GENERAL FEATURES 357 



the pedunculi, by a canal, termed the aquaeductus cerebri, 

 which connects the third ventricle, in the diencephalon, with 

 the fourth ventricle, in the hind-brain. 



Still lower and more posteriorly that is, below and behind 

 the mid-brain is the rhombencephalon or hind-brain. It also 

 is separable into dorsal and ventral portions, and between 

 them is the cavity of the hind-brain, called \hefourth ventricle 

 (Fig. 136). The dorsal portion is the cerebellum ; it lies im- 

 mediately below the posterior parts of the cerebral hemi- 

 spheres and above and behind the fourth ventricle. The 

 ventral part of the hind-brain consists of an upper part, called 

 the pons, which is continuous with the pedunculi of the mid- 

 brain, and a lower part, called the medulla oblongata, which is 

 continuous, below, with the spinal medulla. 



When the brain was removed, the dissector noticed that 

 the cerebral hemispheres occupied the anterior and middle 

 fossae of the cranium and that, more posteriorly, they lay on 

 the tentorium cerebelli a fold of dura mater which separated 

 them from the hind-brain (Figs. 32, 35). The dissector noted 

 also, after the removal of the tentorium cerebelli, that the 

 hind-brain occupied the posterior fossa of the cranium, 

 and that the mid- brain passed from the posterior fossa 

 to the middle fossa through an oval notch, the incisura 

 tentorii. 



The brain is surrounded by three membranes the dura 

 mater, the arachnoid, and the pia mater ; and between the 

 arachnoid and the pia mater lie the main trunks of the blood 

 vessels of the brain. 



The dura mater was examined during and after the removal 

 of the brain from the cranial cavity (pp. 99-102); but, 

 before the arachnoid, the pia mater, and the blood vessels 

 which lie between them, are studied, the dissector must be 

 acquainted, not only with the main subdivisions of the brain, 

 but he must have also a good knowledge (i) of the names of 

 the fissures and sulci of the cerebral hemispheres, (2) of the 

 names and positions of their various borders, surfaces, and 

 lobes, and (3) of the position of the cerebral nerves. He 

 should, therefore, obtain a brain from which the membranes 

 have been detached and in which the mid-brain has been 

 divided horizontally so that the lower part of the mid-brain 

 and the hind-brain can be removed. He should obtain also 

 a cerebral hemisphere which has been separated from its 

 in 23 & 



