864 A N A T M Y [NEEVOUS primary optic vesicle. The stem of the prolongation, at first hollow, becomes solid, and forms the optic nerve and tract, whilst the expanded distal end forms the nervous elements of the retina. The antero-lateral part of the anterior cerebral vesicle is prolonged forward as two hollow processes, the hemisphere vesicles, which become the cerebral hemispheres, and are separated from each other by a median longitudinal fissure ; whilst the hollow in the interior of each forms the lateral ventricle. In the floor of each hemisphere yesicle is developed a large grey mass, striated with bundles of nerve fibres, the corpus striatum, which lies immediately in front and to the outer side of the optic thalamus; a curved band, the tcenia semicircularis, is formed along the junction of the thalamus with the corpus striatum, and at the inner and anterior end of this band, immediately behind the anterior pillars of the fornix, the two lateral ventricles become continuous with each other and with the third ventricle through ike foramen of Monro. Thereof and side walls of each hemisphere vesicle form a grey expansion or mantle, which is at first smooth, but subsequently becomes divided into lobes and convolutions, separated from each other by fissures. A deep gap or fissure now appears on the inner wall of each hemi sphere vesicle, and is bounded above by a longitudinal band of fibres, which, continuous anteriorly with the anterior pillar of the fornix, joins its fellow in the middle line to form the body of the fornix, and then again diverging from its fellow passes backwards, downwards, and forwards as One posterior pillar of the fornix or the tcenia hippocampi. A transverse arrangement of fibres then forms in each hemisphere vesicle, above the plane of the fornix, which, reaching the mesial plane, joins its fellow, connects the two hemi spheres together, and forms the corpus callos-um. In the hinder part this corpus rests upon the upper surface of the fornix, but more anteriorly it lies some distance above the fornix, and then bends down in front of it. Hence there is enclosed between the fornix and the antero-inferior part of the corpus callosum two thin layers of grey matter, one belonging to the inner surface of each hemisphere vesicle, and called the septum lucidum. Between these two layers is a narrow space, the fifth ventricle, which, unlike the other ventricles, is not derived from the cerebro-spinal tube, but is merely a portion of the longitudinal median fissure shut in by the develop ment of the corpus callosum and fornix. Each hemisphere vesicle also gives off from its anterior part a hollow process, which expands in front into a bulbous dilatation, named the olfactory bulb, from which the nerves of smell arise, whilst the stalk of the bulb solidifies and forms the olfactory peduncle. -*.** Owing to the great development of the mantle of the hemisphere vesicles in the human brain, and the size and complexity of the convolutions, these parts of the hemispheres grow forward so as to overlap the olfactory bulbs and peduncles, and backward, so as to conceal not only the corpora striata and optic thalami, but also the corpora quadrigemiua, crura cerebri, cerebellum, pons, and medulla oblongata, so that when the human brain is looked at from above, none of these structures can be seen. It is only when the brain is turned over and its base exposed that the medulla, pons, cerebellum, and crura are visible ; and before the corpora quadrigemina, optic thalami, and corpora striata can be exposed, portions of the hemi sphere substance must be removed. The great growth of the hemi sphere vesicle leads also to a great expansion of the central hollow or lateral ventricle, which is prolonged forwards, backwards, and downwards as the anterior, posterior, and descending cornua. In the descending cornu is a projection, the hippocamus major, along which the tenia nippocampi of the fornix runs ; in the posterior cornu is a smaller eminence, the hippocampus minor; and at the junction of these two cornua is a third elevation, the eminentia collateralis. Immediately investing the spinal cord and encephalon a vascular membrane, the pia mater, is developed, processes from which dip into the fissures between the two halves of the cord and between the cerebral convolutions. A broad band, the velum interpositum, which possesses two marginal fringes, the choroid plexuses, is admitted into the lateral ventricle through the gap or fissure in the inner wall of each hemisphere vesicle. This fissure is bounded above by the arch-shaped fornix, with its tacnia hippocampi. When the two hemispheres are in situ, and the two halves of the fornix are joined together to form the body of that structure, the fissure, with its contained velum interpositum, passes across the mesial plane from one hemisphere to the other, having the fornix and tcenia; for its^roof, and the optic thalami and corpora quadrigcmina for its floor ; it is known as the great transverse fissure of the cerebrum. . ,_. ; _. MEMBRANES OP BRAIN AND SPINAL CORD. These nerve centres are invested by three membranes or meninges, which lie between them and the bones that form the walls of the cranial cavity and spinal canal. The membranes are named dura mater, arachnoid mater, and pia mater. Dura mater. The most external membrane, named dura from its firmness, consists of a cranial and a spinal subdivision. The cranial part is in contact with the inner table of the cranial bones, and is adherent along the lines of the sutures and to the margins of the foramina, which transmit the nerves, more especially to the foramen mag num. It forms, therefore, for these bones an internal periosteum, and the nieningeal arteries which ramify in it are the nutrient arteries of the inner table. As the growth of bone is more active in infancy and youth than in the adult, the adhesion between the dura mater and the cranial bones is greater in early life than at maturity. From the inner surface of the dura mater strong bands pass into the cranial cavity, and form partitions between certain of the subdivisions of the brain. A vertical longitudinal mesial band, named, from its sickle shape, falx cerebri, dips between the two hemispheres of the cerebrum. A smaller sickle- shaped vertical mesial band, the falx cerebelli, attached to the internal occipital crest, passes between the two hemi spheres of the cerebellum. A large band arches forward in the horizontal plane of the cavity, from the transverse groove in the occipital bone to the cHnoid processes of tho sphenoid, and is attached laterally to the upper border of the petrous part of each temporal bone. It separates the cerebrum from the cerebellum, and, as it forms a tent- like covering for the latter, is named tentorium cerebelli. Along certain lines the cranial dura mater splits into two layers, to form tubular passages for tho transmission of venous blood. These passages are named the venous Hood sinuses of the dura mater, and they are lodged in the grooves on the inner surface of the skull referred to in the descrip tion of the cranial bones. Opening into these sinuses aro Fio. 68. Dura mafer and cranial sinuses. 1, Falx cerebri ; 2, tentorlnm. 3. S, superior longitudinal sinus; 4, lateral sinus; 5, internal jugular vein; 6, occi pital sinus ; 6 , torcular Uerophili ; 7, infe or longitudinal ainus; 8, veins of Galon ; 9 and 10, superior and inferior petr^al sinus; 11, cavernous sinus ; 12, circulnr sinus, which connects the two cavernous sinuses together ; 13, ophthal mic vein, from 15, the eyeball ; 14, crista galli of ethmoid bone. numerous veins, which convey from the brain the blood that has been circulating through it ; and two of these sinuses, called cavernous, which lie at the sides of the body of the sphenoid bone, receive the ophthalmic veins from the eyeballs situated in the orbital cavities. These blood sinuses pass usually from before backwards : a superior longitudinal along the upper border of the falx cerebri as far as the internal occipital protuberance; an inferior longitudinal along its lower border as far as the tentorium, where it joins the straight sinus, which passes back as far as the same protuberance. One or two small occipital sinuses, which lie in the falx cerebelli, also pass to join tho straight and longitudinal sinuses opposite this protuberance ; several currents of blood meet, therefore, at this spot, and as Herophilus supposed that a sort of whirlpool was formed in the blood, the name torcular Ilerophili has been used to express the meeting of these sinuses. From
the torcular the blood is drained away by two large
