964 SPECIAL PHYSIOLOGY. 



occurs between the cephalic and cervical portion of the embryo, a pos- 

 terior projecting angle is formed between the spinal cord and the pos- 

 terior cerebral vesicle ; this corresponds with the cervical tuberosity of 

 the embryo. The middle cervical vesicle, or mesencephalon, slightly 

 bent forwards and downwards across its middle, is, at first, the largest, 

 but grows relatively slower than the others. After a time, it is de- 

 veloped, on its dorsal aspect, into the corpora quadrigemina, which 

 form proportionally large masses, and are at first hollow a condition 

 which is permanent in Birds, but not in Mammalia. On the under-side, 

 the peduncles of the cerebrum are formed ; between these parts a 

 cavity remains, which ultimately shrinks into the small canal connect- 

 ing the fourth ventricle with the middle ventricles of the brain, named 

 the aqueduct of Sylvius. 



The anterior cerebral vesicle is, at first, more prominent laterally, 

 though smaller, than the middle one ; it is also at first smooth, but 

 soon exhibts a median sulcus, and grows far more rapidly than the 

 others, being destined to form the cerebrum. It soon bends directly 

 downwards. The portion immediately in front of the middle vesicle, 

 named the diencephalon, forms the two optic thalami, which originally 

 consist of a single hollow mass, but afterwards become solid ; they 

 are divided by a fissure, which remains as the third ventricle, and com- 

 municates behind with the Sylvian aqueduct. The pineal 'gland is 

 either an offshoot from the thalami, or it is derived from the pia mater. 

 The optic nerve also originates in a part of this vesicle. The pituitary 

 body, or hypophysis cerebri, in both its nervous part, and its posterior 

 thyroid-like portion, is said to arise from the base of the brain, or to 

 be in part developed from the pia mater. The prosencephalon, or por- 

 tion of the anterior vesicle in front of the optic thalami, gives origin 

 to the corpora striata, upon which the cerebral hemispheres, with the 

 corpus callosum, the fornix, and the ventricles, are rapidly evolved. 

 The corpora striata, and the hemispheres, are, it is said, at first sepa- 

 rated by a slight constriction. 



The hemispheres are developed from before backwards, leaving be- 

 tween them the cavity of the third and lateral ventricles, which, for a 

 time, open freely into the yet hollow corpora quadrigemina. Gradu- 

 ally the hemispheres overlap the optic thalami, and then, in the higher 

 Vertebrata, the corpora quadrigemina, and, lastly, even the cerebellum. 

 At first smooth on the surface, and composed of thin walls inclosing a 

 large cavity, the hemispheres, by degrees, become thicker, and marked 

 on the surface with the primary grooves or fissures, which subdivide 

 them into frontal, parietal, occipital temporal, and central lobes, and 

 afterwards with the secondary sulci between the convolutions the gray 

 matter on the surface also gradually becoming thicker. The cerebral 

 hemispheres, developed on each side of the middle line, are first con- 

 nected only at their base and anterior part, by rudimentary commis- 

 sural structures of nervous substance: these are the commencing pe- 

 duncles, which may be traced, as white bands, upwards from the cord, 

 the anterior commissure, and the rudimentary transverse commissure 

 or corpus callosum. But, as the hemispheres grow backwards, the 

 transverse commissural fibres of the latter extend in the same direc- 



