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Neurons are nourished by the lymph which bathes them, and the lymph is derived from the blood circulating through the vessels. The central nervous system and the sensory and sympathetic ganglia are alike in the lack of lymphatic vessels. In every case, in mammals, the nerve cell lies surrounded by a pericellular lymph space, and it is thus bathed on every side by the nutrient lymph. The arterial supply of the cord is very plentiful. The two posterior spinal arteries and the anterior spinal artery extend through the length of the cord, and receive with each nerve root branches from the cervical, lumbar or intercostal arteries. The anastomosis of these arteries is complex and efficient. It is not conceivable that any vertebral lesion short of actual crushing could bring about a diminution of the arterial supply to the cord through direct pressure. The effects upon the spinal circulation indirectly produced through vaso-motor impulses may be discussed at a later time. From these longitudinal arteries and their anastomotic branches arise the arteries which enter the cord. These smaller arteries are of two classes, centripetal and centrifugal. Both sets are composed of terminal arteries, thus no anastomosis is found within the cord; there is, however, a certain amount of overlapping of the areas of distribution of the vessels. The centrifugal arteries arise chiefly from the anterior artery. The branches enter the cord by way of the anterior fissure. They break up into fine branches which supply the gray matter, for the most part, though a few small branches supply the white matter in the immediate neighborhood of the centrifugal system. From the posterior arteries arise a few branches of the centrifugal system also. These enter the posterior median septum and are distributed to the posterior white matter and to the central gray matter. The centripetal arteries arise from both the anterior and the posterior longitudinal arteries. The branches from these pass around the cord, break up into finer terminal branches, and enter the cord at right angles to its surface. These finer terminals supply the white matter of the anterior and lateral funiculi, and in part the posterior funiculi. The white matter is less richly supplied with arteries than the gray. This double arterial supply presents no explanation. That some clinical significance is attached to this vascular supply appears apparent when one remembers the frequency with which lesions of the gray matter are found without any apparent disturbance of the gray matter, and how often the gray matter is diseased without there being any recognizable lesion of the white matter. This clinical phenomenon may, of course, be interpreted otherwise than by artery supply. Both centrifugal and centripetal arteries are supplied with vaso-motor nerves, which seem to be derived from the sympathetic ganglia of the same or adjoining segments. It is not known whether the centrifugal and centripetal arteries have identical vaso-motor innervation. It is very evident that the disturbances which interfere with vaso-motor impulses to the viscera, skin, etc., may also interfere with the vaso-motor impulses to the corresponding spinal areas. The veins of the cord are very plentiful. The fissural veins drain the areas of the centrifugal arteries, while the root veins and the smaller veins emerging from the surfaces drain the areas supplied by the centripetal arteries. All of these veins unite in forming the spinal venous plexus. This plexus is made up of freely anastomosing veins. They have been rather vaguely divided into six longitudinal veins, but the anastomosis is so rich and the variations are so many that it is difficult to trace them as exactly six through the extent of the cord. This venous plexus is drained into the vertebral, lumbar, sacral, and intercostals veins. The veins of the central nervous system, including the spinal cord, are without valves. The vessels of the medulla and pons resemble those of the cord. The vertebral arteries unite to form the basilar. From the two vertebrals below their union, from the basilar, and from the anterior and the posterior spinal arteries the arteries of the medulla and pons are derived. They may be divided into three sets. These sets are composed of terminal arteries, though their areas of distribution overlap to a certain extent. The median arteries pass through the substance of the medulla and pons, and divide into fine branches distributed to the nuclei of the cranial nerves. The beginnings of the nerve roots also are partly supplied by the median arteries. The root arteries enter the medulla and pons with the nerve roots. They include central and peripheral branches. The former are distributed with the median arteries to the nuclei of the cranial nerves, the latter are distributed to the white matter in the neighboring areas. The lateral medullary and pontine arteries pass around the external surface, and are distributed to the olives, the restiform bodies, and the pyramids. Branches from the inferior cerebellar arteries are distributed to the restiform bodies and the formatio reticularis. The veins of the medulla and pons follow the arterial arrangement for the most part, and are drained into the superior and inferior petrosal sinuses or into the basilar plexus. The cerebellum is furnished by three arteries on each side. The superior cerebellar is derived from the basilar. It is distributed to the superior surface. A small portion of the posterior border escapes from the supply of the superior cerebellar, which supplies also the geniculate bodies, the quadrigeminates, the tela choroidea, a part of the third ventricle, and sends some branches to the posterior surface of the pons. There are two inferior cerebellar arteries, the anterior and the posterior. The anterior is from the basilar, also. It is distributed to the inferior anterior aspect of the cerebellum and the anterior inferior border. The posterior inferior artery is given off from the vertebral artery before it unites with its fellow to form the basilar. The posterior inferior cerebellar artery supplies the medial part of the hemisphere, the inferior part of the vermis; it communicates with the superior cerebellar artery upon the posterior superior surface. There is a fairly free anatomosis among the larger branches of the three cerebella arteries. The cerbellar veins are called by the same names as the arteries. The internal cerebellar veins empty into the superior and inferior veins. The superior veins empty into the great cerebral veins, for the most part, but several smaller veins associated with these empty into the superior petrosal sinus. The inferior veins run upward into the transverse or straight sinus, or into the inferior petrosal and occipital sinuses. The circulation through the brain presents many peculiarities. The vertebral arteries unite to form the basilar, the basilar unites with the internal carotids by way of the posterior communicating arteries, the anterior cerebral, from the internal carotid passes forward, and the left is united with the right by means of the anterior communicating artery. Thus a complete circle is formed which surrounds the brain stem. From this arterial circle (circle of Willis, in the old naming) the arteries are derived which supply the brain, and this circle is practically the last anastomosis of these arteries. There is a certain amount of overlapping in their areas of distribution, but no anastomosis. The brain is supplied with blood by two distinct systems, the ganglionic system and the cortical system. There is very little overlapping in the areas of distribution of the two systems. Between them lies an area of the brain which is poorly supplied with blood, and this area is subject to the diseases of malnutirion in the aged or in those in whom any cause of enfeebled cerebral circulation is found. The ganglionic arteries are small. They are distributed for the most part in an efferent manner, and supply the base of the brain and the basal ganglia. They include the antero-median and the postero-median, the right and left antero-lateral and the right and left postero-lateral. Thus there are six chief arteries which, with their branches, compose the ganglionic system. The antero-median is a branch of the anterior cerebral. It supplies the region of the optic chiasma, the rostrum of the corpus callosum and the head of the caudate nucleus. The antero-lateral arteries are derived from the middle cerebral. They are distributed to the optic thalamus, the corpus striatum and the internal capsule. One of the branches, the lenticulo-striae, is of interest because of its liability to hemorrhage in elderly people or in those subject to artero-sclerosis. It is called the “artery of cerebral hemorrhage” for this reason. Since it supplies the striatum and the internal capsule, the paralysis which results from this hemorrhage is very widespread and is contra-lateral. The postero-lateral arteries are from the posterior cerebral. They supply the posterior parts of the thalamus, the geniculates, the quadrigeminates, and the pineal body. These areas overlap the area of distribution of the superior cerebellar artery to a certain extent. The postero-median artery is derived fro the posterior cerebral and the posterior communicating arteries. It supplies the medial parts of the thalamus and the third ventricle, the cerebral peduncles and the space between the peduncles. There are two choroidal arteries, an anterior and a posterior. The anterior arises from the internal carotid and follows the optic tract to the inferior end of the choroidal fissure. It supplies the choroids plexus of the inferior horn of the lateral ventricle, and gives collateral branches to the optic tract and to the hippocampus and hippocampal gyrus, the dentate, crus of the fornix, and posterior part of the internal capsule. The posterior choroidal include two or more arteries on each side. These arise from the posterior cerebral and they pass forward in the transverse and choroidal fissues to be distributed to the choroid plexuses of the third and lateral ventricles. The cortical arteries are those which supply the cortex of the hemispheres. These arteries give off a few branches which supply the ganglionic system, but after these few branches are lost no further relationship between the two systems is evident. Not only is there no anastomosis between the two systems, but there is almost no overlapping of their areas of distribution. The anterior cerebral artery arises from the internal carotid. It passes forward toward the longitudinal fissure, and the right anterior cerebral is joined to the left anterior cerebral by a very short anastomotic branch, the anterior communicating artery. The antero-median ganglionic artery arises from the anterior cerebral. There are four branches of the cortical system from the anterior cerebral, the anterior, middle and posterior internal frontal arteries, and the internal orbital artery. The anterior internal frontal supplies the anterior part of the gyrus cinguli and superior frontal gyrus on the medial aspect of the brain, and the superior and middle frontal gyri on its outer aspect. The middle internal frontal artery supplies the middle part of the gyrus cinguli, the paracentral lobule, the upper part of the superior frontal and the precentral and postcentral gyri. The posterior internal frontal artery supplies the corpus callosum, the posterior part of the gyrus cinguli, of the paracentral lobule, the precuneus, and the superior parietal lobule. The internal orbital artery supplies orbital gyri, the optic chiasma, the olfactory bulb, tract, the roots of the olfactory tract, and the parolfactory area. These four arteries are distributed to the cortical area as far posteriorly as the occipito-parietal sulcus. The middle cerebral artery is the largest branch of the internal carotid, and it receives the direct current of its blood stream. Since the left internal carotid receives the most direct current of the blood stream from the ascending aorta, the middle cerebral artery is more apt to receive foreign substances carried in the blood stream than in any other artery in the body. For this reason the area of distribution of the left middle cerebral artery, and especially its ascending frontal branch, is apt to suffer from embolism. Since this area includes the kinesthetic area of the cortex, the embolus in this region results in a contra-lateral paralysis. The middle cerebral artery (Sylvian artery) runs in the lateral fissure of the cerebrum (Sylvian fissures). It gives off the antero-lateral artery, and opposite the insula (Island of Reil) breaks up into six terminal branches. The lateral orbital (inferior external frontal) is distributed to the anterior and posterior orbital gyri and the inferior frontal gyrus). The ascending frontal includes two chief branches, which follow the central sulcus (fissure of Rolando) as far as the precentral sulcus, then follows this to supply the anterior central gyrus and a part of the middle frontal gyrus. The ascending parietal is distributed to the posterior central gyrus (ascending parietal convolution) and the neighboring superior and inferior parietal lobules. The parieto-temporal arteries include three chief branches. Two of these are distributed to the superior, middle and the upper part of the inferior temporal gyri. The posterior branch follows the posterior ramus of the lateral cerebral fissure (fissure of Sylvius) to its termination. It then divides into two branches, one of which passes upward to the supra-marginal, post-parietal and angular gyri, while the other supplies the posterior part of the temporal lobe. The right and left posterior cerebral arteries are formed by the division of the basilar arteries. They are joined to the internal carotids by the posterior communicating arteries. The posterior cerebral arteries give off the postero-median and the postero-lateral ganglionic arteries, and two or more posterior choroidal arteries. Its cortical branches are three, the occipito-parietal, the calcarine, and the temporal. The occipito-parietal supplies the cuneate and lingual gyri, the lateral and superior gyri of the occipital lobe. The temporal branches are three in number, the anterior, middle and posterior temporal branches. These are distributed to the gyrus hippocampus, the fusiform gyrus, and the inferior temporal gyrus. The arteries described lie upon the surface of the brain. From them arise many very fine terminal arteries which penetrate the brain and are distributed to its gray matter. These arteries do not penetrate very deeply into the brain, and the white matter receives most of its comparatively scant blood supply from the ganglionic system. The arteries of the brain are now known to receive vaso-motor nerves from the sympathetic system. (Fig. 21.) The fibers arise as white rami from the upper thoracic segments of the cord, and pass with the sympathetic chain to the superior cervical ganglion. Here the medullated fibers terminate by entering into the formation of the superior cervical pericellular baskets. The axons of the sympathetic cells pass by way of the carotid plexus to the arteries of their distribution. Certain neuroglia cells, called “podasteroids,” seem to be concerned in the nutrition of the brain. These cells lie along the pericellular lymph spaces, partly inclosing them, and partly supporting the blood vessels, especially the capillaries. These podasteroids send prolongations to the walls of the vessels, and rest upon them by a footlike expansion. (Figs. 22, 23.) Under certain abnormal conditions, such as hemorrhage, poisoning, etc., the podasteroids are found to be swollen and the footlike expansions are filled with debris, bits of blood clot, etc. It is, however, probable that the most efficient regulation of the cerebral circulation is through changes in the general blood pressure. The brain is inclosed in the dense skull, the brain itself is almost fluid, the lymph and blood are fluids, and therefore the cranial contents are practically noncompressible. Vaso-constriction or vaso-dilaton of the cerebral vessels must, therefore, be less efficient in modifying the cerebral circulation than are changes in the systemic blood pressure. This is most efficiently modified by variations in the splanchnic circulation, by variations in the heart beat, and by variations in the pulmonic blood supply. The veins of the brain have several peculiarities. They have no valves. They are enclosed in bony channels called sinuses. They are surrounded by perivascular lymph spaces. They are very large in proportion to the arteries whose blood they receive. They have very free anastomoses. The veins empty at a recurrent angle. The veins of the skull have many pouchlike diversions. Enissary veins through the skull assist in preserving a constant intracranial pressure in the presence of variations in the systemic pressure and of overfilling of the cerebral vessels. The sinuses are formed by folds of the dura mater. They are lined with endothelium continuous with that of the vascular system as a whole. The sinuses are as follows: The superior sagittal sinus (superior longitudinal sinus) extends from the foramen caecum to the confluens sinuum (torcular Herophili). It lies in the triangle formed by the inner layer of the dura mater as it tips into the longitudinal sinus, and the outer layer of the dura as it remains attached to the skull. This sinus receives the blood from the superior cerebral veins, the diploe, the dura mater, and, in its posterior portion, from the pericranial tissues. The inferior sagittal sinus (inferior longitudinal sinus) lies in the fold made by the dipping dura as it is folded back upon itself at the inferior or free edge of the falx cerebri. It terminates in the straight sinus. It receives the veins of the falx cerebri, and sometimes a few from the mesial surface of the brain. The straight sinus lies in the triangle formed by the two layers of the falx and the tentorium cerebelli. It terminates at the confluens sinuum (torcular Herophili). The occipital sinus lies in the angle of the attached margin of the falx cerebelli as the superior sagittal sinus lies in the angle of the falx cerebri. It communicates with the spinal veins and terminates in the confluens sinuum. The transvers sinuses arise from the confluens, and pass outward in the triangle formed by the tentorial fold of the dura and the skull. At the base of the petrous portion of the temporal bone the superior petrosal sinus empties into the lateral sinus. It then communicates with the occipital sinus, and unites with the inferior petrosal sinus to form the internal jugular vein. The cavernous sinus lies along the side of the sella tursica. It is a continuation of the ophthalmic veins, and receives the blood of the spheno-parietal sinus, a small sinus which follows the posterior border of the lesser wing of the sphenoid bone. The right and left cavernous sinuses are joined by the anterior and posterior sinus intercavernouses. The cavernous sinus terminates by dividing into the superior and the inferior petrosal sinuses. The termination of the superior petrosal in the transverse sinus has already been mentioned. The right and left inferior petrosal sinuses are joined by the basilar plexus of veins, and unite with the lateral sinus to form the internal jugular veins. |