Page:The American Cyclopædia (1879) Volume III.djvu/199

 BRAIN 193 portions, one belonging to each hemisphere. Of this complicated structure it can only be said here that it begins at the optic thalaraus, proceeding anteriorly to the base of the brain, where it turns suddenly upward and forward, thus forming the corpora albicantia or mammil- laria, and, ascending toward the corpus callo- sum, passes along its lower surface, spreading laterally into what is called its " body ; " it again descends at the back part of the brain, some of its fibres going to the posterior lobes, and others crossing the hippocampi to be connected with the middle lobes; it thus connects those parts of the convolutions of one side beneath the corpus callosum. Other probably commissural structures are the pong Tarini, in the angle formed by the diver- gence of the crura cerebri, and probably con- necting their fibres ; the innermost fibres of the optic tracts are evidently commissural, connect- ing the quadrigeminal and geniculate bodies of opposite sides; the tuber cinereum is a layer of gray matter, containing many nerve tubes, extending from the mammillary bodies to the posterior curves of the corpus callosum, and forming intimate connections with the for- nix, optic tracts and thalami, and the pituitary body. The fibres connecting the cerebrum with the cerebellum are very few ; the princi- pal, if not the only ones, are those going to the testes from the cerebellum. An organ of such importance as the brain must require a large supply of blood ; this is afforded by the great carotid arteries, coming directly from the aorta, and the vertebral branches of the subclavians, which meet at the base of the organ, freely communicating with each other. These arte- ries, coming so directly from the aortic arch, are prevented from injuring the delicate brain: 1, by the blood ascending against gravity; 2, by the curving of the vessels like the letter S before they enter the cranium, thus scatter- ing the force of the stream in different direc- tions; 3, by the minute subdivision of the ves- sels before they enter the cerebral substance. The impure blood returns through the jugular veins ; hence any compression of these vessels by a tight neck stock, or the like, impedes the whole cerebral circulation, causing, it may be, dangerous congestions. If the blood could he shut off completely from the brain, death wouW instantly ensue ; and to prevent the possibility of this accident, the vertebral arteries are pro- tected by the bony canals of the cervical trans- verse vertebral processes from all danger of compression or ordinary injuries. The brains of persons who have died by hanging always exhibit great venous congestion. The veins of the dura mater are quite remarkable by pour- ing their contents into the large canals enclosed between its layers, the sinuses; these, unlike ordinary veins, cannot be distended beyond a certain point, and, as they all empty their blood into the internal jugular vein, any ob- struction in this or in the superior vena cava very speedily produces an uncomfortable dis- tention in the head. These sinuses are the su- perior longitudinal, corresponding to the supe- rior margin of the falx cerebri, commencing near the root of the nose (crista gallt) and ter- minating in the cavity called torcular HeropJii- li, near the internal occipital protuberance ; the inferior longitudinal sinus runs along the lower border of the falx, and ends in the straight sinus, which runs in the median line at the meeting of the falx and the tentorium, and opens into the torcular ; the lateral sinuses ex- tend from the torcular downward and forward to the jugular veins. This is the largest sinus, and its canal is deeply hollowed out of the oc- cipital and temporal bones ; that of the right side is generally the larger. The sinuses are sometimes the seat of dangerous inflammation. Between the layers of the falx cerebelli are the opcipital sinuses, opening into the torcular ; the petrosal sinuses, running along the petrous por- tion of the temporal bone, open into the lateral sinuses ; the cavernous sinuses are on each side of the sella turcica, communicating with the petrosal by the transverse sinus, and with each other by the circular sinus. From this arrange- ment of the sinuses, communicating freely with the external vessels, may be understood the signal advantages of local depletion in relieving vascular fulness within the head, and also the utility of cold applications for similar purposes. There are 12 pairs of nerves belonging strict- ly to the brain, which differ from spinal nerves only in their distribution and in coming through openings in the skull instead of between the vertebras ; all, except the first, proceed from the spinal cord itself, or from its prolongation in the brain (the medulla oblongata). These nerves are : 1, the olfactory, or nerve of smell ; 2, the optic, or nerve of vision ; 3, motores oculorum, the motor nerves of all the muscles of the orbit, except the superior oblique and the external rectus; 4, the pathetici ; 5, the trifacial or tri- geminus, the general sensory nerve of the head and face; 6, the abducentes oculorum; 7, the facial, the motor nerve of the face ; 8, the audi- tory, or nerve of hearing ; 9, the glossopharyn- geal, supplying part of the sensory fibres of the tongue, and presiding over the movements of swallowing; 10, the pneumogastric, or par vagum; 11, the spinal accessory (the last two combined presiding over the functions of res- piration and phonation) ; and 12, the hypoglos- sal, the motor nerve of the tongue. Philosoph- ical anatomists have combined these nerves in various ways, separating the three nerves of special sense, and classifying the others in groups resembling spinal nerves, with their anterior motor, and their posterior sensitive roots. As the skull may be considered as com- posed of three cranial vertebra?, we have the olfactory, optic, and auditory special nerves, making their way out through the three ver- tebrae which may be called by the same name, corresponding to the three primary vesicles which are developed into the brain. Of the intervertebral, analogous to spinal nerves, the