PHYSIOLOGICAL ANATOMY OF THE CAPILLARIES. 279 



iological view, is that the capillaries are the vessels which 

 have but a single, homogeneous tunic ; for in these the blood 

 is brought in closest proximity to the tissues. Vessels which 

 are provided, in addition, with a muscular, or muscular and 

 fibrous coats, are to be regarded as either small arteries, or 

 venous radicles. This view is favored by the character of 

 the currents of blood, as seen in microscopic observations on 

 the circulation in transparent parts. Here an impulse is 

 observed with each contraction of the heart, until we come 

 to vessels which have but a single coat, and are so narrow as 

 to allow the passage of but a single line of blood-corpuscles. 



Physiological Anatomy of the Capillaries. If the arteries 

 be followed out to their minutest ramifications, they will be 

 found progressively diminishing in size as they branch, and 

 their coats, especially the muscular, becoming thinner and 

 thinner, until at last they present an internal structureless 

 coat, provided with oval longitudinal nuclei ; a middle coat 

 formed of but a single layer of circular muscular fibres, the 

 oval nuclei of which are at. right angles to the nuclei of the 

 internal coat ; and an external coat composed of a very thin 

 layer of longitudinal fibres of the white inelastic tissue. Eobin 

 calls these the third variety of capillary vessels ; but they are 

 large, -^ to -^-J-g- of an inch in diameter, become smaller as 

 they branch, and -undoubtedly possess the property of con- 

 tractility, which is particularly marked in the arterial system. 

 Following the course of the vessels, when they are reduced in 

 size to about ^J^ of an inch, the external fibrous coat is lost, 

 and the vessel then presents only the internal structureless 

 coat, and the single layer of muscular fibres. These are 

 called by Eobin, capillaries of the second variety. They 

 become smaller as they branch, and finally lose the muscular 

 coat, and have then but the single amorphous tunic, with its 

 longitudinal nuclei. These, the capillaries of the first variety 

 of Robin, 1 we shall consider as the true capillary vessels. 



1 Dictionnaire de Medecine, etc., Paris, 1858 (Capillaire). This division of the 



