A. Pray what do you understand by a Tendon?

B. A Tendon is a Part confifting of nervous Fibres, devoid of any parenchymous or fleshy Subftance, invested in a Membrane common to all the Mufcles; and form what we call the Head C, and Tail D, of all Muscles; or those Parts by which they arise from, or are inferted into the Bones of the feveral Limbs of the Body: The Number of Fibres in every Tendon is equal to that of the Fibres of the Mufcle, and are the fame Fibres with them; they are those white, hard, compacted Bodies, which we vulgarly call Sinews.

A. What Part is that you call a Membrane, pray?

B. A Membrane is a Web of feveral Sorts of Fibres, interwoven for the covering and wrapping up of fome Parts; hence they are elastick, and of an exquifite Sense; here the innumerable Divifions, Windings, ferpentine Progreffions, and frequent Inofculations of Veins with Veins, and Arteries with Arteries, make a moft agreeable Embroidery, and delicate Net-work, covering the whole Membrane.

A. Pray, Sir, what is the Ufe of Membranes ?

B. To cover and wrap up the Parts; to ftrengthen them; to fave them from external

Of the Membranes and Nerves. 273 nal Injuries; to preferve the natural Heat ; to join one Part to another; to cause an exquifite Sense; to feparate a Humour (by its Glands) for moistening the Parts, &c.

A. Are there not divers Kinds of Membranes ?

B. Yes; as the Scarf-skin, covering the whole Body; the Skin of the Body it felf; the Meninges of the Brain; the Pleura in the Breaft; the Pericardium involving the Heart; the Periosteum investing the Bones; the Tunicles, or Coats of the Veffels, as the Stomach, Bladder, Veins, Arteries, Inteftines, Testicles, &c. are all Membranes. of different Kinds.

A. Pray what do you call a Nerve?

B. A Nerve is a long and fmall Bundle of very fine Pipes, or hollow Fibres, wraped up in the Membranes of the Brain, from whence they have their Beginning.

A. What do you find to be the Ufe of the Nerves?

B. It is very probably fuppofed, they are the immediate Organs of all Sensation; for to every external Organ of Sense, as the Eye, the Ear, the Nofe, the Tongue, are detached one or more Pair of thofe Nerves from the Brain; alfo thofe Nerves, which proceed from the fpinal Marrow, are spread through all Parts of the Mufcles and their Membranes, and to every Point in the SuT perficies

perficies of the Body; and thereby make the whole fenfible.

A. Pray by what Means do the Nerves render the Parts fenfible?

B. It is imagined, by the Motion of an exceeding fine and invifible Fluid they contain, called the animal Spirits; by which Impreffions are communicated to the Mind (whofe Seat is in the Brain) from all Parts of the animal Body.

A. How many, or what Quantity of Nerves may there be in the Body?

B. There be ten Pair, which proceed from the medullary Subftance of the Brain, which are chiefly diftributed to all the Parts of the Head and Neck; from the spinal Marrow there proceed through the Vertebræ, thirty Pair to all the other Parts of the Body; and thus in all there are forty Pair; for they come forth originally by Pairs.

A. Pray what do you call Arteries?

B. They are thofe Pipes, Tubes, or Channels, which convey the Blood from the Heart to all Parts of the Body.

A. Fray what is their Compofition or Texture?

B. They confift, or are composed of three Coats, viz. 1. The first feems to be a Web of fine Blood Veffels and Nerves, for nourishing the Coats of the Artery. 2. The fecond is made up of circular, or ra-. ther

t

Of the Arteries and Veins.

275 ther fpiral Fibres; of which there are more or fewer Lays, according to the Bignefs of the Artery. 3. The third and inmoft Coat is a fine, denfe, tranfparent Membrane, which keeps the Blood within its Channels; the Arteries branch out into various Ramifications, and become invifibly fmall at laft.

A Pray whence is that Pulfation we find in feveral Parts of our Bodies?

B. This is a Motion of the Arteries only; the Blood being ejected from the left Ventricle of the Heart into the great Artery, fome and fome at a Time, preffeth the Sides of the Artery, and caufes an intermitting Dilatation thereof; which is continued by the conftant pulfive Motion of the Blood, and the Spring or Elafticity of the Artery, acting on it; hence therefore enfues a conftant alternate Dilatation and Contraction of the Coats of an Artery; and is what we properly call the Puife.

A. Pray what is the Difference between Arteries and Veins ?

B. Veins are only a Continuation of the capillary Arteries at their Extremities, and convey the refluent Blood back again to the Heart; in their Return they unite their Channels as they approach the Heart, and form at last the three large Trunks, viz. the Vena Cava Afcendens, Defcendens, and the Vena Porta, as they are called. T 2

A. And,

A. And, pray, how happens it that Veins have no Pulfe?

B. Becaufe the Blood is poured into them at the Anaftomafis in one continued even Stream; and because it moves from a narrower Channel to a wider; the Preffure of the Blood against the Sides of the Veins being less than that against the Sides of the Arteries.

A. Pray what do you call those Ducts or tubular Veffels, you mentioned under the laft Head of folid Parts?

B. Those small, fine, hollow Pipes or Tubes difperfed through all Parts of the Body; which convey, fome a Lymph, called Lymphæducts; others a milky Liquor, called lacteal Veins; and others, convey other Juices and Humours of the Body to their proper Places.

A. Having thus heard you on the Nature and Ufe of the more fimple Solids; I shall be obliged to you for a small Account of the more compound folid Parts, which you call the great Organs of Life: And, pray, what is the Nature of the Brain.

B. The whole Subftance of the Brain is divided into two Parts; that which lies in the Fore-part of the Skull, called the Cerebrum; and that lying in the Back-part, called the Cerebellum. In the Cerebrum, or Fore-part of the Brain, there are obferved two Kinds of Subftances, the external and

inter