800 



NEUROLOGY 





fossa and presents a bluish-gray area, the locus cseruleus, which owes its color 

 to an underlying patch of deeply pigmented nerve cells, termed the substantia 

 ferruginea. At the level of the colliculus facialis the sulcus limitans widens into 

 a flattened depression, the superior fovea, and in the inferior part of the fossa appears 

 as a distinct dimple, the inferior fovea. Lateral to the fovese is a rounded elevation 

 named the area acustica, which extends into the lateral recess and there forms a 

 feebly marked swelling, the tuberculum acusticum. Winding around the inferior 

 peduncle and crossing the area acustica and the medial eminence are a number of 

 white strands, the striae medullares, which form a portion of the cochlear division of 

 the acoustic nerve and disappear into the median sulcus. Below the inferior fovea, 

 and between the trigonum hypoglossi and the low r er part of the area acustica is a 

 triangular dark field, the ala cinerea, which corresponds to the sensory nucleus 

 of the vagus and glossopharyngeal nerves. The lower end of the ala cinerea is 

 crossed by a narrow translucent ridge, the funiculus separans, and between this 

 funiculus and the clava, is a small tongue-shaped area, the area postrema. On 

 section it is seen that the funiculus separans is formed by a strip of thickened 

 ependyma, and the area postrema by loose, highly vascular, neuroglial tissue con- 

 taining nerve cells of moderate size. 



THE MID-BRAIN OR MESENCEPHALON. 



The mid-brain or mesencephalon (Fig. 681) is the short, constricted portion which 

 connects the pons and cerebellum with the thalamencephalon and cerebral hemi- 

 spheres. It is directed upward and for- 

 ward, and consists of (1) a ventro- 

 lateral portion, composed of a pair of 

 cylindrical bodies, named the cerebral 

 peduncles; (2) a dorsal portion, consist- 

 ing of four rounded eminences, named 

 the corpora quadrigemina; and (3) an 

 intervening passage or tunnel, the cere- 

 bral aqueduct, which represents the 

 original cavity of the mid-brain and 

 connects the third with the fourth ven- 

 tricle (Fig. 710). 



The cerebral peduncles (peduncidus 

 cerebri; cms cerebri) are two cylindrical 

 masses situated at the base of the&rain, 

 and largely hidden by the temporal 

 lobes of the cerebrum, which must be 

 drawn aside or removed in order to 

 expose them. They emerge from the 

 upper surface of the pons, one on either 

 side of the middle line, and, diverging 

 as they pass upward and forward, dis- 

 appear into the substance of the cere- 

 bral hemispheres. The depressed area 

 between the crura is termed the inter- 

 peduncular fossa, and consists of a layer 

 of grayish substance, the posterior 



perforated substance, which is pierced by small apertures for the transmission of 

 bloodvessels; its lower part lies on the ventral aspect of the medial portions of the 

 tegmenta, and contains a nucleus named the interpeduncular ganglion (page 802) ; 

 its upper part assists in forming the floor of the third ventricle. The ventral sur- 



FIG. 710. Coronal section through mid-brain. (Sche- 

 matic.) (Testut.) 1. Corpora quadrigemina. 2. Cere- 

 bral aqueduct. 3. Central gray stratum. 4. Interpedun- 

 cular space. 5. Sulcus lateralis. ti. Substantia nigra. 7. 

 Red nucleus of tcgmentum. 8. Oculomotor nerve, with S', 

 its nucleus of origin, a. Lemniscus (in blue) with a' the 

 medial lemniscus and a" the lateral lemniscus. b. 

 Medial longitudinal fasciculus, c. Raph6. d. Temporo- 

 pontine fibers, e. Portion of medial lemniscus, which runs 

 to the lentiform nucleus and insula. /. Cerebrospinal 

 fibers, g, Frontopontine fibers. 



