The Oviijin and Development of the Lymphatic System. 61 



as shown in fig. 8, grow (1) over the scapular region and (2) over 

 the occipital region. The lymphatics from the ventral border of the 

 apex are most interesting and complex. They grow ventralward as 

 shown in figs. 8, 16 and 17 and form a very extensive plexus along the 

 course of the external jugular vein, lateral to the sternocleidomastoid 

 muscle. This is the superficial cervical plexus and it becomes a laro-e 

 group of superficial lymphatic glands in the adult pig (Clark 24). 



The cervical plexus not only connects with the apex of the sac, but, 

 as is shown faintly in fig. 16, its caudal end has a group of vessels 

 that connects with vessels that follow the external jugular vein. In a 

 mesial sagittal section of injected pigs 5 to 6 cm. long it is plain that 

 a very abundant plexus of lmyphatics from the root of the sac stalk 

 completely surrounds the external jugular vein, and this external 

 jugular plexus has numerous connections with the cervical plexus at 

 its caudal end. The plexus around the external jugular vein empties 

 mainly into the sac stalk, but a single vessel may join the external 

 jugular vein. It must be said in general that the cervical plexus 

 develops from the jugular sac in two places: (1) from the apex of 

 the sac over the sternocleidomastoid muscle, and (2) from the sac 

 stalk along the external jugular vein. 



From the cervical plexus there are seven sets of lymphatic capillaries 

 (fig. 16). From the cerebral end of the cervical plexus vessels grow 

 (1) behind the ear, and into a new facial plexus from which vessels 

 grow (2) between the eye and the ear, and (3) over the face. From 

 the ventral border of the cervical plexus there is (4) a set of sub- 

 maxillary lymphatics which supplies the lower jaw and tongue and 

 (5) superficial lymphatics for the skin of the neck. From the caudal 

 end of the cervical plexus grow (6) the superficial lymphatics of the 

 arm, and (7) the superficial lymphatics of the thorax. 



Thus to sum up the superficial lymphatics for the jugular sacs : from 

 the sac stalk and anterior curvature arise the pharyngeal, Eustachian 

 and nasal vessels, from the apex of the sac arise the suprascapular 

 and occipital lymphatics, while from the cervical plexus arise the pos- 

 terior auricular, temporal, facial, submaxillary, anterior cervical and 

 the superficial lymphatics of the arm and thorax. These groups of 

 lymphatics begin as distinct plexuses of vessels which arise from the 

 sac or from the secondary cervical plexus which in turn comes from 

 the sac. In fig. 8 is shown particularly well how distinct the groups 

 may be at the start. Subsequently all of the groups anastomose and 



tlm'o 





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