anteriorly extends downwards, putting in its 

 appearance at the anterior part of the base of the brain ; 



