THE \l-:i.'KOGLIA 753 



various. Functionally, they are necessarily of two classes, motor and sensory. 

 Efferent fibres terminate chiefly upon muscle-cells forming the so-called end-plates. 

 The forms of termination in which nerve impulses originate, those of the senary or 

 afferent !i)nvs. vary from the simple or free termination of the telodendria upon 

 the .surfaces of epithelial cells to the most elaborate and complicated forms of the 

 so-called end-organs or corpuscles. If medullatcd, the axone always loses its 

 myeliii sheath upon approaching its termination, and necessarily the terminal twigs 

 are bare. 



The supporting and connective tissue of the nervous system is of two main 

 varieties white fibrous connective tissue and neuroglia. White fibrous tissue 

 alone supports and binds together the peripheral system, and it is the chief support- 

 ing tissue of the central system. As connective tissues, these two varieties are quite 

 similar in structure, each consisting of fine fibrilhi . either dispersed or in bundles, 

 among which are distributed the nuclei of the parent syncytium. In both tissues 

 nuclei are frequently found possessing varying amounts of cytoplasm which has not 

 yet been transformed into the essential fibrils. 



In addition to its enveloping membranes, which are wholly of white fibrous tis- 

 sue, the white fibrous tissue supporting the central system within is quite abun- 

 dant. It is all sent in from without, either as ingrowths of the pia mater, the most 

 proximal of the membranes, or is carried in with the blood-vessels, of the walls of 

 which it is an abundant component. Practically, the neuroglia as a connective tissue 

 proper differs from white fibrous tissue only in origin and in its chemical or staining 

 properties. Based upon the latter, there are methods of technique by which the 

 two may be distinguished. White fibrous tissue is derived from the middle germ 

 layer or the mesoderm, while neuroglia comes from the ectoderm. The epithelium 

 lining the central canal of the spinal cord and the ventricles of the encephalon, with 

 which the canal is continuous, is the remains of the mother tissue of the neuroglia, 

 and in the adult is the only vestige indicating its origin. The cells of this epithe- 

 lium are known as ependymal cells, and they are usually classed as a variety of 

 neuroglia. 



In its development the nervous system is precocious. It is the first of the systems to begin 

 differentiation, and is the first to assume form. The first trace of the embryo appears on the 

 developing ovum as the embryonic area, and the rapidly proliferating cells of that area shortly 

 become arranged into the three germinal layers:- an outer layer or ectoderm, a middle layer or 

 mesoderm, and an inner layer or enloderm. Early in the process of this arrangement there is 

 formed along the axial line of the embryonic area a thickened plate of ectodermal cells, the 

 neural plate. In the proliferation of these cells the margins of the neural plate, which lie parallel 

 with the long axis of the embryonic area, rise slightly above the general surface, forming the 

 neural folds, and the floor of the plate between the folds undergoes a slight invagination, the 

 process result ing in the neural groove (fig. 559). As development proceeds and the embryonic area 

 assumes the form of the distinct embryo, the neural folds or lips of the groove gradually con- 

 verge, and, beginning anteriorly, finally unite. Thus the groove is converted into the neural 

 tube, extending along the dorsal mid-line and enclosed within the body of the embryo by the 

 now continuous ectoderm above. For a time the neural tube remains connected with the inner 

 surface of the general ectoderm along the line of fusion by a strand of ectodermal cells, the neural 

 crest. This crest is derived from the ridges of cells which composed the transition between the 

 lips of the original groove and the general ectoderm, and whose fusion aided in the closure of 

 the tube. The essential elements of the entire nervous system together with the neuroglia tis- 

 sue are derived from the cells of the neural tube and the cells of the neural crest. 



Even before the caudal extremity of the tube is entirely closed, the cephalic portion under- 

 goes marked enlargement and becomes differentiated into three vesicular dilations, the primary 

 vesicles. By a series of further dilations, flexures of its axis, and localised thickenings of its walls, 

 the portion of the tube included in the three primary vesicles developes into the encephalon or 

 brain. The remainder of the tube becomes the spinal cord. 



The walls of the posterior vesicle give rise to the rhombencephalon, the cerebellum developing 

 from its dorsul wall and the pons and medulla oblongata from its ventral wall. Its cavity 

 persists and enlarges into the fourth ventricle of the adult. 



From the middle primary vesicle comes the mesencephalon, the corpora quadrigemina being 

 formed from its dorsal and the cerebral peduncles from its ventral wall. 



The anterior or first primary cerebral vesicle undergoes greater changes than either of the 

 others. At an earlv period three diverticula are given off from its anterior extremity, two lateral 

 and one mesial. The lateral are the primary optic vesicles and the mesial is the rudiment 

 of the cerebral hemispheres or tclencephalon. The remaining portion of t lie vesicle becomes 

 the diencephalon, or inter-brain, its lateral walls thickening into the thalami, the posterior 

 portion of its dorsal wall giving off a diverticulum. the epiphysis or pineal body, and from its 

 ventral wall projects a diverticulum which becomes the hypophysis with its tuber cinereum. 

 The stalks of the two optic vesicles represent the course of the optic nerves, while from their 

 extremities are developed the retinae, portions of the ciliary bodies, and portions of the iris of 

 each ocular bulb. 

 48 



