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THE NERVE SYSTEM 



unite end to end, and at the site of this union a vadum (or shallow uprising of the 

 floor of the fissure) or even a complete isthmus may be demonstrated in the adult 

 brain. Only three cases of bilateral interruption are on record. 



The occipital fissure (fissura occipitalis) is a deep cleft across the dorsimesal 

 border trans-secting the occipitofrontal arc at about 5 cm. (2 inches) from the 

 occipital pole, and extending upon both the mesal and the convex surfaces. On 

 the meson it attains a length of 3 to 3.5 cm. (li to If inches) (to its junction with 

 the calcarine fissure) while its lateral extent is shorter (2 to 2.5 cm., to 1 inch). 

 It is quite deep throughout and usually shows a number of interdigitating sub- 

 gyres. 



* The calcarine fissure (fissura calcarina) is a slightly arched fissure which is 

 usually joined with the occipital fissure at the apex of the cuneus and extends 

 caudad to the occipital pole, ending in a bifurcation. The fissure is composed of 

 two integers which may be partially or completely separated (by a vadum or an 

 isthmus); the caudal segment may then be distinguished as the postcalcarine 

 fissure. 



The occipital and calcarine fissures join to form a Y-shaped junction; the two 

 limbs of the Y embrace the cuneus, while the stem is continued as the occipito- 

 calcarine stem for a distance of about 3 cm. This fissural stem is allotted to the 

 occipital fissure by some and to the calcarine fissure by other authors. As there 

 is no greater frequency of confluence with one as against the other, so far as present 

 statistics go, it is preferable to assign no special relationship for this stem to one 

 or the other principal fissure. 



I. Frontal Lobe. FISSURES OF THE FRONTAL LOBE. 1. The lateral surface 

 is bounded by the dorsimesal arched border, by the fronto-orbital (or superciliary) 

 border, by the sylvian fissure (in part), and by the central fissure. The principal 

 fissures marking this surface demarcate four gyres: (1) the precentral, (2) super- 

 frontal, (3) medifrontal, and (4) subfrontal gyres. The fissures are (1) the pre- 

 central, (2) superfrontal, and (3) subfrontal fissures. In addition must be described 

 certain fissures which are intragyral and of more or less constant occurrence. 



The Precentral Fissural Complex (sulcus praecentralis). Two fissural integers 

 which are sometimes joined extend more or less parallel with the central fissure. 

 The mesally situated piece is usually of zygal (yoke-shaped) shape or triradiate, and 

 usually anastomoses with the superfrontal fissure. From its position it is termed 

 the supercentral or superior precentral fissure (sulcus praecentralix superior}. The 

 laterally situated piece is of longer extent, sometimes straight or slightly sinuous, 

 sometimes arched like an inverted L, or T-shaped. It usually anastomoses with 

 the subfrontal fissure. The two precentral segments demarcate the precentral 

 gyre from the remaining three gyres of the lateral surface of the frontal lobe. 



The superfrontal fissure (sulcus frontalis superior} usually springs from the 

 supercentral and pursues a sinuous course frontad, to become lost, as a rule, in 

 the zigzag or transverse ramifications of the prefrontal region. It is usually quite 

 ramified and often anastomoses with other fissures. It demarcates the superfrontal 

 from the medifrontal gyre. 



The subfrontal fissure (sulcus frontalis inferior} is most often confluent with the 

 precentral, less often with the supercentral fissure. It proceeds frontad in an 

 arched course, to end either in a bifurcation or by anastomosing with other fissures 

 (radiate fissure, orbitofrontal fissure, or medifrontal fissure). The subfrontal 

 fissure demarcates the medifrontal from the subfrontal gyre. 



Both the superfrontal and medifrontal gyres are characterized by a more or 

 less pronounced longitudinal subdivision by less constant fissural segments. They 

 are: (1) the paramesal fissure (sulcus paramedialis) occupying an intermediate posi- 

 tion between the superfrontal fissure and the dorsimesal border, in the super- 

 frontal gyre, more often composed of a series of short segments which become lost 



