A nerve or a nerve and one or more fasciculi in the cerebro spinal axis, linked together end to end, make up a conduction path. The successive orders of neurones in the path are linked by contact (synapsis). In an efferent path the axones of one order touch the dendrites and cell-bodies of the next order below them; e.g., the fibers of the pyramidal tract (third order) form contacts with the intercalated neurones in the cord (second order) and these with the motor nerve neurones (first order). While the axones touch the dendritic side of the neurones next in order above them in an afferent conduction path, in the fol lowing order: the sensory fibers of a spinal nerve (as a sacral), the dorsal root of that nerve and the fasciculus gracilis (first order); the medial fillet neurones (second order), and the thal amo-cortical neurones (third order). Afferent and efferent conduction paths are very numerous. To be fully functional they must be linked together into cycles or arcs; so that, for instance, the afferent path from a muscle, and the joints moved by it, is connected into a circuit with the efferent path which bears the motor impulses to that muscle. Such linking of affer ent and efferent paths occurs at various levels in the nervous system. In the spinal cord and cranial nerve nuclei the simplest reflex arcs are completed. Longer and more compli cated arcs are formed by associations in the medulla, in the basal nuclei of the cerebrum, in the nuclei and cortex of the cerebellum and in the cerebral cortex. In fact the nervous system is very largely made up of reflex arcs and the associative and commissural neurones that relate them one to the other. On account of this fact, all those regions of the cerebral cortex not immediately connected with the afferent or efferent conduc tion paths were named, by Paul Flechsig, the association centers.
and others claim that neurones are joined together by fusion or "concrescence;" they claim actual con tinuity of the neurones forming a conduction path. In higher animals Heald has positively disproved this view of Bethe. He shows that the boundaries of the individual neurones are definite and unmistakable and do not fuse with one another. The relation is one of contact, synapsis. However, the complex ity of the contacts varies: (1) The simple end-brushes touch the dendrites or cell-body of the adjacent neurone. (2) The telo dendria are flattened and moss-like (Cajal), as in cerebellar cor tex. (3) The telodendria are shaped into cups, as the acustic cups of the trapezoid body. (4) Complicated end-brushes inter weave with equally complex dendrites, forming glomeruli, as in the olfactory bulb. (5) "Climbing fibers," the telodendria of corticipetal fibers, entwine about the dendrites of the Purkinje cells in cerebellar cortex. (6) Pericellular networks and baskets are formed by telodendria about the pyramids of cerebral cortex and Purkinje cells of cerebellum.
The white matter of the cerebro-spinal axis is made up chiefly of bundles of medullated axones imbedded in neuroglia and supported by connective tissue. The fibers possess no neuri lemma.
The gray matter of the central nervous system is composed of cell-bodies and dendrites, chiefly, but also contains axones.
These nerve elements are supported by connective tissue and blood-vessels and are imbedded in a great abundance of neu orglia. The nerve fibers in the gray matter are to a large extent non-medullated and naked.
The cerebral gray substance (substantia grisea cerebri) is con veniently divided into three groups or classes: I. Cortical.
incloses the white medulla, the centrum sentiovale. The cortex varies in thickness, 3 mm. being the average. Thickest On the surface of the gyrus, it grows thinner to the bottom of the sulci. In the floor of the small sulci near the frontal and occipital poles of the cerebral hemispheres, the thickness is only 1.5 mm.; but in the crown of the paracentral gyrus the cortex measures 5— mm. It is said to be slightly thicker in the left hemisphere: 3-43 per cent. thicker in left inferior frontal gyrus than in right (Mellus). According to H. Wagner, it has an area of 187,000— 2 2 1,000 sq. mm. If the average thickness is 3 mm., the en tire mass of cortex equals from 561-663 cc., a little more than half the bulk of the cerebrum. Its specific gravity iis• 1033. According to the amount of blood present in it, the cortex is pinkish- or yellowish-gray in color.
Various methods of investigation have shown that the cortex is divided into many regions or centers, each having its character istic structure and specific function. These centers have been located by pathologic and experimental studies in man and lower animals, by a study of myelinization in children from the fourth month in utero to the fourth month after birth (Flechsig), and by a careful microscopic study of sections of mature cortex (Campbell, Cajal, Bolton, Vogt, Mott, Brodmann, Griinbaum and Sherrington, etc.). G. Elliot Smith recognizes 28 types of cortex with the naked eye. Flechsig distinguishes 36 typical regions by his method. The "primary centers," Nos. r to to, are medullated before birth. They include the regions in immediate connection with the afferent and efferent conduction paths: the common sensory and motor regions, and the gusta tory, olfactory, visual and auditory. The "intermediate centers," Nos. II to 31, begin to medullate one month after birth, and the "final centers," Nos. 32 to 36, gradually medul late later, progressing certainly until the twentieth year and, according to Kaes, until about the forty-fifth year. The "final centers" comprise the psychic regions. Brodmann maps out 43 types of cortex; Alfred W. Campbell, 18 types (Figs. 74-79) Cortical or Cerebral Localization (Figs. 74 and 75).—In the following study of the cerebral cortex I shall use very exten sively the recent work of Dr. Alfred W. Campbell, entitled, "Histological Studies on the Localization of Cerebral Function," Cambridge, England. In this epoch-making work we are shown that certain cortical areas have a characteristic histological structure that distinguishes them from all other areas. This will be referred to later under "cell and fiber lamination of the cortex," but to appreciate this histological evidence of localiza tion one should thoroughly study the above work.