physical phenomena of the organism, as well as the tissues which are its seat. Then we shall consider the blood globule and its circulation, and finally, prepared by our knowledge of the accessories, we can more readily comprehend the more intricate relations of the internal and external coverings, and especially the epithelium of the genital organs, as well also as our point of departure, the ovum.

PART SECOND.

NERVOUS SYSTEM.

I. NERVOUS SYSTEM IN GENERAL.

1. Anatomical Elements. The nerve globule partakes

Fig. 7.-Nerve globules, their prolongations, nuclei, and nucleoli.*

two prolongations, striking out in

a,a,

of the general properties of the living globule;

its dimensions

are very small (one to eight-hundredths of a millimetre); but it attains in certain regions larger proportions, and may even with a little care be seen with the naked eye. The nerve globules are looked upon as cells having an envelope (?) enclosing protoplasmic elements, a nucleus, and a nucleolus.

These globules are generally stellate, that is to say, provided with prolongations (Fig. 7) ; at this present time globules having one prolongation are called unipolar, those having the same direction or

From the deep portion of the gray substance of the convolutions of the cerebellum. d, Cells from the posterior portion of the gray substance of spinal cord (dorsal). In all these globules the prolongations are more or less torn.

B

"

oftener in opposite directions, are bipolar; but most of them are multipolar, and may have as many as ten prolongations. These prolongations are ordinarily quite long, and constitute the nerve fibres. (Fig. 8.) These fibres are composed of a thin envelope (vv) (forming Schwann's sheath) encircling a medullary substance (myeline, mm) which may easily be decomposed into little drops of fat, and in the centre of this a thin axis cord (a) discerned with difficulty, the axis cylinder. Some fibres may be reduced to simple axis cylinder and to the peculiar sheath of Schwann without any medullary substance (fine fibres or filaments). The membrane of Schwann and the medullary sheath serve only for the protection and isolation of the axis cylinder. The axis cylinder thus appears to be the most important part of the nerve tube. Finally there is found in certain nerves, and especially in the branches of the great sympathetic, flat, pale, or amorphous fibres, rarely fibrillary, and furnished with very distinct nuclei (Fig. 8, A): (gray or gelatinous fasciculus); these are the fibres of Remak, which some physiologists (Morel) consider as belonging to the connective tissue, though the nerve character of these fibres is indicated by the history of the development of the nerve fibre, and by the study of the pale nerve elements in the lower animals.

770.

Fig. 8. Gray and white nerve fibres

1 Recent histological researches by Ranvier appear to show that the nerve tubes are formed of cells joined together at the ends. Не has also ascertained that the substance of Schwann does not form a continuous cylindrical axis, as has been hitherto supposed, but exhibits at regular intervals constrictions in the shape of rings.

A, Gray fasciculus, gelatinous, from the mesentery, treated by acetic acid. B, White primitive fibre, from crural nerve. a, Axis cylinder exposed. v,v, Fibre, with its medullary sheath, becoming varicose and oozing out in drops at m,m. C, Primitive fibre from brain, containing no myeline. 300 diam. (Virchow, "Cellular Pathology.")

It might be added that in certain little trunks, isolated from the great sympathetic nerve system, the number of these pale fibres is so large, and the number of tubes with medullary substance so small, we are obliged (especially in the splenic nerves) to consider Remak's fibres as true nerve fibres.

If these prolongations of the nerve globules are followed up carefully, the nerve tubes will be observed, after a shorter or longer distance, to be connected, in fact joined, with a neighboring or a distant globule, or sometimes with several of these. Thus in the spinal cord there are globules whose ramifications connect them with other globules. Sometimes the nerve fibres, on the other hand, terminate in muscles (motorial end-plates), or even in organs which are at present but problematical (tactile bodies), and which are specially found in the skin. It may also be noticed that generally nerve fibres are only commissures or bridges projecting from

La

These constrictions, placed at distances varying according to the dimensions of the tubes, enclose segments which are called interannular segments. Each of these appears to represent a cell; indeed, in the centre of each, and on the inner surface of the substance of Schwann is found a flat oval nucleus (Fig. 9) floating in a sea of protoplasm, with which the tissue is lined. Farther in is found the myeline, which, considered in regard to general morphology, bears the same relation to the interannular segment as the fat in an adipose cell does to the cell. The signification of the cylindrical axis, which runs uninterruptedly through the whole series of segments, has not yet been definitely ascertained from the standpoint of general morphology. The study of the degeneration of the nerves after section, seems to confirm the foregoing conclusions as to the nature of the interannular segments, without, however, yielding us any more precise information as to the nature of the axis cylinder, which is, notwithstanding, the essential element of the nerve tube. Indeed, it seems probable that the other appearances are simply due to the artificial methods used in the preparations.

A

searches.*

*A, Nerve tube under low magnifying power. a, Constriction. b, Nucleus of interannular segment. c, Axis cylinder. B, The constriction and part of interannular segment, seen under a higher power (prepared with osmic acid). a', Constriction. b', Nucleus in segment. c, External nucleus in sheath.

a nerve globule to an element of another variety, or simply to another nerve globule.

These nerve fibres seem to be only a physiological supplement to the globule from which they originate; every excitation of the fibre is retained by the globule, and vice versa: the fibre disconnected from its globule undergoes a degeneration (fatty) more or less complete.

2. Life of the Nervous System.This physiological whole (globule and its prolongations) lives and is nourished: the nerve centres, composed practically of globules, needs an enormous quantity of material, and gives back to the surrounding media (by means of the blood) a large quantity of refuse matter. The mass of nerve fibres (nerves) consumes likewise some materials, and produces refuse matter; they in other words are fed; they are very vascular, and when the supply of blood is shut off, phenomena resembling decomposition may be observed.

It will be noticed, farther on, that the materials consumed by the muscles during their activity are principally hydro-carbons (sugars and fats) and also albuminoids in small quantity. On the other hand, the nerve element seems to require albuminoid substances; and the more intense is the nerve work, the greater will be the amount of refuse material, from the combostion of the albuminoids (especially urea), in the excretions, in the urine, and in the products of the liver. According to Biasson (1868) the amount of urea excreted by man varies according to the amount of cerebral activity. Again, Oscar Liebreich has shown that, in animals who have been made to die by pain, after cutting the sensitive roots of one side of the spinal cord, this side (reduced to inertia) would consume less protagon than the other side. Protagon, whose composition is not yet defined, seems to be a compound of fatty phosphates and neurine, and serves for the nutrition of the nervous system, to which it is carried by the blood globule. According to Austin Flint, Jr., the excrementitial product formed by the disassimilation of the brain and of the nerves, at the expense of protagon, is represented by cholesterine, separated from the blood by means of the liver, and then thrown into the intestinal canal. This view is based upon a number of experiments, which show, moreover, that the excretion of cholesterine is in direct ratio with the nervous activity. The common expression, "to feel bilious," seems justified by one of the elements of the bile, viz., cholesterine.

These acts of nutrition produce in the nerves a disengagement of forces, which are brought to light by electrical currents; this phenomenon, though not directly observed in the nerve globules, is very evident in the peripheral nerves.