THE VASCULAR SYSTEM 259 



bladder from which pass two pairs of small vessels. Each pair 

 accompanies the hypoglossal nerve of one side and enters the sinus 

 thyroideus of that side. Between the radials on either side of the 

 copula further bladder-like extensions of the saccus occur, which are 

 drained by a pair of sinuses lying dorsal to the cerato-hyals. From 

 their position they may be named the hyoidean sinuses (s.ly.hy.). 

 Each joins the sinus mandibularis profundus of its side and enters 

 the jugular sinus. On the dorsal side of the branchial cartilages on 

 either side, and filling the space between them with the exception of 

 that occupied by the M. rectus cervicis, is the branchial sinus (s. ly.br.). 

 This also arises from the saccus copularis, but flows into the T. peri- 

 pharyngeus. Its course between the posterior end of the branchial 

 cartilages and the peri-pharyngeal trunk is sinuous and follows the 

 lingual vein. 



Tht jugular sinus (s.ly.j.) lies directly behind the corner of the 

 mouth. Into it drain (i) the combined SS. mandib. prof, and hyoi- 

 deus, (ii) the direct continuation of the S. circum. scap., and (iii) 

 numerous small vessels from the occipital region, the thymus gland, 

 and the antrum petrosum laterale. It discharges directly into the peri- 

 pharyngeal trunk. 



It must be noted here that the large space into which the posterior 

 cornu of the hyoid projects is labelled by Driiner 'Sinus lymphaticus'. 

 It is, however, not a true lymph sinus. Of the several injections that 

 have been made in the course of the present investigation, in none 

 has the injection mass used entered this space, neither has it been 

 possible to find any opening therefrom. It therefore seems to be a 

 closed sac of connective tissue whose sole function is to allow the free 

 extension of the hyobranchial cartilages. 



Extending posteriorly from the copula in the middle line is a median 

 sinus connecting the saccus copularis with the Truncus peri-pharyn- 

 geus. It also receives many tributaries from the floor of the mouth. 



G. Lymph hearts and their Relation to the Blood-vessels (PI. XVII). 



As previously mentioned there is a series of contractile vesicles along 

 each side of the body in the sulcus lateralis, which place the lymphatic 

 system in communication with the V. lateralis. There are normally 

 fifteen pairs of these hearts. Four pairs are post-sacral, i.e. caudal, and 

 the remaining eleven pairs lie between the sacrum and the posterior 

 border of the scapula. There is some variation in size — those lying 

 in the caudal and sacral regions being the largest, while those in the 

 middle of the trunk are the smallest. Each heart lies embedded in 

 the muscles directly behind the myoseptum, at the lateral extremity 



