VII.-Some peculiarities of the blood-vascular system of the porbeagle shark ( lamna cornubica )

Before the middle of last century, owing very largely to the prevalence of injection as a method of research, the coarse anatomy of the blood vascular system drew to itself a considerable measure of attention. Amongst the peculiarities of the system that more than others aroused interest and speculation during this period, were certain meshworks of arteries and veins that formed localised plexuses or retia of fine anastomosing vessels. These retia are found upon vessels supplying many different regions of the body, and occur somewhat haphazard, in genera belonging to practically all the leading groups of Vertebrates. A good summary of their distribution will be found in Milne Edwards’ ‘Leçons sur la Physiologic,’ vol. 3, 1858; it is sufficient here to point out that their structure and composition is far from being similar in all cases (Müller, 17, p. 275); and that, although there is noticeable a tendency to their formation in aquatic air-breathing animals, both birds and mammals, their occurrence depends neither on blood relationship nor on a universal similarity of habit, for they are to be found in creatures of such different mode of life as fish, birds, ungulates, cetacea, edentates and lemurs. It is a remarkable fact that in spite of the curiosity that these structures originally aroused, little attention appears to have been bestowed upon them for the last fifty or sixty years. This lapse of interest may no doubt be attributed to the predominant importance assumed in more recent times by purely morphological anatomy, although when looked at from a physiological standpoint, there is no question that these vascular adaptations are of very considerable interest and deserve more study than they have hitherto received.