226 THE VOYAGE OF H.M.S. CHALLENGER. 



Around the oesophageal opening upon the under surface the fibres from the crura meet at its 

 posterior part, and pass on to the central tendon on each side of the vena cava ; upon the upper surface 

 the fibres from the right crus divide and run along each side of it, and end behind the vena cava. 

 The vena cava is fibrous on the under surface of the diaphragm ; the upper is not, but it receives 

 a few fibres from the left side. 



In Arctoccphalus as in Phoca the diaphragm has a costal and a vertebral origin. The former 

 arises by fleshy slips from the ensiform sternebra and not from the spade- shaped cartilage attached 

 to it, posteriorly from the posterior surface of the 8th rib, and from the posterior and inner 

 surfaces of the 9 th to the 14th ribs. These also interdigitate with the transversalis. The latter 

 origin consists of two crura, the left crus arises by muscular fibres from the body of the 1st and 2nd 

 lumbar vertebra; and the disc between, and by tendon from the 2nd and 3rd lumbar vertebras 

 and the disc between. The right crus is larger than the left, and arises by muscular fibres from 

 the 1st, 2nd, and 3rd lumbar vertebras and the discs between, and by tendon from the 3rd and 4th 

 lumbar; the crura expand and form an oval slip which fits into the back of the central tendon. 

 The tendon is V-shaped, and the crura are attached to its dorsal side. The fibres from the ensiform 

 cartilage and the ribs pass towards the anterior part of the tendon, those from the 14th rib meet the 

 central tendon midway between the opening for the vena cava and this rib, and the gap between is 

 filled in by fibrous tissue. The oesophagus is in the apex of the central tendon, the vena cava to 

 the right, and the aorta between the crura. 



In both specimens the left phrenic nerve pierces the diaphragm half an inch to the right of the 

 vena cava ; the right goes through the same spot on the other side, which is one and a half inch to 

 the left of the vena cava, and they supply the muscle. 



The Deep Muscles of the Back. 



The muscles may be considered in the following groups : — The serratus posticus, splenius, 

 erector spinas, complexus and transverso-spinales, interspiuales, intertransversales and interzyga- 

 pophyses, and the short postero-cranio-vertebral muscles. 



The Serratus posticus in Phoca vitulina is a very thin muscular band, and arises from the 2nd 

 to the 5th dorsal vertebras. The fibres course downwards and backwards, and are inserted by a 

 short aponeurosis into the lower borders of the 5th to the 9th ribs, outside the tendons of the ilio- 

 costalis. In the large Phoca vitulina it arises by thin tendons from the same vertebras and from 

 the intervals between the vertebras by thin aponeuroses, and is inserted into the posterior borders 

 of the 6th to the 10th ribs by fine tendons. As it is not mentioned by Murie in Otaria and 

 Trichechus, I conclude it is absent. It is supplied by the external branches of the dorsal spinal 

 nerves. 



The Splenius in Phoca vitulina, is not a double muscle as in human anatomy, neither is it strap- 

 shaped, but triangular. It is hidden by the cephalo-humeral, rhomboideus-capitis and cervicis, 

 and arises from the ligamentum nuchas, by muscular fibres in its posterior part, and by a fine 

 aponeurosis in its anterior. It extends posteriorly from where the fibres of the rhomboideus 

 cervicis begin to take a transverse course from the middle line of the neck, which is about one 

 inch anterior to the vertebral anterior angle of the scapula, and terminates anteriorly at the back 



