278 Dr. Waller on some Tissues of the Animal Frame, 



conveys them to the venous branches. The movement of the 

 blood in the gland may be compared to that in small whirl- 

 pools; and at first it does appear as if the blood were animated 

 by a circular movement, in a small spherical cavity at the cen- 

 tre of the gland, without its being contained in any capillary 

 vessel. This appearance however is illusory, and created by 

 the rapid motion of the blood in a coil of transparent vessels, 

 which are twisted up on this spot into a spherical shape. The 

 limits of this vascular nucleus are well-defined, and are gene- 

 rally found to occupy half the diameter of the gland, or about 

 ToW tn of an inch. When engorged with blood it is easy to 

 perceive the coils of capillaries ; and their continuity may be 

 traced to the ends, which, as in fig. 2, bring the blood and 

 convey it away. The two extremities are generally found at 

 opposite sides of the gland, as in fig. 2. When perfectly 

 empty it is almost impossible to detect these coils ; but there 

 may be seen, however, a distinct outline, which probably con- 

 sists of the membrane, upon which is secreted the fluid con- 

 tained in the external transparent area. The rate at which 

 the blood moves in these coils is constantly varying ; some- 

 times it is so quick that the globules are only perceptible by 

 a flitting appearance, and in a few moments afterwards it be- 

 comes stationary. In favourable circumstances, when the red 

 globules are scanty and the circulation slow, one of them will 

 arrive in the gland, and may be followed in the various gyra- 

 tions it performs ; at one moment ascending in the superficial 

 curves,at other times descending and passing into others placed 

 deeper. Some idea may be formed in this way of the consider- 

 able extent of the capillary convolutions. The blood does not 

 circulate with equal rapidity in different glands at the same mo- 

 ment ; for there may be seen at the same time some in which 

 the blood is stationary, and others in which it moves with ex- 

 treme quickness. The external area, which, divested of the 

 epithelial scales, would be transparent, comprises the other 

 half of the diameter of the gland: within this the contents are 

 perfectly motionless. In some there appears a radiated struc- 

 ture distinct from the epithelial covering, constituting separate 

 compartments or cells in its interior : they contain a fluid 

 substance void of granules or other particles perceptible by 

 the microscope. 



In general no opening is to be perceived on the surface of 

 the gland for the passage of the fluid after it has been secreted ; 

 but in favourable instances a small opening with scales of epi- 

 thelium around its borders can be seen, descending apparently 

 into the cavity of the gland. Although it is difficult to demon- 

 strate the existence of an aperture or duct, it is easy to ascer- 



