338 DISCOVERY REPORTS 



indeed, be mentioned here that there is a general tendency, noted by most previous 

 authors, towards a plexiform arrangement of all the venous channels of the body: the 

 interstices of all the tissues all over the body of a Rorqual foetus are filled with these 

 anastomosing venous channels. The brachial vein enters the brachiocephalic trunk by 

 a short wide stem formed by the brachial, axillary plexus and internal mammary, and 

 this may perhaps be considered to represent the subclavian vein. 



The two brachiocephalics join to form a wide precava entering the right auricle (Fig. 3). 



The posterior thoracic artery and vein constitute the main trunks supplying and 

 draining the great thoracic rete and the anterior intercostal areas. 



Between the 5th and 6th rib (Fig. 4 A) the posterior thoracic artery turns dorsally, 

 and running upwards and outwards, supplies by numerous twigs the longissimus 

 dorsi and semispinalis muscles above the 5th dorsal vertebra. Between each pair of 

 ribs anterior to this and in front of the 1st rib, which is cervical in its attachment, the 

 posterior thoracic artery has a branch to the dorsal musculature similar to that between 

 the 5th and 6th rib (Fig. 4 A). These are of increasing calibre as the series passes 

 forwards and the first of them, which is directed more obliquely than those behind it, 

 is a fairly stout trunk serving the muscles above the neck by numerous tufts of branches. 

 The dorsal musculature behind the 6th rib is supplied by a series of very much smaller 

 trunks arising from the segmental intercostal arteries of the dorsal aorta (Fig. 4 A). It is 

 thus seen that the blood supply to the dorsal muscles becomes increasingly generous 

 towards the anterior end of the body. 



The posterior thoracic vein emerges from the neural canal behind the 7th dorsal 

 vertebra. It passes downwards between the 7th and 8th ribs (Fig. 4 B), where it turns 

 forwards, and after passing ventral to the head of the 6th rib, accompanies the posterior 

 thoracic artery forwards to the anterior end of the thorax, where it has the relations 

 already described. 



A large thin-walled venous sinus accompanies the spinal cord latero-ventrally on 

 either side within the neural canal of the vertebral column. Between the vertebrae, 

 within the neural canal, the two venous sinuses on either side receive cross connections, 

 so that there is a ladder-like system of large venous channels lying in the neural canal 

 ventral to the spinal cord. These sinuses extend as far forward as the 7th cervical 

 vertebra, where, on each side, they turn ventrally to form the first thoracic intraspinous 

 vein of the series, shortly to be described, draining into the posterior thoracic vein. 

 Anterior to that point, up to the foramen magnum, the neural canal is filled with a plexus, 

 which is entirely venous, embracing the spinal chord and entering the skull through 

 the foramen magnum to form a vascular mass against the hinder surface of the brain. 

 Throughout the whole of their course in the neural canal these two large sinuses receive 

 veins between the vertebrae entering the canal by the foramina between the neural 

 spines (Fig. 4 B). Between the 7th and 8th dorsal vertebrae the vein traversing the 

 foramen between the neural spines is the first of the series draining from the neural 

 venous sinus into the posterior thoracic vein. It turns forward, as has been already 

 stated, ventral to the 6th rib to form the posterior thoracic vein itself. Similar intra- 



