STRUCTURE OF CAPILLARIES. 



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and agminated intestinal glands and adjacent mucous membrane, etc., the 

 small blood-vessels and capillaries commonly receive a coating of connective 

 tissue corpuscles, which are s-imilar to those of the retiform tissue and con- 

 nected with the fine trabeculse of the network, by which the vessels are 



Fig. XCVIII. A SMALL ARTERY A, WITH A CORRESPONDING VEIN B, TREATED WITH 

 ACETIC ACID, AND MAGNIFIED 350 DIAMETERS (after Kolliker). 



a, external coat with oblong nuclei ; & nuclei of the transverse muscular tissue of the 

 middle coat (when seen endwise, as at the sides of the vessel, their outline is circular); 

 7, nuclei of the epithelium- cells ; 8, elastic layers of the inner coat. 



thus supported. On the smallest capillaries the corpuscles are but sparingly 

 distributed, but nevertheless afford a continuous covering to the vessel by 

 their finely reticulating outrunners. This coating is named by His, who 

 has most fully described and figured it, the adventitia capillaris. 



Vital properties. From the share which the capillaries take in many vital actions 

 both healthy and diseased, and especially from the part they have been supposed to 

 play in the process of inflammation, much pains has naturally been bestowed to find 

 out whether they are endowed with vital contractility. There is still, however, a 

 difference of opinion on this question ; and, while this property evidently exists in 

 vessels, however small, provided with a muscular coat, it has not been shown by 

 equally direct evidence, to belong to the more simply constructed capillaries ; and it 

 must be confessed, that the proofs commonly adduced of the existence of vital con- 

 tractility in these vessels, are ambiguous and inconclusive. These proofs are chiefly 

 the two following : viz., 1st, That stimulants, such as alcohol, oil of turpentine, 

 pepper, and ice or ice-cold water, applied to the frog's foot or mesentery, cause the 

 capillary vessels to shrink in diameter, and that this contraction is speedily followed 

 by their dilatation beyond their natural capacity ; the shrinking of the vessels being 

 attributed to the direct operation of the stimuli on their contractility, and their sub- 

 sequent dilatation to the temporary exhaustion of that property, consequent on its 

 previous undue excitation. 2udly, That, when the vessels are preternaturally dilated 

 in the way above described; or by the action of ammonia or common salt, they 



