•1594 A MANUAL OF ANATOMY 



Monro, on its internal surface. The anterior pillars of the fornix and the 

 anterior commissure are to be again looked to. The two foramina of Monro 

 are to receive careful consideration, each being situated between the anterior 

 pillar of the fornix in front and the anterior tubercle of the optic thalamus 

 behind. It will be evident to the dissectors that the third ventricle com- 

 municates freely with the two lateral ventricles by means of the foramina 

 of Monro. If attention is now directed to the posterior part of the third 

 ventricle, the dissectors will see the upper opening of the aqueduct of Sylvius, 

 by which aqueduct the third ventricle communicates with the fourth ventricle. 

 Above the upper opening of this aqueduct, and beneath the stalk of the 

 pineal body, the dissectors will have no difficulty in seeing the posterior 

 commissure. 



Pineal Body. — This body is situated upon the mesencephalon at the pos- 

 terior part of the third ventricle, and should receive careful attention. In 

 connection with it the following parts are to be noted : (i) the stalk, peduncle, 

 or habenula, the dorsal portion of which forms the stria pinealis, or medul- 

 laris, already referred to in connection with the internal surface of the optic 

 thalamus ; (2) the pineal recess, which passes backwards above the posterior 

 commissure into the stalk, or habenula, of the pineal body (but this recess, 

 like the optic, infundibular, and suprapineal recesses, can Ijest be studied 

 from plaster, or metal, casts of a mesial section through the brain) ; and 

 (3) the trigonum habenula;, which will come more fully into view presently. 



Mesencephalon, or Mid-brain. — The mid-brain may be exposed from above 

 by displacing backwards the anterior part of the cerebellum until the corpora 

 quadrigemina are sufficiently exposed. This should be done with the greatest 

 care, in order not to injure (i) the valve of Vieussens, immediately below the 

 lower pair of quadrigeminal bodies, with the superior peduncle of the cere- 

 bellum on either side ; and (2) the slender fourth cranial nerves, which issue 

 from the valve of Vieussens, and thereafter turn round the crura cerebri, 

 one on either side. 



The corpora quadrigemina now fall to be studied. They will easily be 

 recognised as four rounded eminences, arranged in pairs, upper and lower, 

 and the crucial groove which separates them from one another will attract 

 attention. The internal geniculate body will be recognised as a small oval 

 mass on the lateral aspect of the mesencephalon, under cover of the pulvinar 

 of the optic thalamus. The superior and inferior brachia are to be examined, 

 also upon the lateral aspect of the mesencephalon. A. reference to the text 

 may guide the dissectors in their examination of these brachia. 



At this stage the dissectors are advised to read the description of the optic 

 tract and its connections as given in the text. They are to make themselves 

 familiar with the mesial and lateral roots of the optic tract, and are to note very 

 carefully that (i) the pulvinar of the optic thalamus, (2) the external genicu- 

 late body, and (3) the upper quadrigeminal body constitute the lower visual 

 centres, as distinguished from the higher, or cortical, visual centre in the 

 cortex of the cuneate and lingual gyri close to the calcarine fissure on the 

 mesial surface of the occipital lobe. The trigonum habenulae, previously 

 referred to, will now be fully exposed. 



To complete the dissection of the mesencephalon the crura cerebri are to 

 receive the most scrupulous attention at this stage, and it will be necessary 

 to carefully reverse the preparation for this purpose — at all events, for re- 

 vision. 



The mesencephalon is now to be divided transversely between the upper 

 and lower pairs of quadrigeminal bodies. This procedure will enable the 

 dissectors to remove the following parts, namely, the pons Varolii, cerebellum, 

 and medulla oblongata. On examining the cut surface the aqueduct of 

 Sylvius will be seen in section. The division of each crus cerebri into two 

 by the crescentic mass of grey matter, called the substantia nigra, may be 

 evident, the upper or dorsal part being the tegmentum, and the lower or 

 ventral part the crusta or pes. The oculo-motor sulcus on the inner surface 

 of the crus for the third or oculo-motor nerve is to be noted, and on the outer 

 surface will be seen the lateral sulcus. The description given of the crus 

 cerebri in the text ia to be carefully read. 



