CHAP, xni.] CIRCULATION. 211 



degree that it approaches the capillaries. It slackens, on the 

 one hand, because the cardiacal impulsion is transmitted with a 

 diminishing intensity; on the other hand, because the total 

 capacity of the arterial ramifications is greater than that of the 

 trunks whence they emanate. 



At last the arterial blood, in a continuous and uniform current, 

 reaches the fine capillary ramifications, penetrating into the web 

 of the tissues, and connecting the arterial tree with the venous 

 tree. But this fine capillary network is the seat of pheno-" 

 mena very important, and entirely bound up with the primordial 

 acts of nutrition. 



C. Capillary Vessels. In many invertebrates, as we have seen, 



the circulatory system is composed solely of some principal 



1 vessels, beyond which the blood circulates in the interstices of 



the organs, in lacunae, without peculiar membraneous partitions. 



| On the contrary, in other invertebrates there are true capillary 



i vessels. Thus it is, for example, in the annelates (annulata), in 



I which the finest vascular ramifications have their autonomy. In 



I certain insects there exist also fine canalicules, comparable with 



the capillaries. In the cephalopal mollusks, we see, at many 



points of the body, the arteries terminating in veritable capillaries 



formed of a homogeneous membrane, which is bestrewn with 



elongated nuclei. 



Such is, verily, the texture of the true capillaries in the 

 I vertebrates. They are fine canalicules, having nearly the 

 1 diameter of a sanguineous globule. The slimmest allow the 

 I globules only to pass in file. Their diameter varies between 

 O mm ,007 and O mm ,030 ; they are constituted solely by a single 

 | coat, with a thickness varying from O mm ,001 to O mm ,002. This 

 I coat, everywhere homogeneous, is besprinkled with ovoid nuclei, 

 I the main diameter whereof is directed toward the axis of the 

 I vessel. If we follow the capillaries, either in the direction of 

 the arteries or in the direction of the veins, we see their diameter 

 ) enlarging little by little. It passes from O mm ,030 to O mm ,.070, 

 ' and a second membrane, or coat, clothes the first at the exterior. . 



p 2 



