THE NERVOUS SYSTEM 1391 



portion of the limbic lobe which lies below the splenium of the 

 corpus caUosum, this portion being known as the isthmus of the 

 limbic lobe. 



The occipital lobe is pjTamidaJ, and presents an apex and three 

 surfaces — external, internal, and inferior. 



The apex forms the occipital pole of the cerebral hemisphere. 



External Surface. — This surface presents t\vo sulci, transverse 

 occipital and lateral occipital. The transverse occipital sulcus is 

 formed by the bifurcation of the posterior end of the ramus 

 occipitalis of the intrapanetal sulcus, and it crosses the upper part 

 of the occipital lobe transversely. Its upper limb Ues a Uttle behind 

 the external part of the parieto-occipital fissure, from which it is 

 separated by a portion of the arcus parieto-occipitlais ; and its lower 

 limb is behind the post-parietal gyrus. The lateral occipital sulcus 

 is situated on the external surface of the occipital lobe, and extends 

 almost horizontally from behind forwards. It divides the external 

 surface of the lobe into two parts, upper and lower, which are 

 connected with the parietal and tempor£il lobes by annectant gyri. 



Internal Surface. — The internal or mesial surface presents the 

 calcarine fissure. This is a deep cleft which commences on the 

 internal aspect of the occipital pole in a bifurcated manner. It 

 takes a curved course forwards, pcissing at first upwards and then 

 downwards, and it terminates by entering the limbic lobe beneath 

 the splenium of the corpus callosum. It is joined at a point 

 anterior to its centre by the internal part of the paneto-occipital 

 fissure, and between the two fissures is the cuneus. The calcarine 

 fissure may be regarded as being composed of two parts, anterior, 

 representing the portion in front of the internal part of the parieto- 

 occipital fissure, and posterior, representing the portion behind 

 that fissure. The anterior calcarine fissure gives rise to the hippo- 

 campus minor, or calcar avis, on the inner wall of the posterior 

 comu of the lateral ventricle. 



The gyri of the internal surface are two in number, namely, the 

 cuneus and the gyrus lingualis. 



The cuneus is triangular, and is wedged in between the posterior 

 calcarine fissure and the internal part of the parieto-occipital fissure. 

 The gyrus lingualis (infracalcarine gyrus) is situated between the 

 calcarine fissure above and the posterior part of the collateral fissure 

 below. Anteriorly it becomes narrow, and joins the hippocampal 

 gyrus. The lower portion of this gyrus is visible on the inferior 

 surface of the lobe. 



Inferior Surface. — ^The inferior or tentorial surface presents the 

 posterior part of the occipito-temporal gyrus, internal to which is 

 the posterior part of the collateral fissure, and internal to this again 

 there is the lower portion of the gyrus lingualis. 



Temporal Lobe. — ^The temporal (temporo-sphenoidal) lobe is 

 prominent, and of large size. It is situated below the posterior 

 horizontal limb of the fissure of Syhius, and behind the stem of 

 that fissure. Superiorly it is bounded by (i) the horizontal portion 



