THE MID-BRAIN. 



673 



divides into two strands, the peduncles of the pineal gland, or striae pine.alis ; these 

 extend on either side along the optic thalamus at the junction of its mesial and 

 upper surfaces (see page 671) to the anterior pillars of the fornix, with which they 

 blend. The two stalks join together at their posterior extremity, in front of the 

 pineal gland, forming a sort of festoon, and the base of the gland is connected to 

 their posterior margin at the point of junction. 



Structure. The pineal gland consists of a number of follicles, lined by epithe- 

 lium, and connected together by ingrowths of connective tissue. The follicles con- 

 tain a transparent viscid fluid and a quantity of sabulous matter named acervulus 

 cerebri, composed of phosphate and carbonate of lime, phosphate of magnesia and 

 ammonia, with a little animal matter. These concretions are almost constant in 

 their existence, and are present at all periods of life. They are found upon the 

 surface of the pineal body and occasionally upon its peduncles. 



Morphologically the pineal gland is regarded as the homologue of the structure 

 termed the pineal eye of the lizards. In these reptiles the epiphysis cerebri is 

 attached by an elongated stalk and projects through the parietal foramen. Its 

 extremity lies immediately under the epidermis, and on microscopic examination 

 presents, in a rudimentary fashion, structures similar to those found in the eyeball. 



III. The Mid-Brain. 



The mid-brain, or mesencephalon, is the constricted portion of the brain which 

 connects the pons Varolii with the inter-brain and hemispheres, and hence it is 

 frequently called the isthmus cerebri. It is developed from the third cerebral 

 vesicle, the cavity of which becomes the aqueduct of Sylvius. It comprises the 

 crura cerebri, the corpora quadrigemina, the geniculate bodies, and the iSylvian 

 aqueduct. Its direction is from before backward and downward. In front and 

 above it is continuous with the inter-brain ; below with the pons. Its two surfaces 

 are ventral and dorsal. They are free, but concealed : the ventral surface by the 

 apices of the temporal lobes which overlap it ; the dorsal, by the overhanging cere- 

 bral hemispheres. The ventral surface when exposed by drawing aside the 

 temporal lobes, is seen to consist of two cylindrical bundles of white matter which 

 emerge from the pons and diverge as they 

 pass forward and outward to enter the inner 

 and under part of either hemisphere. They 

 are the crura cerebri or cerebral peduncles, 

 and between them is a triangular area, 

 already described as part of the interpe- 

 duncular space (see page 656) ; near the point 

 of divergence of the crura the roots of the 

 third nerve are seen to emerge in several 

 bundles from a groove, the sulcus oculo- 

 motorius. The dorsal surface is not visible 

 until a considerable portion of the cerebral 

 hemispheres and other overlying structures 

 have been removed. It then presents four rounded eminences placed in pairs, two 

 in front and two behind, and separated from one another by a crucial depression. 

 These are termed the corpora or tubercula quadrigemina. The ventral and dorsal 

 surfaces meet on the* side of the mid-brain, and are separated from each other by a 

 furrow, the lateral groove, which runs from below upward and forward (Fig. 359). 



If a cross section be made through the mesencephalon (Fig. 360) it will be seen 

 that each lateral half is divided into two unequal portions by a lamina of deeply 

 pigmented gray matter, named the substantia nigra ; of these the postero-superior 

 portion is named the tegmentum, and the antero-inferior the crusta or pes. The 

 substantia nigra is curved on section with its concavity upward, and extends from 

 the lateral groove externally to the oculomotor sulcus internally. The two crustse 

 are quite separate from one another, but the two halves of the tegmentum are joined 



43 



C. Quadrigemina. 

 Aqueduct. 



Interpetluncular space. 

 FIG. 360. Transverse section of the mid-brain. 



