NERVOUS TISSUE. 57 



cated to be adequately treated here. All that need be said in this place is that, 

 as might be expected, the rigor is stronger the more powerful and more healthy 

 the muscles are, and, consequently, is both more powerful and more lasting in 

 cases of sudden or violent death. It also sets in later in such cases, while in 

 emaciated and exhausted subjects it is more rapid and transient: as is also the 

 case, according to Hunter, in animals which have been hunted to death. In 

 some instances of violent death in persons of robust frame, the rigor mortis has 

 not entirely disappeared till the end of the first week after death. In rare 

 cases, as in some instances of death from lightning, the muscles are found rigid 

 immediately, and in other cases rigor commences in a few minutes, but usually 

 not till six or seven hours after death. The cessation of rigidity in the muscles 

 must be regarded as the commencement of putrefactive changes. 



NEKYOUS TISSUE. 



The Nervous Tissue is composed chiefly of two different structures, the gray 

 or vesicular, and the white or fibrous. It is in the former, as is generally sup- 

 posed, that nervous impressions and impulses originate, and by the latter that 

 they are conducted. Hence the gray matter forms the essential constituent of 

 all the ganglionic centres, both those separated in the ganglia, and those aggre- 

 gated in the cerebro-spinal axis ; while the white matter is found in all the 

 commissural portions of the nerve centres, and in all the cerebro-spinal nerves. 

 Besides these two principal kinds of nervous matter, there is found a third 

 structure chiefly in the sympathetic system called the gelatinous nerve-tissue. 



The nervous substance is again divided into two different systems. The first 

 is connected directly with the great central mass inclosed in the skull and 

 spine. This is called the cerebro-spinal system, and is divided into the brain (in- 

 cluding the medulla oblongata), the spinal cord, the cranial nerves, the spinal 

 nerves, and the ganglia connected with both those classes of nerves. The 

 second, called the sympathetic system, is not directly connected with the brain 

 or spinal cord, though it is so indirectly by means of its numerous communi- 

 cations with the cranial and spinal nerves. It consists of a double chain of 

 ganglia, with the branches which go to and come from them. 



A third method of division of the nervous system is based upon the functions 

 which it performs. On this principle it is divided into the nervous system of 

 animal life and the nervous system of organic life the former subserving the 

 higher functions of volition, sensation, &c., the latter those of growth and nu- 

 trition. It is clear that the former qualities reside mainly in the cerebro-spinal 

 system, while the intimate connection between the sympathetic nerve and the 

 great viscera renders it highly probable that the sympathetic system has mainly 

 to do with the organic functions. Consequently the cerebro-spinal system was 

 designated the system of animal life, and the sympathetic the system of organic 

 life. But the distinction, though true to a certain extent, is by no means com- 

 plete, as the student may easily see by consulting the works of modern phy- 

 siologists. 



The gray or vesicular nervous substance is distinguished by its dark reddish- 

 gray color and soft consistence. It is found in the brain, spinal cord, and 

 various ganglia, intermingled with the fibrous nervous substance, but is never 

 found in the nerves. It is composed, as its name implies, of vesicles, or cor- 

 puscles, commonly called nerve-corpuscles or ganglion-corpuscles, containing 

 nuclei and nucleoli; the vesicles being imbedded either in a fine granular sub- 

 stance, as in the brain, or in a capsule of nucleated cells, as in the ganglia. 

 Each vesicle consists of an exceedingly delicate membranous wall, inclosing a 

 finely granular material, part of which is occasionally of a coarser kind, and of 

 a reddish or yellowish-brown color. The nucleus is vesicular, much smaller 

 than the vesicle, and adherent to some part of its interior. The nucleolus, 

 which is inclosed within the nucleus, is vesicular in form, of minute size, and 



