THE BRAIN. 



409 



tract, triangle, and the pardlfactory area (Brocae), the posterior 

 portion forms the anterior perforated substance, etc. 



Primary Fissures (Figs. 121 and 122). The vesicle walls are 

 of uniform thinness up to the second month. During their rapid 

 growth in the second and third months they become thrown into 

 folds which encroach upon the cavity and present on the surface 

 of the vesicle (or hemisphere) the primary fissures. The cause 

 of the infoldings is, perhaps, the resistance of the slower growing 

 cranium. The primary fissures are best developed in the third 

 month. They consist of the chorioidal fissure, already mentioned, 

 the hippocampal fissure which, surrounding it, is concentric with 



Fig. 120. Diagrammatic sagittal section of vertebrate brain. 

 (Morris's Anatomy after Huxley.} 



a. Corpora quadrigemina. b. Mid-brain, c. Pineal body. d. Cerebellum (hind-brain). 

 e. Medulla oblongata (after-brain), f. Pons Varolii (hind -brain), g. Lateral ventricle. 

 h. Cerebral hemisphere, i. Corpus striatum. j. Olfactory diverticulum. k. Pedunculi 

 cerebri. 1. Thalamus, m. Inter-brain, n. Hypophysis, o. Interventicular foramen. 4. 

 Fourth ventricle, s. Aqueduct of cerebrum. 3. Third ventricle. 



it, and the fossa lateralis cerebri (Sylvii). The hippocampal 

 fissure (Fig. 121) begins near the frontal pole on the medial sur- 

 face of the hemisphere vesicle and extends backward, downward 

 and, lastly, forward toward the temporal pole. It divides the 

 medial surface into a broad marginal gyrus and the dentate gyrus, 

 the latter is between it and the chorioidal fissure. In its whole 

 course it is parallel with the chorioidal fissure (Figs. 119 and 121). 

 The ventricular eminence caused by the hippocampal fissure is 

 called the hippocampus. The hippocampal fissure gives off two 

 branches at its most posterior part, which represent the occipito- 

 parielal and the calcarine fissures (Fig. 121). Both branches pro- 

 duce ventricular eminences, but only that of the calcarine per- 



