Chap, xlii.] CIRCULATIOlir. 2li 



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

 one hand, because the cardiacal impnlsioB is transmitted with a 

 diminishing intensity ; on the other hand, because the total 

 capacity of the arterial ramifications is gi-eater than that of the 

 trunks whence they emanate. 



At last the ai-terial 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 prijnordial 

 acts of nutrition. 



C. Capillary Vessels.— In many invertebrates, as we have §een, 

 the circulatory system is composed solely of some principal 

 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 

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

 which the finest vascular ramifications have their autonomy. In 

 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 

 vertebrates. They are fine canalicules, having nearly the 

 diameter of a sanguineous globule. The slimmest allow th^ 

 globules only to pass in file. Their diameter varies between 

 0°"°,007 and 0"""',030 ; they are constituted solely by a single 

 coat, with a thickness varying from 0""°,001 to 0""°,002. This 

 coat, everywhere homogeneous, is besprinkled with ovoid nuclei, 

 the main diameter whereof is directed toward the axis of the 

 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 0°"°,030 to 0»°',070; 

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



p 2 



