cortical or epithelial globules (AAA), from which some absorb, others excrete, and finally others are impermeable or neutral. In the interior, towards the middle far from the surface (Fig. 6, B), are found a group of globules, relatively permanent; viz., the nerve globules, which by means of their prolongations are in communication with the peripheral globules so as to be excited by one set and to react upon another (reflex actions). Thus the blood globules travel from the periphery towards the centre, and vice versû (Fig. 6, C C); and this circular current draws towards the centre the elements of nutrition absorbed by certain globules from the surface, and draws the decayed portions of the globules at the centre towards the globules upon the surface, which are then thrown off (excremental secretions). The blood globule thus acts as a medium of exchange, the same process in lower animals being effected by imbibition.

Fig. 6.

Diagram of the organism.*

Though these are the more simple forms of globular activity, yet it must not be forgotten, that these phenomena are also linked with those belonging to chemistry and physics, which likewise should be studied at the same time; as for instance the blood globule seems to be of service to the nerve globule by establishing for purposes of nutrition a communication between this deep-seated globule and those at the surface; but its circulation requires the intervention of the nerve globule, which may excite the muscular fibre, and thus give rise to certain mechanical phenomena of hydrostatics, etc.

Now it may be noticed that the collection of the phenomena of animal life constitutes a living chain that must be artificially broken for convenience of study. The most striking phenomena is the wandering of the blood globule; it might most naturally seem that the commencement of our study should be with this phenomenon; but we prefer to commence first with the nerve globule, which will lead us to study, secondly, the non-globular forms (muscles) with which it is connected; and subsequently the movements and other mechanical and

AAA, Globules from the surface, epithelium. B, Central nerve-globules, with prolongations coming from or going to the surface. CC, The circle of cir culation of the blood.

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

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

*a,a, 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.

A

B

m.

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.

1

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. He 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

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.

B

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* 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.