THE ELOOD-VESSELS. 343 



E 



Section IV.— ANC^OLOGY, 



OR, DESCEIPTIOX OF THE BLOOD-YESSELS AXD 

 ABSOEBENT VESSELS. 



The vascular system, as a -whole, comprehends two sets of vessels, 

 viz., those carrying blood and those carrying lymph or chyle. The 

 first, constituting the sanguiferous system, includes the heart or central 

 propelling organ and the peripheral channels for the blood, viz., the 

 arteries, capillaries and veins. The absorbent system includes the 

 smaller and larger lymphatic and lacteal vessels, together with the 

 lymphatic and mesenteric glands with wliich many of these vessels are 

 connected. The descriptive anatomy of the heart is given along with 

 that of the thoracic viscera in the second volume : the account of the 

 minute structure of the blood-vessels and of the lymphatic vessels and 

 glands will be found in the part of the same volume which treats of 

 the General Anatomy. Under the present division, therefore, will be 

 brought only the descriptive anatomy of the principal blood-vessels and 

 absorbent vessels. 



1.— BLOOD-YESSELS. ARTERIES AND VEINS. 



The descriptive anatomy of the blood-vessels includes an account of 

 their form, position, mode of division, distribution, anastomosis with 

 each other, and relation to other parts. Seeing, however, that the 

 blood-vessels are subject to frequent variations, while the most constant 

 forms and modes of distribution are described as the normal, it will be 

 necessary also to make frequent reference to the more important 

 varieties which have been observed. 



The varieties of blood-vessels may consist either of a deviation from 

 the usual size of the channels or from their usual position and their 

 connection with other vessels. Thus, they may be described as con- 

 sisting in differences of origin from the main stem, or from a branch, 

 or from quite another source than that which is the most common 

 or usual. But some varieties are so common that it becomes doubtful 

 which form is to be described as normal. 



Many of the vascular varieties are not only compatible with life, 

 but cause no disturbance whatever in the performnuce of the ordinary 

 functions of the body. Others are of such a nature as to be coni- 

 patible only with the conditions of the circulation subsisting during 

 foetal or intra-uterine life, and therefore prove fatal at birth. Some 

 are of considerable interest from their frequency, and others from their 

 existing in situations which are liable to diseases requiring surgical 

 operations. 



Many vascular varieties repeat forms which are natural in diflFerent 

 species among the lower animals ; others are obviously due to the per- 

 sistence of early foetal forms of distribution ; and not a few are expli- 

 cable on the supposition of abnormal enlargement or diminution of 

 naturally existing vessels. 



