THE FRONTAL LOBE. 145 



extremity of the lobe. The middle part of this fissure, which mainly causes the 

 projection of the collateral eminence in the lateral ventricle (see p. 125), is formed 

 independently of the two extremities. 



The calloso-margiiial fissure (Huxley) is an extensive fissure of the mesial 

 surface, which begins below the rostrum of the corpus callosum close to the anterior 

 perforated space, and sweeping round the genu of the callosum runs parallel to that 

 body, separated from it by the gyrus fornicatus, as far as a little behind the middle 

 of the hemisphere, where it turns obliquely upwards, and ends at the upper margin 

 of the hemisphere a short distance behind the commencement of the fissure of 

 Kolando (fig. 102). Both the anterior and the posterior parts of this fissure are deve- 

 loped independently of and are often permanently distinct from the middle part. 

 The anterior part or pretimbic fissure (fig. 102, pr.-l.} sweeps round the genu of the 

 corpus callosum, and when distinct from the middle part passes obliquely upwards 

 towards the upper margin of the hemisphere. In any case it usually sends a well- 

 marked ascending branch towards the margin (fig. 102, pr.-l. asc.). The posterior 

 part of the calloso-marginal (paracentral fissure of Wilder, fig. 102) hooks round the 

 inflected end of the fissure of Rolando, and is curved round the paracentral lobule, 

 which it bounds behind, below, and in front : the last by an ascending ramus, which 

 comes off at the junction of the posterior with the middle part of the calloso- 

 marginal fissure. 



The six fissures which have been described are used by anatomists to map out the surface 

 of the brain into regions to which the name of " lobes " has, not very appropriately, been 

 applied. In all, seven lobes are enumerated, viz., the frontal, parietal, occipital, temporal, 

 and liniMc, the island of Re,il, or central lobe, and the olfactory bulb and tract, or olfactory 

 lobe. It must, however, be understood that these so-called lobes have by no means an equal 

 morphological value, nor do they correspond precisely with the functional differentiations 

 of the hemisphere which can be made out as the result of experiments on animals, and 

 c-linical and pathological observations in man. 1 The distinction is further artificial because 

 the so-called lobes are in many places not marked off from one another otherwise than by 

 imaginary lines. Nevertheless it is found convenient for purposes of anatomical description 

 to consider the surface of the brain as thus constituted, and they will accordingly be here 

 described in the order in which they have been above enumerated. 



THE FRONTAL LOBE. The fissure of Rolando, passing obliquely down- 

 wards from the upper margin of the brain towards the Sylvian fissure, marks off the 

 anterior part of the hemisphere constituting in man nearly one-third of the whole 

 as the frontal lobe. This term, however, includes not only the part of the external 

 surface which is thus marked off, but the corresponding adjacent part (marginal con- 

 volution) of the mesial surface, as far as the calloso-marginal fissure, and also the 

 under or orbital surface of this anterior part of the brain. In the description of the 

 fissures and convolutions within the lobe, these three surfaces will be separately 

 considered. 



SULCI AND GYEI OF THE ExTEKNAL SURFACE. The precentral sulctis 

 (pre-Rolandic sulcus of Broca) (fig. 101, p.c.inf.,p.c.sup. and p.c.m.) has a direction 

 parallel with that of the fissure of Rolando, from which it is separated by 

 the ascending frontal gyrus. It is sometimes complete, but more usually is 

 subdivided into two or three separate portions by annectent gyri, which connect 

 the ascending frontal with the superior and middle frontal respectively ; a third 

 annectent gyrus passes below the lower end of the fissure, and unites the ascending 

 frontal with the third frontal. The uppermost portion (sulcus precentralis mesialis, 

 fig. 101, p.c.m,.} cuts the upper border of the hemisphere, and appears on the 

 mesial surface (fig. 102, p.c.m.} often as a well-marked vertical fissure, which 



1 Certain of them it is true, such as the olfactory and the occipital lobe and to a less marked degree 

 the temporal lobe, appear to have a localized association with the functions of certain special sense 

 organs. 



VOL. III. L 



