Page:The American Cyclopædia (1879) Volume XII.djvu/241

NERVE CELL NERVOUS SYSTEM 233 which is usually terminated by a slightly round- ed extremity. Nerves are divided- into motor and sensitive, according to the preponderance of the two kinds of nervous filaments in their tissue, and whether they are distributed to muscular or sensitive organs. A nerve distrib- uted to muscles is a motor nerve, and its irrita- tion produces a muscular contraction ; one dis- tributed to the integument is a sensitive nerve, and its irritation causes a painful sensation. Thus the seventh pair of cranial nerves, or the facial, belongs to the. motor nerves, and animates the superficial muscles of the face. The fifth pair, on the contrary, belongs to the sensitive nerves, and communicates sensibility to the integument of that part of the body. In point of fact, however, but few if any of the nervous trunks are exclusively motor or exclusively sensitive, since they generally re- ceive filaments of both kinds, either from their own roots or from other neighboring nerves. Thus the facial nerve has a certain degree of sensibility, which it derives from communications with the fifth pair ; and one portion of the fifth pair itself is motor in character, and animates the muscles of mastica- tion. When both kinds of filaments are min- gled together in a nerve in similar or nearly similar proportions, it is said to be a "mixed nerve," and is at the same time motor and sensi- tive. This is the case with all the spinal nerves, the branches of which are distributed both to the muscles and the integument of the body and limbs. When cut across, a nerve at once ceases to perform its functions. It can no longer transmit the nervous influence in either direction, and accordingly the parts to which it is distributed become paralyzed and insensi- ble. The nerve may however reunite after such an injury, and its natural functions thus become reestablished. The substance may even be regenerated when a considerable portion has been cut out. This reunion and regenera- tion of divided nerves takes place most readi- ly in young animals. Vulpian has found the sciatic nerve regenerated, in very young rats, after the excision of a little more than two inches of its length, in 17 days ; and in young cats sensibility has returned in the tongue, after excision of one inch of the lingual nerve, in 14 days. In adult animals, however, and in the human subject, a longer time is required for the regeneration of a divided nerve ; and the full restoration of its function is often not complete until after the lapse of several months. (See NERVOUS SYSTEM.) NERVE CELL, a form of animal cell found in the gray nervous matter of the nervous centres (see GANGLION), and also in the peripheral ex- pansion of some of the cranial nerves. Nerve cells are rounded or ovoid-shaped bodies, often with slender, elongated, tapering or branching processes extending from their periphery in va- rious directions. They vary in size from ^^ to vws of an inch in diameter. They consist of a soft granular substance, usually containing a considerable quantity of gray pigment, and a well defined round or oval nucleus, with a large and distinct nucleolus. Their branching pro- cesses often become continuous with the ulti- Nerve Cells from the Eetina of the Ox, magnified 850 times. mate nerve fibres, which are mingled in profu- sion with the nerve cells in the gray substance of nervous centres. They are regarded as the most important anatomical elements of the ner- vous centres, where nervous power originates, or the nervous impressions are finally received. NERVII, an ancient people, one of the most warlike tribes of Belgic Gaul. Their territory extended from the Sambre to the. ocean, and was partly covered by the Ardennes. Baga- cum (Bavay) was their capital, and Camaracum (Oambrai) one of their towns. They claimed a Germanic origin. They joined the Belgic league against Caesar, and, though numbering 60,000 men capable of bearing arms, were almost annihilated by him (57 B. C.). NERVOUS SYSTEM. One of the most impor- tant and remarkable characteristics that distin- guish the physical organization of animals from that of plants, is the existence in the former of a peculiar tissue or substance called nerve. The special endowment of this substance is what is termed sensibility, or that faculty by which an animal is capable of receiving impres- sions from external objects, and by which con- sequently it is brought into relation with the surrounding world. In most animals this tis- sue is so disposed as to form a distinct and complete apparatus called the nervous system. Its development is always in direct relation to the complexity of the physical organization and the degree of physical endowment. In the lowest forms of animal creation the existence of nervous matter is as yet undemonstrated, but it no doubt exists in many species where it has not yet been discovered, owing to the deli- cacy of its tissues and the imperfect means of observation. The tissue out of which the