172 TOPOGRAPHICAL ANATOMY OF THE 



An inspection of the lateral aspect of the brain emphasises the 

 preponderating size of the cerebrum, and affords partial views of the 

 cerebellum, pons and medulla oblongata. 



On regarding the brain from its ventral aspect, a considerable 

 number of structures are visible. At the present moment it will suffice 

 to recognise only the most conspicuous of these. The most posterior is 

 the medalla oblongata, a dorso-ventrally flattened column somewhat 

 similar to the spiual cord in appearance, and continuous therewith 

 without any definite line of demarcation. The artificial limits of the 

 two structures may be taken as occurring at a transverse plane on a 

 level with the foramen magnum. Immediately in front of the medulla 

 there is a rounded, transverse baud, the pons,^ which can be 

 readily followed laterally into the hemispheres of the cerebellum. 

 Appearing from underneath the pons are two thick, rounded cords or 

 columns, the cerebral pednncles^-^ which, after a diverging course, 

 disappear into the ventral face of the cerebral hemispheres. 



Crossing the anterior end of each cei'ebral peduncle obliquely is a 

 strongly marked white cord, the optic tract, which appi'oaches and 

 meets the tract from the other side and unites therewith at the optic 

 chiasma? The rest of the ventral view is occupied by parts of the 

 cerebral hemispheres, separated by a continuation of the longitudinal 

 fissure. 



The cranial meninges — Of the three membranous coverings of 

 the brain, the dura mater has already been examined (page 138). 



Continuous with, and structurally similar to, the corresponding 

 membrane of the spinal cord, the arachnoid of the brain (arachnoidea 

 encephali) is, for the most part, intimately related to the pia mater. 

 Over the summits of the convolutions of the brain the two membranes 

 are so inter-related as to constitute a single membranous covering;'* 

 but in other situations they are separated by a subarachnoid space 

 (cavum subarachnoidale) of variable depth, across which passes a fine 

 felt- work of fibres. In some places, for example at the base of the brain, 

 the subarachnoid space is of considerable dimensions and forms the 

 subarachnoid cisterns (cisternse subarachnoidales). A cistern of some 

 size exists between the medulla and the cerebellum (cisterna cerebello- 

 medullaris), and communicates with the interior of the fourth ventricle 

 by openings in the lateral recesses of the ventricle. A much smaller 



^ Pons [L.], a bridge. 



- Fedunculus (dim. of j^es) [L.], a little foot. 



3 xlaafxa. (chiasnia) [Gr.], the maik or figure X, two crossing lines. 



* There are those who hold that only two meninges sliould be recognised — a 

 leptomeninx (comprising the arachnoid and pia mater together) and a pachymoiinx 

 (dura mater). 'Kstttos (leptos) [Or.], thin, delicate. Traxi^s (pachys) [Gr.], thick, stout. 



