322 ZOOLOGY FOR MEDICAL STUDENTS chap. 



This phenomenon observed during the development of the Dogfish 

 and other vertebrates — -the conversion of longitudinal ridges into rows 

 of pocket-valves^ — appears to be the repetition of a process which has 

 taken place during the evolution of this phylum, and one readily sees 

 the physiological advantage of replacing a complicated muscular and 

 nervous apparatus, with its greater liability to failure through pathological 

 interference, by an apparatus purely mechanical and automatic. 



The blood passing forwards from the heart is distributed through- 

 out the tissues by a system of vessels to which the early anatomists 

 gave the name arteries, meaning literally air-tubes, owing to the fact 

 that at death the strongly muscular walls of these vessels commonly 

 contract and drive the blood out of them so that when opened they 

 contain only air. This expulsion of blood from the arteries at death 

 gives them a characteristic pale colour in a dissection, in striking contrast 

 with the deep colour of the veins which as a rule remain gorged with 

 blood. 



Before sketching out the arrangement of the main arteries in the 

 Dogfish it is advisable to have a clear grasp of the general plan of 

 arrangement of the main arterial trunks in the region of the vertebrate 

 pharynx as these are of particular morphological importance (cf. Fig. 

 120, p. 293). 



(i) Of aortic arches the normal number is six and these are de- 

 nominated according to the visceral arch in which they are situated — 

 I, II, III, IV, V, VI or Mandibular, Hyoid, First Branchial, Second 

 Branchial, and so on. Where the arch is provided with gills the aortic 

 arch is not a wide-open channel throughout but has intercalated in its 

 course the capillary respiratory network. The ventral portion of the 

 arch, supplying this network, is known as the afferent branchial vessel 

 while the dorsal portion which drains the blood from the network is 

 known as the efferent vessel. Of the aortic arches the first (Mandibular) 

 is in most vertebrates a transitory structure in the embryo and soon 

 disappears : the second (Hyoidean) also usually disappears or becomes 

 much modified. 



(2) There is a tendency for the main longitudinal vessels — dorsal and 

 ventral aorta — to undergo a process of splitting from before backwards 

 into right and left halves which with growth become displaced outwards 

 so as to remain in comparative proximity to the gill-clefts. This is 

 clearly an arrangement for the economy of tissue by diminishing the 

 length of the afferent and efferent vessels. 



(3) The paired vessels arising by this process of splitting are con- 

 tinued forwards into the head as the carotid arteries. The prolongations 



