330 MORPHOLOGY OF THE CEREBRAL CONVOLUTIONS. 



CONVOLUTIONS OF THE OCCIPITAL LOBE IN MAN. 



We have represented on Plate XXXVI, fig. 5, the structure of the occipital 

 lobe as found in a Negro brain, and I think it can be clearly seen that the arrange- 

 ment is similar to that of the Chimpanzee, except that the fissures are tortuous and 

 pushed out of shape. The convolution, 2, is the premier pli de passage siiperieur 

 externe which separates, O 1 , into two separate fissures, the parietooccipital, P 0, or 

 internal perpendicular, and an external portion, O l , the external perpendicular or 

 " transverse" fissure of Ecker. By comparing this brain with that of the Chim- 

 panzee, Plate XXXIX, fig. 4, it will be seen that the fissura perpendicularis ex- 

 terna of the apes is represented by a tortuous fissure which passes around the con- 

 volution, 2, and then some distance forward and downward. The posterior portion 

 of the interparietal, i p, is seen joining O 1 across the gap or separation of the walls 

 of the fissures produced by the development of gyrus, 2. The inferior branch of 

 the occipital arch, fissura occipitalis inferior, O 2 , is well marked off and may be dis- 

 tinctly recognized in fig. 1, Plate XLII and fig. 5, Plate XXXVI. These two fis- 

 sures, O 1 and O 2 , mark off the occipital lobe in the same manner as in the monkeys, 

 the modifications consisting simply of greater tortuosity of the fissures, and the 

 crowding of the occipital backward and on to the lateral surface. Within the limits 

 cut off by the primary arch, we find as in Cynocephalus, Chimpanzee, etc., a second 

 occipital arch, O 1 ' and O 2 ', which is in every way identical with the same sulcus 

 found in these brains, and divides in the same manner the lateral surface of the 

 occipital lobe into three gyri, a superior occipital, S. Oc, a postero-median, M. Oc, 

 an inferior, I. Oc. The primary and secondary arches are well developed, and the 

 only difference between the occipital as thus developed, compared with that of the 

 higher apes, consists in the more marked evidence of crowding of the fissures and 

 convolutions, producing increased tortuosity. 



The brain of a mulatto is represented in Plate XXXVI, fig. 6. Here the same 

 type is still followed, except that the secondary arch, O 1 ' O 2 ', has separated into 

 two parallel-running fissures, the fissurse occipitales longitudinales of Ecker, and 

 posteriorly we have a vertical furrow, t. ca 1 , which has developed as a vegetative 

 repetition of the transverse calcarine, t. ca, which, as we have previously pointed 

 out, has been displaced posteriorly by the development of a bridging gyrus, until it 

 appears, as can be seen in Plate XLIV, fig. 1, as a fissure of the lateral surface of the 

 occipital lobe. Otherwise the relations are not markedly changed. 



It will be unnecessary to go into further written detail, but an attentive study 

 of the plates of the human brain in which the occipital lobe is shown, will prove 

 conclusively that the arrangement of the fissures and convolutions in this lobe 

 so far from being entirely different from those found in the monkeys, is founded 

 upon precisely the same fundamental morphological type. 



Viewed in this light the human occipital lobe assumes an entirely new aspect, 

 and apparently obscure, irregular and unimportant points receive a new interpre- 

 tation and fresh interest. The occipital lobe is not an irregular and variable de- 



