ENPages
(seq. 51)
22 Lect. 2d
Direction. The Large Artery of an Ox will prove this, which may be easily acquired at any Time. the more it is stretched, the stronger is its Elasticity; whereas Muscles are not Elastic, altho' they possibly may have some elastic Fibres in their Texture. The Property of Muscles is to be red, not elastic; but the Coats of an Artery are elastic & not red, therefore they are not muscular, but of an elastic Texture like the Ligamentum Colli or white Leather in Brutes - The Coat of an Artery is of some considerable Thickness, & Anatomists divide it into an inner Coat, which has a smooth internal Surface, with a fine Polish, except where Branches go off - This is very thin, & closely un- ited to the next outer Coat. The Fibres are of a very fine Texture their Course not to be seen. The Uses of this Coat are 2. first to give free Course to the Motion of the Blood, 2d. to prevent the red Globules from transuding. In the dead Subject, or soon after Death these do transude, but it is not because the Coat has its Pores more opened, but because the Blood has lost its Tenacity Density. The next Coat is composed of Circular Fibres, having a slight adhesion to each other of the same Substance. The outside Coat is made up of Fibres, that run in every Direction of the same ligamentous Substance - Arteries lie in the Interstices of parts surrounded with cellular Membrane, which some have described as
(seq. 53)
23 Lect. 2d
the nervous Coat of the Artery, so called by the Antients. This is not properly a Coat; but a Connecting Medium, between the Artery & Muscles, every Fibre being surrounded in the same Manner, this was their fourth Coat, a fifth Coat is added by some in particular parts, as the Pericardium, & Peritoneum, tho' very improperly. The Coats of an Artery have Blood Vessels like other Parts, called Vasa Vasorum, but the Blood in these is said to be not red. These may be seen in a Preparation first injected minutely with red, then the red Injection forced out, & green thrown in its Room, which being of a darker Colour permits the red Vasa Vasorum to be seen - Blood enters into the Combination of every Part, but of some in no great Quantity, or not so as to carry red Globules; hence these Vessels appear white. Nervous Filaments are also bestowed on them, tho' not to be easily demonstrated, & are supposed to serve some important Use. The whole Bo- dy of the Artery is elastic; & has different Strata or Layers for different Purposes, & from their being in all Directions, they bring the Artery to its former State, & strengthen the Artery both later- ally & longitudinally, which is more expanded than lengthened by the Blood - The internal Coat answers two purposes; it both gives a smooth Surface, & does not hinder the Circulation, & is dense in Texture, that the finer Parts of the Blood may not transude. After Death the red Globules will transude, but not
(seq. 55)
24 Lect. 2d
in the living Body because the Blood is thoroughly mixed with the Gluten, which gives it some Degree of tenacity. 'tis probable that when the red Globules transude after Death that the Pores are not altered, but that the glutinous Part is in a Coagulated state the Particles being attracted to each other; thus the remaining Part fluids being thinner [?]
The Bile transudes in the living Body, tho' the Blood does not as may be seen by opening a Body immediately after Death, all the Parts near the Gall Bladder being of a Yellowish Hue. There is a twofold Motion of the Arteries viz. Systole & Diastole The 1st. is their Contraction, the other their swelling or Dilating & this Motion is owing to a Gush of Blood from the Heart which drives the Blood on before it. & the Artery yields by its Elasticity to this greater Quantity of Fluids pressing quaque versum hence the Artery becomes more capacious, & also something longer, these Convolutions may be seen in the Lungs of a Snake or a Viper, which are very long, & have an Artery running their whole Length. The Systole of the Heart is the Diastole of Arteries, & vice versa. But the Diastole of Arteries takes up much less Time, than their Systole, there being some considerable Time between die Strokes of the Pulse - Dr. Haler made an Experiment in Order to know the Force, which the Heart exerts in throwing the Blood into the Arterial System