ANATOMY. 



medullary cUord, about the size of the 

 ibre-finger, arising within the cranium 

 from the medulla oblongata ; leaving that 

 cavity at the foramen magnum occipitale, 

 and continued along the canal left in the 

 spine to the upper lumbar vertebrae, 

 where it terminates by forming the cauda 

 equina. 



It sends oft' a pair of nerves at each in- 

 terval between two vertebrae. It is co- 

 vered immediately by pia mater and tuni- 

 ca arachnoidea, and more loosely by a 

 sheath of dura mater, which lines the 

 whole spinal canal. It is plentifully sup- 

 plied with blood vessels. The nerves 

 come off from this body in numerous 

 threads, quite separate from each other at 

 first, but uniting afterwards. The cauda 

 equina consists of the medulla spinalis, 

 entirely resolved into a bundle of such 

 threads. 



Structure of the Nerves. The nerves 

 are soft, white, and fibrous chords, nearly 

 of a cylindrical shape, arising from the 

 brain, or medulla spinalis. When they 

 leave the brain, the pia mater collects the 

 fibres into larger or smaller fasciculi. 



The medullary filaments of the nerves 

 are covered by a vascular membrane, call- 

 ed by lieil neurilema, which detaches pro- 

 cesses from its inner surface, to surround 

 and invest the smaller divisions and fibres 

 of the medullary substance. By immers- 

 ing a nerve in alkali, its medulla is dissolv- 

 ed, and the containing membranous tubes, 

 formed of neurilema, are left. Acids dis- 

 solve the neurilema, and leave the medul- 

 lary fibres. These organs receive a con- 

 siderable supply of blood from vessels ra- 

 mifying on their neurilema. 



By maceration in water, and careful dis- 

 section, a nervous trunk may be separated 

 into numerous threads ; and each of these, 

 when examined in a microscope, seems to 

 be an assemblage of proportionably small- 

 er fibres. Greater magnifying powers 

 shew those fibres, which before appeared 

 simple, to be composed of still smaller 

 threads ; and it is doubtful, whether the 

 ultimate nervous fibre can be discovered. 

 All that is said, therefore, of the form, 

 course, &c. of these ultimate fibres is 

 wholly conjectural. The fibres do not pro- 

 ceed in a straight uninterrupted course, 

 but join frequently with each other. 



A nerve divided in tke living subject 

 retracts : the medulla is expressed from 

 its extremities, by the contraction of its 

 membranes, in the form of globules. If 

 the Annual be killed at some distance of 

 time from the operation of dividing a 

 nerve, the divided extremities are rather 



swoln, and are connected by a newly 

 formed matter. Anatomists have disput- 

 ed greatly, whether or not this be a real 

 nerve. As this question can hardly be 

 decided by merely anatomical testimony, 

 it appears most philosophical to inquire, 

 whether the new matter will perform the 

 functions of a nerve ; and this has been 

 completely proved by the experiments of 

 Dr. llaighton, in the first part of the Phi- 

 losophical Transactions for the year 1795 



In some parts of the nervous system, 

 little tubercles, or knots, called ganglia, 

 are found in the course of the nerve, and 

 are usually formed by the concurrence 

 of Severn branches. These bodies are of 

 various figures, but generally flattened. 

 They partake more of the red colour than 

 tUe trunks of the nerves on which they 

 are formed, as they possess more nume- 

 rous blood-vessels. They contain nervous 

 fibres, surrounded by a firm vascular sub- 

 stance. 



By the term, origin of a nerve, we un- 

 derstand its connection with the brain or 

 spinal marrow. This end is called its 

 sensorial extremity, being considered as 

 the point to which it conveys the impres- 

 sions made on it by external objects, and 

 from which it receives the commands of 

 the will, to be transmitted to the organs 

 which it supplies. 



There is considerable difference in, 

 form, structure, and consistence, betweea 

 the individual nerves. 



The nerves are arranged in pairs, as 

 they are exactly similar on both sides of 

 the body. Hence any pair of nerves con- 

 sists of the right and left nerve. 



They are sometimes divided into those 

 of the brain, and those of the medulla spi- 

 7ialis ; or into the nerves of the organs of 

 sense, the n-rves of motion, and the mixed 

 nerves; or, according to the nature of the 

 parts which they supply, into voluntary 

 and involuntary nerves. 



The quantity of nerves distributed to 

 the different structures in the body varies 

 greatly. The organs of sense receive the 

 most copious supply viz. the eye, the 

 nose, labyrinth of the ear, ends of the 

 fingers, glans penis et clitoridis, and the 

 rest of the skin. Muscles have also a 

 large share of nerves : the blood-vessels 

 are much more sparingly furnished. The 

 nerves of the viscera are very small in 

 proportion to the size of the organs. 

 Bones, cartilages, tendons, ligaments, 

 membranes, marrow, fat, have no discerni- 

 ble nerves. 



Nerves ramify through the body some- 

 thing like? arteries : thus, a nervous trunk 



