Page:The American Cyclopædia (1879) Volume III.djvu/194

 188 BRAIN planes ; it is freely supplied with blood vessels, and is perforated for the passage of nerves, and, according to Arnold and Pappenheim, has some branches between its own laminee. It forms the internal periosteum of the skull, and is closely applied to the cranial bones, and in some places firmly adherent, especially in youth and old age. From it processes are Fio. 8. The Brain enclosed In the Dura Mater. given off, which serve as partitions between the cerebrum and cerebellum behind, and be- tween the cerebral and cerebellar hemispheres ; these processes are the falx cerebri, which sep- arates the great hemispheres, extending on the median line from the forehead to the occiput, along the sagittal suture; it is falciform in shape, its lower border concave and correspond- ing to the convexity of the corpus callosvm, and its upper border enclosing the great longi- tudinal sinus ; narrow in front, and deep be- hind, having the inferior longitudinal sinus along its posterior border. The tentorium cere- belli extends horizontally between the posterior cerebral lobes and the cerebellum ; it is attach- ed to the falx cerebri, and to the occipital and petrous portion of the temporal bones along the groove for the lateral sinus ; in the cats and some other leaping animals, this membrane is partially replaced by bone, doubtless to prevent injury from sudden shocks. Between the lobes of the cerebellum descends vertically from the tentorium the falx cerebelli, containing the oc- cipital sinuses. Next to the dura mater, which also furnishes sheaths for the nerves and ves- sels at their origins, lies' the arachnoid, the serous membrane of the cerebro-spinal cavity ; it consists of two layers, the outer one closely adherent to the dura mater, and the inner one loosely to the pia mater; the space between th two layers is the arachnoid cavity, and that between it and the pia mater, the sub-arach- noid cavity; resembling other serous mem- branes, the arachnoid is liable to become in- flamed, with the effusion of fluid into one or both of the above cavities, especially toward the base of the brain. The sub-arachnoid space is filled with what is called the " cerebro-spinal fluid," varying in quantity from two or three ounces in the .healthy adult condition to ten or twelve ounces in old age. It keeps during life the opposed arachnoid surfaces in contact ; it is most abundant where the brain has shrunk either from disease or advanced age. From the experiments of Magendie it appears that its presence is necessary for the healthy action of the nervous centres ; when removed, it is quick- ly formed again ; it is a limpid, alkaline fluid, doubtless secreted by the pia mater, and affords mechanical protection to the brain and spinal cord by the interposition of its yielding medium between them and the bony parietes which surround them ; its accumulation at the base of the brain is highly favorable for the protection of the large nerves and vessels there situated. This fluid exists in an increased quantity in the brains of idiots ; and whenever the cranial or spinal walls are deficient, as for instance in spina bifida, an accumulation of the fluid becomes prominent at the part, thereby protecting the nervous substance. The third membrane imme- diately investing the brain is the pia mater, composed of white fibrous tissue and blood ves- sels ; in the skull it is very delicate and very vascular ; it adheres to the surface of the cere- bral and cerebellar hemispheres, and sends in- numerable minute vessels to their substance ; it sinks into the fissures and sulci, and pene- trates into the ventricles, forming the choroid plexuses and the velum interpositum ; its minute ramifications are sometimes incrusted with sandy particles, consisting principally of phos- phate of lime. The pia mater is the medium of nutrition to the nervous substance and to the arachnoid ; and hence any inflammation of these membranes would be communicated to the superficial gray matter of the brain, the seat of its physiological activity. Along each side of the longitudinal sinus it is common to find a series of depressions in the dura mater ; these are due to the presence of whitish granules, called Pacchionian glands, from their first describer, of an albuminous material, aris- ing probably from a deposit of granular lymph among the vessels of the pia mater ; they are found principally along the edge of the great longitudinal fissure of the hemispheres, pushing the arachnoid before them, and even projecting into the sinus. They are generally considered morbid structures, and the result of local irrita- tion of a chronic character; if the products of disease, they do not seem to interfere in the least with the functions of the brain. The brain of the adult human male, comprising the whole contents of the cranium us far as the occipital foramen, will average in weight about 50 oz. ; that of the adult female, about 45 oz. ; the maximum weight of the healthy organ is about 64 oz., and the minimum about 31 oz. ; in cases of idiocy it has been found weighing only 20 oz. According to Bourgery, if the brain be divided into 204 parts, the cerebral hemispheres would weigh 170, the cerebellum 21, and the medulla and sensory ganglia 13 ; on the same scale, the spinal cord would weigh 7. In proportion to the body's weight, the brain of man would weigh -% part ; in the average of mammalia this proportion would be T ^B ; in