520 THE NERVOUS SYSTEM. 
Owing to the subdivision of the superior and middle frontal convolutions in the 
inanner indicated, the typical arrangement of the convolutions in the anterior part 
of the outer surface of the frontal lobe is in five horizontal tiers, and not in three 
tiers, as formerly described. ! 
Asa rule, the sulci on the outer surface of the frontal lobe during the process of development 
appear in the following order: (1) sulcus preecentralis inferior ; (2) suleus frontalis inferior ; 
(3) sulcus precentralis superior and sulcus frontalis superior; (4) sulcus frontalis medius ; 
(5) suleus paramedialis. This gives an indication of the relative morphological importance of 
these sulci. 
On the mesial aspect of the frontal lobe there is an elongated more or less 
continuous convolution, called the gyrus marginalis. It les between the supero- 
mesial margin of the hemisphere and the calloso-marginal fissure. In the forepart 
of this gyrus one or two curved sulci are usually present. They are termed the 
sulci rostrales. The posterior part of the marginal convolution is more or less 
completely cut off from the portion which lies in front. This part is called the 
paracentral lobule, and into it the upper end of the fissure of Rolando is prolonged 
as it turns over the supero-mesial border of the hemisphere. . 
On the orbital aspect of the frontal lobe there are two sulci, viz. the olfactory 
and the orbital. . 
The olfactory sulcus is a straight furrow which runs parallel to the mesial border 
of the hemisphere. It is occupied by the olfactory tract and bulb, and it cuts off a 
narrow strip of the orbital surface close to the mesial border, which receives the 
name of gyrus rectus. 
The orbital sulcus is a composite furrow which assumes many different forms. 
As a general rule it presents a 
shape sunilar to that of the 
letter H, and is then composed 
of three parts, viz. an external 
limb, an internal limb, and a 
transverse limb. ‘The external 
limb (sulcus orbitalis externus) 
curves round the orbital part of 
the inferior frontal convolution, 
so as to limit it on this aspect 
of the brain. The internal limb 
(sulcus orbitalis internus) is fre- 
quently broken up into two 
pieces. It marks off a convolu- 
olfactory sulcus 
internal orbitals 
olfactory sulcus, called the gyrus 
orbitalis internus. Thetransverse 
limb (sulcus orbitalis transversus) 
takes a curved course between 
the internal and external limbs. 
The district in front is termed 
the gyrus orbitalis anterior, and 
that behind the gyrus orbitalis 
posterior. The latter, in its 
outer part, corresponds with the 
orbital operculum. 
PARIETAL Lospe.—The 
parietal lobe forms a consider- 
able part of the external surface 
of the cerebral hemisphere, and 
it also appears on the inner 
surface in the form of the pracuneus or the quadrate lobule. Jn front, it is 
Zu BOdWSL 
ore? sulcus 
Fic. 387.—Gyrt anp Suuct on the tentorial and orbital aspects 
of the cerebral hemispheres. 
bounded by the fissure of Rolando, which separates it from the frontal lobe. — 
Below, it is limited in its forepart by the posterior horizontal limb of the fissure 
of Sylvius. Behind the upturned end of this fissure the surface of the parietal 
tion between itself and the: 
