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The Pineal Gland is placed amidst nervous masses, and seems to have some resemblance to them. consists of tubules, nucleated vesicles, and brain sand, composed of phosphate and carbonate of lime, which is not developed till seven years of age. The habenæ are thought to establish some nervous connexion, but we are as ignorant of its use as when Descartes guessed it to be the seat of the soul.

The Pituitary Body is composed of gland-like vesicles and abundant capillaries. Luschka believes it quite analogous to a structure he has discovered at the other extremity of the cranio-spinal axis, "the coccygeal gland;" and he regards these two, and the suprarenal capsules, "nerve-glands" of the sympathetic.

The Pacchionian Bodies are probably hypertrophied villi of the arachnoid membrane, developed where that membrane is exposed to much friction or stretching, consequent on the respiratory and circulatory movements of the brain.

NUTRITION, which we shall now very briefly review, is a great formative operation, to which all the processes we have hitherto described are but accessory. Its offices are development, growth, and maintenance. All parts, in performing their function, undergo destruction, and the rate of this "wear and tear" is proportional to their functional activity. Thus, every muscular contraction, even so slight as those by which I write, produces the elements of the destruction of muscle, viz., urea, water, and carbonic acid; and every mental act leads to destruction of the nervous matter of the brain, the phosphorus of which is excreted mainly as the phosphates in urine. Mr. Paget states four conditions are requisite for healthy nutrition: 1, a right state of the blood; 2, a regular supply of it; 3, a certain influence of the nervous system; and, 4, a normal state of the part to be maintained. 1. The blood contains, either in a crude

or prepared state, all the matters which each tissue attracts and appropriates; and as Treviramus well expressed it, "each single part of the body, in respect of its nutrition, stands to the whole body in the relation of a secreted substance." This law explains the presence of many parts otherwise apparently useless; but they withdraw from the blood matters whose retention would be injurious. One organ or tissue is complemental to the others. Either albumen or fibrin, or both, are the main nutritive matters. The white cells, too, as has been stated, seem concerned in the process; but the red cells, from not agreeing in size with those of any tissue, are not directly engaged in tissue-forming, but rather carry oxygen for their activity and subsequent destruction. 2. Every surgeon must have seen the effect of a diminished supply of blood in producing want of nutrition and gangrene in a limb; and Hunter's experiments of transplanting cock's spurs to their combs, where they increased enormously, show the effect of an augmented supply of blood. 3. As nutrition goes on in plants, in animals before their nervous system is developed, and in acephalous fœtuses, its influence over the process has been denied. It is, however, proved, for instance, by the sloughing of the cornea which follows division of the trifacial nerve, especially beyond the Casserian ganglion, an effect which seems to show that this regulating power rather belongs to the sympathetic. In many invertebrates, too, the presence and size of ganglia, and certain organs, correspond-as likewise their changes during metamorphosis. 4. A healthy condition of the part is evidently necessary to produce new additions of similar matter; and even in disease, as shall be afterwards noticed, the special condition of the part keeps up a modified nutrition, producing similar structures. It has been objected that complete molecular changes cannot occur in such organs as the brain, which stores up impressions made upon it many years before; but Prof.

Paget, in his classical lectures on Surgical Pathology, eloquently says: "The answer is, because of the exactness of assimilation accomplished in the formative process the effect once produced by an impression upon the brain, whether in perception or in intellectual act, is fixed and there retained; because the part, be it what it may, which has been thereby changed, is exactly represented in the part which, in the course of nutrition, succeeds to it. Thus, in the recollection of sensuous things, the mind refers to a brain in which are retained the effects, or rather the likenesses of changes that past impressions and intellectual acts had made. As, in some way passing far our knowledge, the mind perceived and took cognizance of the changes made by the first impression of an object acting through the sense-organs on the brain; so, afterwards, it perceives and recognizes the likeness of that change in the parts inserted in the process of nutrition. Yet here also the tendency to revert to the former condition, or to change with advancing years, may interfere. The impress may be gradually lost or superseded, and the mind, in its own immortal nature unchanged and immutable by anything of earth, no longer finds in the brain the traces of the past." Growth differs from nutrition in there occurring, besides the renewal of destroyed tissue, the addition of more material, which increases the size, weight, and function of the part. It is but a difference of degree. Growth does not always cease with the apparent maturity of the body, as the heart and arteries enlarge even to hale old age, increased propelling force being then required owing to the rigidity of tissues. The controlling influence of the nervous system and the modifying effects of disease will be hereafter considered.

QUESTIONS FOR EXAMINATION.
JUNIOR.

1. Enumerate the ductless or vascular glands.

2. Describe the capsule of the spleen and its processes.

3. Sketch the position, structure, and supposed functions of the suprarenals.

4. What uses have been attributed to the thyroid body?

5. Which of the ductless glands are most like nervous matter?

SENIOR.

1. Describe the spleen-cells and their surrounding vessels. 2. What facts respectively support the theories that the spleen is a blood-reservoir and a cell-former?

3. What effects on the lungs of an animal does close confinement produce, and how have they been removed?

4. Compare the structures of the thymus and thyroid.

5. What are the requirements for healthy nutrition according to Paget?

INNERVATION.

ANIMALS are most plainly distinguished from the other division of the organic world by a function which controls all other physiological processes, by conveying impressions to, or from, nervous centres along conductors we term nerves.

Neurine, or nervous matter, is the agent employed in this mysterious function. It is a white or grey opaque and soft substance, mechanically supported by areolar tissue or blood-vessels. The specific gravity of that of the brain is 1.039. Its composition varies at different periods, as will be seen by the following analyses of the brain:

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The fatty substances are, besides olein, margarin, and

cholesterin, cerebric and oleophosphoric acids, discovered by Fremy, and which exist in combination with soda, like a soap. The phosphorus is chiefly united with the latter acid, and seems to be proportional to the activity of nervous function, during the exercise of which it is oxidised and excreted as phosphates in the urine.

Nervous matter is of two kinds, readily distinguished by colour, the grey and white, and still better identified as the vesicular and the tubular by the microscope.

Vesicular Neurine covers the surface of the brainhence the ill-applied term of cortical-but lies in the substance of the spinal cord and ganglia. It is softer and more vascular than white neurine, containing more water and less fat. The vesicles are large, contain a nucleus, which is also cellular, possessing a bright nucleolus, and much pigmentary granular matter, which, with that between the vesicles, gives the characteristic dark grey tint. They are ovoid or spheroid in the sympathetic ganglia, and stellate or caudate in the cerebro-spinal centres, the largest specimens occurring in the locus niger of the crus cerebri. It is probable, but can be hardly regarded as certain, that these caudate projections are continued into the nerve-tube. Such is figured by Kölliker,

tic Nerve-Cell.

Spinal Nerve-
Cell.

Ovoid Sympathe Stellate Cerebro- after a drawing by the Marquis Corti, and the continuity may be demonstrated in the ventral ganglion of the leech. Owsjannikow asserts that the cells in the spinal cord are

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