334 



TERMINATION OF SENSORY NERVES. 



division the fibres usually become smaller ; but whilst some of the resulting small 

 fibres may be simple, many are really bundles of fine fibrils. 



These pale fibres bear nuclei, which probably appertain to the prolongation of 

 the primitive sheath ; the nuclei are common at the bifurcations of the fibres, where 

 they are of a triangular or irregular shape. The original dark-bordered fibres which 

 thus undergo division and change, or which may proceed singly to end in a different 

 and special manner, are commonly provided with a tolerably strong connective tissue 

 sheath with nuclei, which, as it stands well apart from the dark borders of the fibre, 

 is very conspicuous (sheath of Henle). This is derived from the perineurium which 

 incloses the funiculi of the nerve-trunks, and, as these part into smaller collections 

 and single fibres, undergoes a corresponding division, and finally sends sheaths along 

 single fibres. Within the sheath of Henle fine longitudinal connective tissue fibres, 

 with interspersed corpuscles, are seen surrounding the nerve -fibre or fibres. This 

 tissue is a prolongation of the endoneurium. 



In further treating of the termination of nerves, it will be convenient to consider 

 the sensory and motor nerves separately. 



TERMINATION OF SENSORY NERVES. 



The sensory or afferent nerves end either in cells or in free nerve-endings, which 

 may be simple or plexiform, and may be enclosed by cells or have an independent 

 distribution. Of the sensory nerves which terminate in cells, the best recognised 

 are those which are found in the organs of special sense. But these nerves may 

 perhaps be regarded as taking origin in the sense-organ rather than ending in it, 

 for if their development is studied it would appear, at least in the case of the 

 olfactory and visual organs, that the nerve-fibres grow from the sense-organ towards 

 the central nervous system and not centrifugally, as is the case with most other 

 nerve-fibres. Their mode of connexion with the cellular elements of the special 

 sense-organs will be studied most conveniently when those organs are themselves 

 treated of. 



Of the ordinary sensory nerves, including those which are devoted to the 

 perception of tactile sensations, some end in ramifications of the axis cylinder, 

 which resolves itself eventually into its ultimate fibrils, and thus penetrates 

 between the epithelium-cells which cover the sensory surface, whilst others 

 terminate in special organs, of which the best known are the tactile corpuscles, 

 simple and compound end-bulbs, the corpuscles of Grandry, which occur in birds, the 

 round end-bulbs of the human conjunctiva, cylindrical end-bulbs, corpuscles of 

 Herbst, also occurring in birds, and the corpuscles of Vater or Pacinian bodies. 



Fig. 389. A. Two TACTILE CELLS IN THE DEEPER PART OF THE HUMAN EPIDERMIS. (Merkel.) 



B. ENDING OF NERVES IN TACTILE DISKS IN THE PIG'S SNOUT. (Ranvier.) 



n, nerve-fibre ; m t terminal menisci or tactile disks ; e, ordinary epithelium-cell ; a, altered epi- 

 thelium cell, to which the meniscus is applied. 



