io6o 



HUMAN ANATOMY. 



The latter, the outwardly bulging- pallium or hemisphere-anlage, is limited below by 

 the optic recess, the entrance into the optic vesicle, and, farther front, by a flattened 

 triangular elevation that marks the earliest rudiment of the co7'pus striatum. The 

 posterior or thalamic region extends backward to the mid-brain, from which it is 

 separated by the slight external constriction and corresponding internal ridge. 

 During the fourth week the demarcations just noted become more definite, so that 

 the primary anterior vesicle is imperfectly subdivided into two secondary compart- 

 ments, the telencephalon, conveniently called the end-brain, and the dienceph- 

 alon. Considered with regard to the details presented by the interior' of the fore- 

 brain, the four areas recognized by His are evident. These are (Fig. 912) the 

 region of the pallium and of the corpus striatum, respectively above and below in 

 the telencephalon, and the region of the thalamus and of the hypothalamus respec- 

 tively above and below in the diencephalon. Between the protruding hemispheres, 

 the telencephalon is closed in front and below by a thin and narrow wall, the lamina 

 terminalis, which defines the anterior limit of the brain-tube. 



While the more detailed account of the further development of these regions 

 will be given in connection with the description of the several divisions of the brain, 



Fig. 912. 



B 



cp Mid-brain 



Mid-brain 



Diencephalon 



/ 



Thalamus 

 Telencephalon 





Pallium 



/ 



Optic vesicle 

 — Otic vesicle 



Corpus 

 striatum 



Hind- 

 brain 



- Spinal cord 



X 



Hind- 

 brain 



Spinal cord 



Reconstruction of brain of human embryo of about four weeks (6.9 mm.); A, outer surface; B, inner surface; 

 /, isthmus; os, aperture of optic stalk ; ct>, cerebral peduncle; cf, cervical flexure; hf, cephalic flexure. Drawn from 

 His model. 



it may be pointed out here, in a general way, that the pallium gives rise to the con- 

 spicuous cerebral hemispheres, which, joined below by a common lamina, expand out- 

 ward, upv/ard and backward and rapidly dwarf the other parts of the brain-tube which 

 are thus gradually covered over. The striate area thickens into the corpus striatum, 

 which appears as a striking prominence on the outer and lower wall of each lateral 

 ventricle. The latter represents a secondary extension of the original cavity of the 

 fore-brain enclosed by the developing cerebral hemisphere, and at first is large and 

 thin-walled and communicates by a wide opening with the remainder of the brain- 

 vesicle. The unequal growth and' thickening, which subsequently modify the 

 surrounding walls, reduce this large aperture until it persists as the small foramen 

 of Monro, by which the lateral ventricle communicates with the third ventricle. The 

 latter represents what is left of the cavity of the fore-brain and, therefore, the.com- 



