The brain as an organ of mind . brauous sheath. present a distmct appearance of fibrillation, are surrounded by a delicate membranous envelope, and the larger fibres are similarly formed by the running together of fibrils and smaller fibres. Nerve fibres thus compounded, both dark bordered andpale, similarly tend to aggregate into cords or bundles ofdifferent sizes, the fibres of which run parallel to one 34 THE STRUCTUllE OF another, and are invested by a sheath. These again, intheir course towards the centre, collect into larger andlarger bundles, the different elements of which are allbound
The brain as an organ of mind . brauous sheath. present a distmct appearance of fibrillation, are surrounded by a delicate membranous envelope, and the larger fibres are similarly formed by the running together of fibrils and smaller fibres. Nerve fibres thus compounded, both dark bordered andpale, similarly tend to aggregate into cords or bundles ofdifferent sizes, the fibres of which run parallel to one 34 THE STRUCTUllE OF another, and are invested by a sheath. These again, intheir course towards the centre, collect into larger andlarger bundles, the different elements of which are allbound together into one white trunk or * nerve (fig. 6),by means of a firm connective tissue envelope, whichsends thinner investing prolongations in amongst theconstituent cords. These nerves of various sizes frequently containwithin the same bundle both ingoing and outgoing fibres,and are then known as mixed nerves . Others containonly sensory, or only motor fibres. In their coursenerves often communicate freely one with another by. Fig. 6.—Portion of the Tnink of a Nerve, consisting of many smaller Cordswrapped up in a common Sheath (Quain after Sir C. Bell), a, the nerve ; b, a singlecord drawn out from the rest. Magnified several diameters. means of branches. Such communicating branches areespecially numerous in the course of the visceral nerves,and, when ma ay occur amon(]^st some particular set ofcords, what is termed a plexus is formed (fig. 7). In theseplexuses the individual nerve fibres do not undergodivision. Some of them merely leave one bundle or cordand pass to another, with the fibres of which they areultimately distributed, either to muscles or to nervecentres. The smaller medullatcd nerve fibres unite, so far as weknow, only near their commencements, and the largermotor fibres only undergo bifurcation near their termina-tions in muscles or glands (fig. 4). The fibrils or ele-mentary constituents of the fibres probably do not divide Ckap. II.] A NEIIVOUS SYSTEM. 35
Size: 3072px × 813px
Photo credit: © The Reading Room / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1, booksubjectbrain, booksubjectpsychologycomparative
