388 PliOF. G. ELLIOT SMITH ON THE 



adds in explanation that lie " calls it s'dlon calcarhi, although the ergot (calcar) 

 from which its name is derived exists only in the Pi'imates " (p. 322). It is clear from 

 this that Broca believes that the postsplenial sulcus represents that sulcus of the Primate 

 brain which produces the calcar, i. e. the true calcarine and not the retrocalcarine sulcus. 



In the brains of most Carnivora and Ungulata. as well as in many other mammals, a 

 series of inconstant and exceedingly variable sulci make their appearance behind the 

 calcarine [i. e. the retrosplenial part of the splenial) sulcus. In the smaller members 

 this so-called *' postsplenial" sulcus tends to run parallel to the " splenial " sulcus. But 

 in the larger Carnivores, especially the Bears and Seals, and the larger Ungulates, such 

 as the Cow, Horse, and Camel, there are usually a series of postsplenial sulci ; and it 

 usually happens that the deepest of these pursues a course which is not parallel to the 

 calcarine (splenial), but inclines obliquely upward aiul backward at an angle with 

 the latter. It not unfrequeutly happens that this "retrocalcarine" sulcus, as we may 

 not inaptly term it, joins the calcarine. The tendency of the retrocalcarine element to 

 pursue a course which is not parallel to the calcarine becomes more pronounced in such 

 mammals (the Seals, for example) as possess a distinct caudal (or occipital) prolongation 

 of the hemisphere. It is, therefore, not surprising to find that in those mammals — viz., 

 the Primates— in which such a caudal extension of the hemisphere is most pronounced 

 (so that the mechanical factors which tend to induce the cortex to become folded in the 

 longitudinal direction are most j)otent), the retrocalcarine sulcus always assumes 

 the longitudinal, and never the vertical, direction *. It is still further determined 

 in this course l:iy the increasing obliquity of the calcarine sulcus, which is brought about 

 partly by the bcickward extension of the hemisphere itself, and partly by the caudal 

 elongation of the corpus callosum. In fact, the direction of the calcarine and retro- 

 calcarine sulci is brought, as the result of the operation of these various factors, so 

 accurately into the same line that in most Primates the less important retrocalcarine 

 element seems to develop merely as a caudal prolongation of the more precocious 

 and stable calcarine sulcus. 



The ])rain of the Camel affords a striking illustration of these facts. The mesial 

 surface of its cerebral hemisphere exhibits a triradiate group of deep sulci behind 

 the splenium of the corpus callosum (fig. 55). Superficially tiiese furrows apj)ear to be 

 jdined to one another; but if their lijjs be separated, it will be found in some cases that 

 the deep cdcarine sulcus is confluent with neither the intercalary sulcus, as in most 

 mammals, nor with the retrocalcarine sulcus, as in most Primates, but is separated from 

 l)oth by deeply submerged gyri, as is frequently the case in the human brain. It 

 is, moreover, noteworthy that the bulging (calcar) which is pushed into the definite 

 jjosterior cornu of the ventricle in this brain is produced wholly by the calcarine, and not 

 by the retrocalcarine nor intercalary sulci. 



* The varijibility of this sulcus clearly shows that it possesses little stability, aud cannot be regarded as a 

 deliuitely fixed landmark like the true calcarine. It probably arises in order to relieve the teasiun of the 

 expanding " visual " cortex, and its form and position are determined by conditions which are chiefly, if not wholly, 

 mechanical, la these respects it presents a most marked contrast, to the calcarine sulcus. 



